This manual is last updated 15 December 2004 for version 1.1.23 of GNU TLS.
Copyright © 2004 Simon Josefsson
Copyright © 2001, 2002, 2003, 2004 Nikos Mavroyanopoulos
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
This document tries to demonstrate and explain the GnuTLS library API. A brief introduction to the protocols and the technology involved, is also included so that an application programmer can better understand the GnuTLS purpose and actual offerings. Even if GnuTLS is a typical library software, it operates over several security and cryptographic protocols, which require the programmer to make careful and correct usage of them, otherwise he risks to offer just a false sense of security. Security and the network security terms are very general terms even for computer software thus cannot be easily restricted to a single cryptographic library. For that reason, do not consider a program secure just because it uses GnuTLS; there are several ways to compromise a program or a communication line and GnuTLS only helps with some of them.
This document tries to be self contained, although basic network programming and PKI knowlegde is assumed in most of it. Peter Gutmann's “Everything you never wanted to know about PKI but were forced to find out”1 is a good introduction to Public Key Infrastructure.
Updated versions of the GnuTLS software and this document will be available from http://www.gnutls.org/ and http://www.gnu.org/software/gnutls/.
In brief GnuTLS can be described as a library which offers an API to access secure communication protocols. These protocols provide privacy over insecure lines, and were designed to prevent eavesdropping, tampering, or message forgery.
Technically GnuTLS is a portable ANSI C based library which implements the TLS 1.0 (See Introduction to TLS, for a more detailed description of the protocols) and SSL 3.0 protocols, accompanied with the required framework for authentication and public key infrastructure. The library is available under the GNU Lesser GPL license2. Important features of the GnuTLS library include:
Additionally GnuTLS provides a limited emulation API for the widely used OpenSSL3 library, to ease integration with existing applications.
GnuTLS consists of three independent parts, namely the “TLS protocol part”, the “Certificate part”, and the “Crypto backend” part. The `TLS protocol part' is the actual protocol implementation, and is entirely implemented within the GnuTLS library. The `Certificate part' consists of the certificate parsing, and verification functions which is partially implemented in the GnuTLS library. The Libtasn14, a library which offers ASN.1 parsing capabilities, is used for the X.509 certificate parsing functions, and Opencdk5 is used for the OpenPGP key support in GnuTLS. The “Crypto backend” is provided by the Libgcrypt6 library.
In order to ease integration in embedded systems, parts of the GnuTLS library can be disabled at compile time. That way a small library, with the required features, can be generated.
A brief description of how GnuTLS works internally is shown at the figure below. This section may be easier to understand after having seen the examples (see examples).
As shown in the figure, there is a read-only global state that is initialized once by the global initialization function. This global structure, among others, contains the memory allocation functions used, and some structures needed for the ASN.1 parser. This structure is never modified by any GnuTLS function, except for the deinitialization function which frees all memory allocated in the global structure and is called after the program has permanently finished using GnuTLS.
The credentials structure is used by some authentication methods, such as certificate authentication (see Certificate Authentication). A credentials structure may contain certificates, private keys, temporary parameters for diffie hellman or RSA key exchange, and other stuff that may be shared between several TLS sessions.
This structure should be initialized using the appropriate initialization functions. For example an application which uses certificate authentication would probably initialize the credentials, using the appropriate functions, and put its trusted certificates in this structure. The next step is to associate the credentials structure with each TLS session.
A GnuTLS session contains all the required stuff for a session to handle one secure connection. This session calls directly to the transport layer functions, in order to communicate with the peer. Every session has a unique session ID shared with the peer.
Since TLS sessions can be resumed, servers would probably need a database backend to hold the session's parameters. Every GnuTLS session after a successful handshake calls the appropriate backend function (See resume, for information on initialization) to store the newly negotiated session. The session database is examined by the server just after having received the client hello7, and if the session ID sent by the client, matches a stored session, the stored session will be retrieved, and the new session will be a resumed one, and will share the same session ID with the previous one.
In GnuTLS most functions return an integer type as a result. In almost all cases a zero or a positive number means success, and a negative number indicates failure, or a situation that some action has to be taken. Thus negative error codes may be fatal or not.
Fatal errors terminate the connection immediately and further sends
and receives will be disallowed. An example of a fatal error code is
GNUTLS_E_DECRYPTION_FAILED. Non-fatal errors may warn about
something, ie a warning alert was received, or indicate the some
action has to be taken. This is the case with the error code
GNUTLS_E_REHANDSHAKE returned by gnutls_record_recv.
This error code indicates that the server requests a re-handshake. The
client may ignore this request, or may reply with an alert. You can
test if an error code is a fatal one by using the
gnutls_error_is_fatal.
If any non fatal errors, that require an action, are to be returned by a function, these error codes will be documented in the function's reference. See Error Codes, for all the error codes.
GnuTLS internally handles heap allocated objects
differently, depending on the sensitivity of the data they
contain. However for performance reasons, the default memory functions
do not overwrite sensitive data from memory, nor protect such objects
from being written to the swap. In order to change the default
behavior the gnutls_global_set_mem_functions function is
available which can be used to set other memory handlers than the
defaults.
The Libgcrypt library on which GnuTLS depends, has such secure memory allocation functions available. These should be used in cases where even the system's swap memory is not considered secure. See the documentation of Libgcrypt for more information.
There are several cases where GnuTLS may need some out of band input from your program. This is now implemented using some callback functions, which your program is expected to register.
An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer.
gnutls_transport_set_push_function
gnutls_transport_set_pull_function
Other callback functions such as the one set by
gnutls_srp_set_server_credentials_function, may require more
complicated input, including data to be allocated. These callbacks
should allocate and free memory using the functions shown below.
gnutls_malloc
gnutls_free
TLS stands for “Transport Layer Security” and is the successor of SSL, the Secure Sockets Layer protocol8 designed by Netscape. TLS 1.0 is an Internet protocol, defined by IETF9, described in RFC 2246 and also in RESCOLA. The protocol provides confidentiality, and authentication layers over any reliable transport layer. The description, below, refers to TLS 1.0 but also applies to SSL 3.0 since the differences of these protocols are minor. Older protocols such as SSL 2.0 are not discussed nor implemented in GnuTLS since they are not considered secure today.
TLS 1.0 is a layered protocol, and consists of the Record Protocol, the Handshake Protocol and the Alert Protocol. The Record Protocol is to serve all other protocols and is above the transport layer. The Record protocol offers symmetric encryption, data authenticity, and optionally compression.
The Alert protocol offers some signaling to the other protocols. It can help informing the peer for the cause of failures and other error conditions. See The Alert Protocol, for more information. The alert protocol is above the record protocol.
The Handshake protocol is responsible for the security parameters' negotiation, the initial key exchange and authentication. See The Handshake Protocol, for more information about the handshake protocol. The protocol layering in TLS is shown in the figure below.
TLS is not limited to one transport layer, it can be used above any transport layer, as long as it is a reliable one. A set of functions is provided and their purpose is to load to GnuTLS the required callbacks to access the transport layer.
gnutls_transport_set_push_function
gnutls_transport_set_pull_function
gnutls_transport_set_ptr
These functions accept a callback function as a parameter. The
callback functions should return the number of bytes written, or -1 on
error and should set errno appropriately.
GnuTLS currently only interprets the EINTR and EAGAIN errno
values and returns the corresponding GnuTLS error codes
GNUTLS_E_INTERRUPTED and GNUTLS_E_AGAIN. These values
are usually returned by interrupted system calls, or when non blocking
IO is used. All GnuTLS functions can be resumed (called
again), if any of these error codes is returned. The error codes
above refer to the system call, not the GnuTLS function,
since signals do not interrupt GnuTLS' functions.
By default, if the transport functions are not set, GnuTLS
will use the Berkeley Sockets functions. In this case
GnuTLS will use some hacks in order for select to
work, thus making it easy to add TLS support to existing
TCP/IP servers.
The Record protocol is the secure communications provider. Its purpose is to encrypt, authenticate and –optionally– compress packets. The following functions are available:
gnutls_record_send
To send a record packet (with application data).
gnutls_record_recv:
To receive a record packet (with application data).
As you may have already noticed, the functions which access the Record protocol, are quite limited, given the importance of this protocol in TLS. This is because the Record protocol's parameters are all set by the Handshake protocol.
The Record protocol initially starts with NULL parameters, which means no encryption, and no MAC is used. Encryption and authentication begin just after the handshake protocol has finished.
Confidentiality in the record layer is achieved by using symmetric
block encryption algorithms like 3DES, AES10, or stream algorithms like
ARCFOUR_12811. Ciphers are encryption algorithms that use a single, secret,
key to encrypt and decrypt data. Block algorithms in TLS also provide
protection against statistical analysis of the data. Thus, if you're
using the TLS 1.0 protocol, a random number of blocks will
be appended to data, to prevent eavesdroppers from guessing the actual
data size.
Supported cipher algorithms:
3DES_CBC
3DES_CBC is the DES block cipher algorithm used with triple
encryption (EDE). Has 64 bits block size and is used in CBC mode.
ARCFOUR_128
ARCFOUR is a fast stream cipher.
ARCFOUR_40
This is the ARCFOUR cipher that is fed with a 40 bit key,
which is considered weak.
AES_CBC
AES or RIJNDAEL is the block cipher algorithm that replaces the old
DES algorithm. Has 128 bits block size and is used in CBC mode. This
is not officially supported in TLS.
Supported MAC algorithms:
MAC_MD5
MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs
128 bits of data.
MAC_SHA
SHA is a cryptographic hash algorithm designed by NSA. Outputs 160
bits of data.
MAC_RMD160
RIPEMD is a cryptographic hash algorithm developed in the framework of
the EU project RIPE. Outputs 160 bits of data.
The TLS record layer also supports compression. The algorithms implemented in GnuTLS can be found in figure compression. All the algorithms except for DEFLATE which is referenced in TLSCOMP, should be considered as GnuTLS' extensions12, and should be advertised only when the peer is known to have a compliant client, to avoid interoperability problems.
The included algorithms perform really good when text, or other compressible data are to be transfered, but offer nothing on already compressed data, such as compressed images, zipped archives etc. These compression algorithms, may be useful in high bandwidth TLS tunnels, and in cases where network usage has to be minimized. As a drawback, compression increases latency.
The record layer compression in GnuTLS is implemented based on the paper TLSCOMP.
Supported compression algorithms:
Some weaknesses that may affect the security of the Record layer have been found in TLS 1.0 protocol. These weaknesses can be exploited by active attackers, and exploit the facts that
Those weaknesses were solved in TLS 1.1 which is implemented in GnuTLS. For a detailed discussion see the archives of the TLS Working Group mailing list and the paper CBCATT.
The Alert protocol is there to allow signals to be sent between peers.
These signals are mostly used to inform the peer about the cause of a
protocol failure. Some of these signals are used internally by the
protocol and the application protocol does not have to cope with them
(see GNUTLS_A_CLOSE_NOTIFY), and others refer to the
application protocol solely (see GNUTLS_A_USER_CANCELLED). An
alert signal includes a level indication which may be either fatal or
warning. Fatal alerts always terminate the current connection, and
prevent future renegotiations using the current session ID.
The alert messages are protected by the record protocol, thus the information that is included does not leak. You must take extreme care for the alert information not to leak to a possible attacker, via public log files etc.
gnutls_alert_send:
To send an alert signal.
gnutls_error_to_alert:
To map a gnutls error number to an alert signal.
gnutls_alert_get:
Returns the last received alert.
gnutls_alert_get_name:
Returns the name, in a character array, of the given alert.
The Handshake protocol is responsible for the ciphersuite negotiation, the initial key exchange, and the authentication of the two peers. This is fully controlled by the application layer, thus your program has to set up the required parameters. Available functions to control the handshake protocol include:
gnutls_cipher_set_priority:
To set the priority of bulk cipher algorithms.
gnutls_mac_set_priority:
To set the priority of MAC algorithms.
gnutls_kx_set_priority:
To set the priority of key exchange algorithms.
gnutls_compression_set_priority:
To set the priority of compression methods.
gnutls_certificate_type_set_priority:
To set the priority of certificate types (e.g., OpenPGP,
X.509).
gnutls_protocol_set_priority:
To set the priority of protocol versions (e.g., SSL 3.0,
TLS 1.0).
gnutls_set_default_priority:
To set some defaults in the current session. That way you don't have
to call each priority function, independently, but you have to live
with the defaults.
gnutls_credentials_set:
To set the appropriate credentials structures.
gnutls_certificate_server_set_request:
To set whether client certificate is required or not.
gnutls_handshake:
To initiate the handshake.
The Handshake Protocol of TLS 1.0 negotiates cipher suites
of the form TLS_DHE_RSA_WITH_3DES_CBC_SHA.
The usual cipher suites contain these parameters:
DHE_RSA in the example.
3DES_CBC in this example.
MAC_SHA is used in the above example.
The cipher suite negotiated in the handshake protocol will affect the Record Protocol, by enabling encryption and data authentication. Note that you should not over rely on TLS to negotiate the strongest available cipher suite. Do not enable ciphers and algorithms that you consider weak.
The priority functions, dicussed above, allow the application layer to enable and set priorities on the individual ciphers. It may imply that all combinations of ciphersuites are allowed, but this is not true. For several reasons, not discussed here, some combinations were not defined in the TLS protocol. The supported ciphersuites are shown in ciphersuites.
In the case of ciphersuites that use certificate authentication, the
authentication of the client is optional in TLS. A server
may request a certificate from the client – using the
gnutls_certificate_server_set_request function. If a
certificate is to be requested from the client during the handshake,
the server will send a certificate request message that contains a
list of acceptable certificate signers. The client may then send a
certificate, signed by one of the server's acceptable signers. In
GnuTLS the server's acceptable signers list is constructed
using the trusted CA certificates in the credentials structure.
The gnutls_handshake function, is expensive since a lot of
calculations are performed. In order to support many fast connections
to the same server a client may use session resuming. Session
resuming is a feature of the TLS protocol which allows a
client to connect to a server, after a successful handshake, without
the expensive calculations. This is achieved by using the previously
established keys. GnuTLS supports this feature, and the
example (see ex:resume-client) illustrates a typical use of it.
Keep in mind that sessions are expired after some time, for security reasons, thus it may be normal for a server not to resume a session even if you requested that. Also note that you must enable, using the priority functions, at least the algorithms used in the last session.
The resuming capability, mostly in the server side, is one of the problems of a thread-safe TLS implementations. The problem is that all threads must share information in order to be able to resume sessions. The gnutls approach is, in case of a client, to leave all the burden of resuming to the client. Ie. copy and keep the necessary parameters. See the functions:
gnutls_session_get_data
gnutls_session_get_id
gnutls_session_set_data
The server side is different. A server has to specify some callback functions which store, retrieve and delete session data. These can be registered with:
gnutls_db_set_remove_function
gnutls_db_set_store_function
gnutls_db_set_retrieve_function
gnutls_db_set_ptr
It might also be useful to be able to check for expired sessions in
order to remove them, and save space. The function
gnutls_db_check_entry is provided for that reason.
A number of extensions to the TLS protocol have been proposed mainly in RFC 3546 (http://www.ietf.org/rfc/rfc3546.txt). The extensions supported in GnuTLS are
discussed in the subsections that follow.
This extension allows a TLS 1.0 implementation to negotiate
a smaller value for record packet maximum length. This extension may
be useful to clients with constrained capabilities. See the
gnutls_record_set_max_size and the
gnutls_record_get_max_size functions.
A common problem in HTTPS servers is the fact that the TLS protocol is not aware of the hostname that a client connects to, when the handshake procedure begins. For that reason the TLS server has no way to know which certificate to send.
This extension solves that problem within the TLS protocol,
and allows a client to send the HTTP hostname before the handshake
begins within the first handshake packet. The functions
gnutls_server_name_set and gnutls_server_name_get can be
used to enable this extension, or to retrieve the name sent by a
client.
The TLS protocol provides confidentiality and encryption, but also offers authentication, which is a prerequisite for a secure connection. The available authentication methods in GnuTLS are:
X.509 certificates contain the public parameters, of a public key algorithm, and an authority's signature, which proves the authenticity of the parameters. See The X.509 trust model, for more information on X.509 protocols.
OpenPGP keys also contain public parameters of a public key algorithm, and signatures from several other parties. Depending on whether a signer is trusted the key is considered trusted or not. GnuTLS's OpenPGP authentication implementation is based on the TLSPGP proposal.
See The OpenPGP trust model, for more information about the OpenPGP trust model. For a more detailed introduction to OpenPGP and GnuPG see Mike Ashley's The GNU Privacy Handbook14.
In GnuTLS both the OpenPGP and X.509 certificates are part of the certificate authentication and thus are handled using a common API.
When using certificates the server is required to have at least one
certificate and private key pair. A client may or may not have such a
pair. The certificate and key pair should be loaded, before any
TLS session is initialized, in a certificate credentials
structure. This should be done by using
gnutls_certificate_set_x509_key_file or
gnutls_certificate_set_openpgp_key_file depending on the
certificate type. In the X.509 case, the functions will
also accept and use a certificate list that leads to a trusted
authority. The certificate list must be ordered in such way that every
certificate certifies the one before it. The trusted authority's
certificate need not to be included, since the peer should possess it
already.
As an alternative, a callback may be used so the server or the client specify the certificate and the key at the handshake time. That callback can be set using the functions:
gnutls_certificate_server_set_retrieve_function
gnutls_certificate_client_set_retrieve_function
Certificate verification is possible by loading the trusted
authorities into the credentials structure by using
gnutls_certificate_set_x509_trust_file or
gnutls_certificate_set_openpgp_keyring_file for openpgp
keys. Note however that the peer's certificate is not automatically
verified, you should call gnutls_certificate_verify_peers,
after a successful handshake, to verify the signatures of the
certificate. An alternative way, which reports a more detailed
verification output, is to use gnutls_certificate_get_peers to
obtain the raw certificate of the peer and verify it using the
functions discussed in The X.509 trust model.
In a handshake, the negotiated cipher suite depends on the
certificate's parameters, so not all key exchange methods will be
available with some certificates. GnuTLS will disable
ciphersuites that are not compatible with the key, or the enabled
authentication methods. For example keys marked as sign-only, will
not be able to access the plain RSA ciphersuites, but only the
DHE_RSA ones. It is recommended not to use RSA keys for both
signing and encryption. If possible use the same key for the
DHE_RSA and RSA_EXPORT ciphersuites, which use signing,
and a different key for the plain RSA ciphersuites, which use
encryption. All the key exchange methods shown below are available in
certificate authentication.
Note that the DHE key exchange methods are generally
slower15 than plain RSA and require Diffie
Hellman parameters to be generated and associated with a credentials
structure. The RSA-EXPORT method also requires 512 bit RSA
parameters, that should also be generated and associated with the
credentials structure. See the functions:
gnutls_dh_params_generate2
gnutls_certificate_set_dh_params
gnutls_rsa_params_generate2
gnutls_certificate_set_rsa_export_params
Sometimes in order to avoid bottlenecks in programs it is usefull to store
and read parameters from formats that can be generated by external programs such
as certtool. This is possible with GnuTLS by using the following
functions:
gnutls_dh_params_import_pkcs3
gnutls_rsa_params_import_pkcs1
gnutls_dh_params_export_pkcs3
gnutls_rsa_params_export_pkcs1
Key exchange algorithms for OpenPGP and X.509 certificates:
RSA:
The RSA algorithm is used to encrypt a key and send it to the peer.
The certificate must allow the key to be used for encryption.
RSA_EXPORT:
The RSA algorithm is used to encrypt a key and send it to the peer.
In the EXPORT algorithm, the server signs temporary RSA parameters of
512 bits – which are considered weak – and sends them to the client.
DHE_RSA:
The RSA algorithm is used to sign Ephemeral Diffie Hellman parameters
which are sent to the peer. The key in the certificate must allow the
key to be used for signing. Note that key exchange algorithms which
use Ephemeral Diffie Hellman parameters, offer perfect forward
secrecy. That means that even if the private key used for signing is
compromised, it cannot be used to reveal past session data.
DHE_DSS:
The DSS algorithm is used to sign Ephemeral Diffie Hellman parameters
which are sent to the peer. The certificate must contain DSA
parameters to use this key exchange algorithm. DSS stands for Digital
Signature Standard.
The anonymous key exchange perform encryption but there is no indication of the identity of the peer. This kind of authentication is vulnerable to a man in the middle attack, but this protocol can be used even if there is no prior communication and trusted parties with the peer, or when full anonymity is required. Unless really required, do not use anonymous authentication. Available key exchange methods are shown below.
Note that the key exchange methods for anonymous authentication require Diffie Hellman parameters to be generated and associated with an anonymous credentials structure.
Supported anonymous key exchange algorithms:
ANON_DH:
This algorithm exchanges Diffie Hellman parameters.
Authentication using the SRP16. The SRP key exchange is an extension to the TLS 1.0 protocol protocol is actually password authentication. The two peers can be identified using a single password, or there can be combinations where the client is authenticated using SRP and the server using a certificate.
The advantage of SRP authentication, over other proposed secure password authentication schemas, is that SRP does not require the server to hold the user's password. This kind of protection is similar to the one used traditionally in the UNIX /etc/passwd file, where the contents of this file did not cause harm to the system security if they were revealed. The SRP needs instead of the plain password something called a verifier, which is calculated using the user's password, and if stolen cannot be used to impersonate the user. See TOMSRP for a detailed description of the SRP protocol and the Stanford SRP libraries, which includes a PAM module that synchronizes the system's users passwords with the SRP password files. That way SRP authentication could be used for all the system's users.
The implementation in GnuTLS is based on paper TLSSRP. The supported SRP key exchange methods are:
SRP:
Authentication using the SRP protocol.
SRP_DSS:
Client authentication using the SRP protocol. Server is
authenticated using a certificate with DSA parameters.
SRP_RSA:
Client authentication using the SRP protocol. Server is
authenticated using a certificate with RSA parameters.
If clients supporting SRP know the username and password
before the connection, should initialize the client credentials and
call the function gnutls_srp_set_client_credentials.
Alternatively they could specify a callback function by using the
function gnutls_srp_set_client_credentials_function. This has
the advantage that allows probing the server for SRP
support. In that case the callback function will be called twice per
handshake. The first time is before the ciphersuite is negotiated,
and if the callback returns a negative error code, the callback will
be called again if SRP has been negotiated. This uses a
special TLS-SRP handshake idiom in order to avoid,
in interactive applications, to ask the user for SRP
password and username if the server does not negotiate an
SRP ciphersuite.
In server side the default behaviour of GnuTLS is to read
the usernames and SRP verifiers from password files. These
password files are the ones used by the Stanford srp libraries
and can be specified using the
gnutls_srp_set_server_credentials_file. If a different
password file format is to be used, then the function
gnutls_srp_set_server_credentials_function, should be called,
in order to set an appropriate callback.
Some helper functions such as:
gnutls_srp_verifier
gnutls_srp_base64_encode
gnutls_srp_base64_decode
Are included in GnuTLS, and may be used to generate, and maintain SRP verifiers, and password files. A program to manipulate the required parameters for SRP authentication is also included. See srptool, for more information.
In GnuTLS every key exchange method is associated with a
credentials type. So in order to enable to enable a specific method,
the corresponding credentials type should be initialized and set using
gnutls_credentials_set. A mapping is shown below.
Key exchange algorithms and the corresponding credential types:
| Key exchange | Client credentials | Server credentials
|
|---|---|---|
KX_RSA
| ||
KX_DHE_RSA
| ||
KX_DHE_DSS
| ||
KX_RSA_EXPORT
| CRD_CERTIFICATE
| CRD_CERTIFICATE
|
KX_SRP_RSA
| CRD_SRP
| CRD_SRP
|
KX_SRP_DSS
| CRD_CERTIFICATE
| |
KX_SRP
| CRD_SRP
| CRD_SRP
|
KX_ANON_DH
| CRD_ANON
| CRD_ANON
|
Several parameters such as the ones used for Diffie-Hellman
authentication are stored within the credentials structures, so all
sessions can access them. Those parameters are stored in structures
such as gnutls_dh_params and gnutls_rsa_params, and
functions like gnutls_certificate_set_dh_params and
gnutls_certificate_set_rsa_export_params can be used to
associate those parameters with the given credentials structure.
Since those parameters need to be renewed from time to time and a
global structure such as the credentials, may not be easy to modify
since it is accessible by all sessions, an alternative interface is
available using a callback function. This can be set using the
gnutls_certificate_set_params_function. An example is shown
below.
#include <gnutls.h>
gnutls_rsa_params rsa_params;
gnutls_dh_params dh_params;
/* This function will be called once a session requests DH
* or RSA parameters. The parameters returned (if any) will
* be used for the first handshake only.
*/
static int get_params( gnutls_session session,
gnutls_params_type_t type,
gnutls_params_st *st)
{
if (type == GNUTLS_PARAMS_RSA_EXPORT)
st->params.rsa_export = rsa_params;
else if (type == GNUTLS_PARAMS_DH)
st->params.dh = dh_params;
else return -1;
st->type = type;
/* do not deinitialize those parameters.
*/
st->deinit = 0;
return 0;
}
int main()
{
gnutls_certificate_credentials_t cert_cred;
initialize_params();
/* ...
*/
gnutls_certificate_set_params_function( cert_cred, get_params);
}
The X.509 protocols rely on a hierarchical trust model. In this trust model Certification Authorities (CAs) are used to certify entities. Usually more than one certification authorities exist, and certification authorities may certify other authorities to issue certificates as well, following a hierarchical model.
One needs to trust one or more CAs for his secure communications. In that case only the certificates issued by the trusted authorities are acceptable. See the figure above for a typical example. The API for handling X.509 certificates is described at section sec:x509api. Some examples are listed below.
An X.509 certificate usually contains information about the certificate holder, the signer, a unique serial number, expiration dates and some other fields RFC3280 as shown in the table below.
version:
The field that indicates the version of the certificate.
serialNumber:
This field holds a unique serial number per certificate.
issuer:
Holds the issuer's distinguished name.
validity:
The activation and expiration dates.
subject:
The subject's distinguished name of the certificate.
extensions:
The extensions are fields only present in version 3 certificates.
The certificate's subject or issuer name is not just a single string. It is a Distinguished name and in the ASN.1 notation is a sequence of several object IDs with their corresponding values. Some of available OIDs to be used in an X.509 distinguished name are defined in gnutls/x509.h.
The Version field in a certificate has values either 1 or 3 for version 3 certificates. Version 1 certificates do not support the extensions field so it is not possible to distinguish a CA from a person, thus their usage should be avoided.
The validity dates are there to indicate the date that the specific certificate was activated and the date the certificate's key would be considered invalid.
Certificate extensions are there to include information about the certificate's subject that did not fit in the typical certificate fields. Those may be e-mail addresses, flags that indicate whether the belongs to a CA etc. All the supported X.509 version 3 extensions are shown in the table below.
subject key id (2.5.29.14):
An identifier of the key of the subject.
authority key id (2.5.29.35):
An identifier of the authority's key used to sign the certificate.
subject alternative name (2.5.29.17):
Alternative names to subject's distinguished name.
key usage (2.5.29.15):
Constraints the key's usage of the certificate.
extended key usage (2.5.29.37):
Constraints the purpose of the certificate.
basic constraints (2.5.29.19):
Indicates whether this is a CA certificate or not.
CRL distribution points (2.5.29.31):
This extension is set by the CA, in order to inform about the issued CRLs.
In GnuTLS the X.509 certificate structures are handled using
the gnutls_x509_crt_t type and the corresponding private keys
with the gnutls_x509_privkey_t type. All the available
functions for X.509 certificate handling have their prototypes in
gnutls/x509.h. An example program to demonstrate the X.509
parsing capabilities can be found at section ex:x509-info.
Verifying certificate paths is important in X.509 authentication. For
this purpose the function gnutls_x509_crt_verify is
provided. The output of this function is the bitwise OR of the
elements of the gnutls_certificate_status enumeration. A
detailed description of these elements can be found in figure below.
The function gnutls_certificate_verify_peers is equivalent to
the previous one, and will verify the peer's certificate in a TLS
session.
CERT_INVALID:
The certificate is not signed by one of the known authorities, or
the signature is invalid.
CERT_REVOKED:
The certificate has been revoked.
CERT_SIGNER_NOT_FOUND:
The certificate's issuer is not known.
Although the verification of a certificate path indicates that the certificate is signed by trusted authority, does not reveal anything about the peer's identity. It is required to verify if the certificate's owner is the one you expect. See RFC2818 and section ex:verify for an example.
A certificate request is a structure, which contain information about an applicant of a certificate service. It usually contains a private key, a distinguished name and secondary data such as a challenge password. GnuTLS supports the requests defined in PKCS #10 RFC2986. Other certificate request's format such as PKIX's RFC2511 are not currently supported.
In GnuTLS the PKCS #10 structures are handled
using the gnutls_x509_crq_t type. An example of a certificate
request generation can be found at section ex:crq.
A PKCS #12 structure PKCS12 usually contains a user's private keys and certificates. It is commonly used in browsers to export and import the user's identities.
In GnuTLS the PKCS #12 structures are handled
using the gnutls_pkcs12_t type. This is an abstract type that
may hold several gnutls_pkcs12_bag_t types. The Bag types are
the holders of the actual data, which may be certificates, private
keys or encrypted data. An Bag of type encrypted should be decrypted
in order for its data to be accessed.
An example of a PKCS #12 structure generation can be found at section ex:pkcs12.
The OpenPGP key authentication relies on a distributed trust model, called the “web of trust”. The “web of trust” uses a decentralized system of trusted introducers, which are the same as a CA. OpenPGP allows anyone to sign anyone's else public key. When Alice signs Bob's key, she is introducing Bob's key to anyone who trusts Alice. If someone trusts Alice to introduce keys, then Alice is a trusted introducer in the mind of that observer.
For example: If David trusts Alice to be an introducer, and Alice signed Bob's key, Dave also trusts Bob's key to be the real one.
There are some key points that are important in that model. In the example Alice has to sign Bob's key, only if she is sure that the key belongs to Bob. Otherwise she may also make Dave falsely believe that this is Bob's key. Dave has also the responsibility to know who to trust. This model is similar to real life relations.
Just see how Charlie behaves in the previous example. Although he has signed Bob's key - because he knows, somehow, that it belongs to Bob - he does not trust Bob to be an introducer. Charlie decided to trust only Kevin, for some reason. A reason could be that Bob is lazy enough, and signs other people's keys without being sure that they belong to the actual owner.
In GnuTLS the OpenPGP key structures
RFC2440 are handled using the gnutls_openpgp_key_t type
and the corresponding private keys with the
gnutls_openpgp_privkey_t type. All the prototypes for the key
handling functions can be found at gnutls/openpgp.h.
The verification functions of OpenPGP keys, included in GnuTLS, are simple ones, and do not use the features of the “web of trust”. For that reason, if the verification needs are complex, the assistance of external tools like GnuPG and GPGME (http://www.gnupg.org/related_software/gpgme/) is recommended.
There are two verification functions in GnuTLS, The
gnutls_openpgp_key_verify_ring and the
gnutls_openpgp_key_verify_trustdb. The first one checks an
OpenPGP key against a given set of public keys (keyring) and
returns the key status. The key verification status is the same as in
X.509 certificates, although the meaning and interpretation are
different. For example an OpenPGP key may be valid, if the
self signature is ok, even if no signers were found. The meaning of
verification status is shown in the figure below. The latter function
checks a GnuPG trust database for the given key. This
function does not check the key signatures, only checks for disabled
and revoked keys.
CERT_INVALID:
A signature on the key is invalid. That means that the key was
modified by somebody, or corrupted during transport.
CERT_REVOKED:
The key has been revoked by its owner.
CERT_SIGNER_NOT_FOUND:
The key was not signed by a known signer.
This chapter is intended to provide some hints on how to use the TLS over simple custom made application protocols. The discussion below mainly refers to the TCP/IP transport layer but may be extended to other ones too.
Traditionally SSL was used in application protocols by assigning a new port number for the secure services. That way two separate ports were assigned, one for the non secure sessions, and one for the secured ones. This has the benefit that if a user requests a secure session then the client will try to connect to the secure port and fail otherwise. The only possible attack with this method is a denial of service one. The most famous example of this method is the famous “HTTP over TLS” or HTTPS protocol RFC2818.
Despite its wide use, this method is not as good as it seems. This approach starts the TLS Handshake procedure just after the client connects on the –so called– secure port. That way the TLS protocol does not know anything about the client, and popular methods like the host advertising in HTTP do not work17. There is no way for the client to say “I connected to YYY server” before the Handshake starts, so the server cannot possibly know which certificate to use.
Other than that it requires two separate ports to run a single service, which is unnecessary complication. Due to the fact that there is a limitation on the available privileged ports, this approach was soon obsoleted.
Other application protocols18 use a different approach to enable the secure layer. They use something called the “TLS upgrade” method. This method is quite tricky but it is more flexible. The idea is to extend the application protocol to have a “STARTTLS” request, whose purpose it to start the TLS protocols just after the client requests it. This is a really neat idea and does not require an extra port.
This method is used by almost all modern protocols and there is even the RFC2817 paper which proposes extensions to HTTP to support it.
The tricky part, in this method, is that the “STARTTLS” request is sent in the clear, thus is vulnerable to modifications. A typical attack is to modify the messages in a way that the client is fooled and thinks that the server does not have the “STARTTLS” capability. See a typical conversation of a hypothetical protocol:
(client connects to the server)CLIENT: HELLO I'M MR. XXX
SERVER: NICE TO MEET YOU XXX
CLIENT: PLEASE START TLS
SERVER: OK
*** TLS STARTS
CLIENT: HERE ARE SOME CONFIDENTIAL DATA
And see an example of a conversation where someone is acting in between:
(client connects to the server)CLIENT: HELLO I'M MR. XXX
SERVER: NICE TO MEET YOU XXX
CLIENT: PLEASE START TLS
(here someone inserts this message)
SERVER: SORRY I DON'T HAVE THIS CAPABILITY
CLIENT: HERE ARE SOME CONFIDENTIAL DATA
As you can see above the client was fooled, and was dummy enough to send the confidential data in the clear.
How to avoid the above attack? As you may have already thought this one is easy to avoid. The client has to ask the user before it connects whether the user requests TLS or not. If the user answered that he certainly wants the secure layer the last conversation should be:
(client connects to the server)CLIENT: HELLO I'M MR. XXX
SERVER: NICE TO MEET YOU XXX
CLIENT: PLEASE START TLS
(here someone inserts this message)
SERVER: SORRY I DON'T HAVE THIS CAPABILITY
CLIENT: BYE
(the client notifies the user that the secure connection was not possible)
This method, if implemented properly, is far better than the traditional method, and the security properties remain the same, since only denial of service is possible. The benefit is that the server may request additional data before the TLS Handshake protocol starts, in order to send the correct certificate, use the correct password file19, or anything else!
To use GnuTLS, you have to perform some changes to your sources and your build system. The necessary changes are explained in the following subsections.
All the data types and functions of the GnuTLS library are defined in the header file gnutls/gnutls.h. This must be included in all programs that make use of the GnuTLS library.
The extra functionality of the GnuTLS-extra library is available by including the header file gnutls/extra.h in your programs.
It is often desirable to check that the version of `gnutls' used is
indeed one which fits all requirements. Even with binary
compatibility new features may have been introduced but due to problem
with the dynamic linker an old version is actually used. So you may
want to check that the version is okay right after program startup.
See the function gnutls_check_version.
If you want to compile a source file including the `gnutls/gnutls.h' header file, you must make sure that the compiler can find it in the directory hierarchy. This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).
However, the path to the include file is determined at the time the
source is configured. To solve this problem, GnuTLS ships
with two small helper programs libgnutls-config and
libgnutls-extra-config that knows about the path to the
include file and other configuration options. The options that need
to be added to the compiler invocation at compile time are output by
the --cflags option to libgnutls-config. The
following example shows how it can be used at the command line:
gcc -c foo.c `libgnutls-config --cflags`
Adding the output of libgnutls-config --cflags to the compilers command line will ensure that the compiler can find the GnuTLS header file.
A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files. For this to work,
the path to the library files has to be added to the library search
path (via the -L option). For this, the option --libs to
libgnutls-config can be used. For convenience, this option
also outputs all other options that are required to link the program
with the GnuTLS libararies. The example shows how to link
`foo.o' with the GnuTLS libraries to a program foo.
gcc -o foo foo.o `libgnutls-config --libs`
Of course you can also combine both examples to a single command by specifying both options to `libgnutls-config':
gcc -o foo foo.c `libgnutls-config --cflags --libs`
Although the GnuTLS library is thread safe by design, some parts of the crypto backend, such as the random generator, are not. Since libgcrypt 1.1.92 there was an automatic detection of the thread library used by the application, so most applications wouldn't need to do any changes to ensure thread-safety. Due to the unportability of the automatic thread detection, this was removed from later releases of libgcrypt, so applications have now to register callback functions to ensure proper locking in sensitive parts of libgcrypt.
There are helper macros to help you properly initialize the libraries. Examples are shown below.
#include <gnutls.h>
#include <gcrypt.h>
#include <errno.h>
#include <pthread.h>
GCRY_THREAD_OPTION_PTHREAD_IMPL;
int main()
{
/* The order matters.
*/
gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
gnutls_global_init();
}
#include <gnutls.h>
#include <gcrypt.h>
#include <errno.h>
#include <pth.h>
GCRY_THREAD_OPTION_PTH_IMPL;
int main()
{
gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pth);
gnutls_global_init();
}
/* The gcry_thread_cbs structure must have been
* initialized.
*/
static struct gcry_thread_cbs gcry_threads_other = { ... };
int main()
{
gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_other);
}
This section contains examples of TLS and SSL clients, using GnuTLS. Note that these examples contain little or no error checking.
The simplest client using TLS is the one that doesn't do any authentication. This means no external certificates or passwords are needed to set up the connection. As could be expected, the connection is vulnerable to man-in-the-middle (active or redirection) attacks. However, the data is integrity and privacy protected.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
/* A very basic TLS client, with anonymous authentication.
*/
#define MAX_BUF 1024
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"
/* Connects to the peer and returns a socket
* descriptor.
*/
int tcp_connect(void)
{
const char *PORT = "5556";
const char *SERVER = "127.0.0.1";
int err, sd;
struct sockaddr_in sa;
/* connects to server
*/
sd = socket(AF_INET, SOCK_STREAM, 0);
memset(&sa, '\0', sizeof(sa));
sa.sin_family = AF_INET;
sa.sin_port = htons(atoi(PORT));
inet_pton(AF_INET, SERVER, &sa.sin_addr);
err = connect(sd, (SA *) & sa, sizeof(sa));
if (err < 0) {
fprintf(stderr, "Connect error\n");
exit(1);
}
return sd;
}
/* closes the given socket descriptor.
*/
void tcp_close(int sd)
{
shutdown(sd, SHUT_RDWR); /* no more receptions */
close(sd);
}
int main()
{
int ret, sd, ii;
gnutls_session_t session;
char buffer[MAX_BUF + 1];
gnutls_anon_client_credentials_t anoncred;
/* Need to enable anonymous KX specifically. */
const int kx_prio[] = { GNUTLS_KX_ANON_DH, 0 };
gnutls_global_init();
gnutls_anon_allocate_client_credentials(&anoncred);
/* Initialize TLS session
*/
gnutls_init(&session, GNUTLS_CLIENT);
/* Use default priorities */
gnutls_set_default_priority(session);
gnutls_kx_set_priority (session, kx_prio);
/* put the anonymous credentials to the current session
*/
gnutls_credentials_set(session, GNUTLS_CRD_ANON, anoncred);
/* connect to the peer
*/
sd = tcp_connect();
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake(session);
if (ret < 0) {
fprintf(stderr, "*** Handshake failed\n");
gnutls_perror(ret);
goto end;
} else {
printf("- Handshake was completed\n");
}
gnutls_record_send(session, MSG, strlen(MSG));
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("- Peer has closed the TLS connection\n");
goto end;
} else if (ret < 0) {
fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
goto end;
}
printf("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++) {
fputc(buffer[ii], stdout);
}
fputs("\n", stdout);
gnutls_bye(session, GNUTLS_SHUT_RDWR);
end:
tcp_close(sd);
gnutls_deinit(session);
gnutls_anon_free_client_credentials (anoncred);
gnutls_global_deinit();
return 0;
}
Let's assume now that we want to create a TCP client which communicates with servers that use X.509 or OpenPGP certificate authentication. The following client is a very simple TLS client, it does not support session resuming, not even certificate verification. The TCP functions defined in this example are used in most of the other examples below, without redefining them.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
/* A very basic TLS client, with X.509 authentication.
*/
#define MAX_BUF 1024
#define CAFILE "ca.pem"
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"
/* Connects to the peer and returns a socket
* descriptor.
*/
int tcp_connect(void)
{
const char *PORT = "443";
const char *SERVER = "127.0.0.1";
int err, sd;
struct sockaddr_in sa;
/* connects to server
*/
sd = socket(AF_INET, SOCK_STREAM, 0);
memset(&sa, '\0', sizeof(sa));
sa.sin_family = AF_INET;
sa.sin_port = htons(atoi(PORT));
inet_pton(AF_INET, SERVER, &sa.sin_addr);
err = connect(sd, (SA *) & sa, sizeof(sa));
if (err < 0) {
fprintf(stderr, "Connect error\n");
exit(1);
}
return sd;
}
/* closes the given socket descriptor.
*/
void tcp_close(int sd)
{
shutdown(sd, SHUT_RDWR); /* no more receptions */
close(sd);
}
int main()
{
int ret, sd, ii;
gnutls_session_t session;
char buffer[MAX_BUF + 1];
gnutls_certificate_credentials_t xcred;
/* Allow connections to servers that have OpenPGP keys as well.
*/
const int cert_type_priority[3] = { GNUTLS_CRT_X509,
GNUTLS_CRT_OPENPGP, 0
};
gnutls_global_init();
/* X509 stuff */
gnutls_certificate_allocate_credentials(&xcred);
/* sets the trusted cas file
*/
gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
GNUTLS_X509_FMT_PEM);
/* Initialize TLS session
*/
gnutls_init(&session, GNUTLS_CLIENT);
/* Use default priorities */
gnutls_set_default_priority(session);
gnutls_certificate_type_set_priority(session, cert_type_priority);
/* put the x509 credentials to the current session
*/
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);
/* connect to the peer
*/
sd = tcp_connect();
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake(session);
if (ret < 0) {
fprintf(stderr, "*** Handshake failed\n");
gnutls_perror(ret);
goto end;
} else {
printf("- Handshake was completed\n");
}
gnutls_record_send(session, MSG, strlen(MSG));
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("- Peer has closed the TLS connection\n");
goto end;
} else if (ret < 0) {
fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
goto end;
}
printf("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++) {
fputc(buffer[ii], stdout);
}
fputs("\n", stdout);
gnutls_bye(session, GNUTLS_SHUT_RDWR);
end:
tcp_close(sd);
gnutls_deinit(session);
gnutls_certificate_free_credentials(xcred);
gnutls_global_deinit();
return 0;
}
Most of the times it is desirable to know the security properties of
the current established session. This includes the underlying ciphers
and the protocols involved. That is the purpose of the following
function. Note that this function will print meaningful values only
if called after a successful gnutls_handshake.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
extern void print_x509_certificate_info(gnutls_session_t);
/* This function will print some details of the
* given session.
*/
int print_info(gnutls_session_t session)
{
const char *tmp;
gnutls_credentials_type_t cred;
gnutls_kx_algorithm_t kx;
/* print the key exchange's algorithm name
*/
kx = gnutls_kx_get(session);
tmp = gnutls_kx_get_name(kx);
printf("- Key Exchange: %s\n", tmp);
/* Check the authentication type used and switch
* to the appropriate.
*/
cred = gnutls_auth_get_type(session);
switch (cred) {
case GNUTLS_CRD_ANON: /* anonymous authentication */
printf("- Anonymous DH using prime of %d bits\n",
gnutls_dh_get_prime_bits(session));
break;
case GNUTLS_CRD_CERTIFICATE: /* certificate authentication */
/* Check if we have been using ephemeral Diffie Hellman.
*/
if (kx == GNUTLS_KX_DHE_RSA || kx == GNUTLS_KX_DHE_DSS) {
printf("\n- Ephemeral DH using prime of %d bits\n",
gnutls_dh_get_prime_bits(session));
}
/* if the certificate list is available, then
* print some information about it.
*/
print_x509_certificate_info(session);
} /* switch */
/* print the protocol's name (ie TLS 1.0)
*/
tmp = gnutls_protocol_get_name(gnutls_protocol_get_version(session));
printf("- Protocol: %s\n", tmp);
/* print the certificate type of the peer.
* ie X.509
*/
tmp =
gnutls_certificate_type_get_name(gnutls_certificate_type_get
(session));
printf("- Certificate Type: %s\n", tmp);
/* print the compression algorithm (if any)
*/
tmp = gnutls_compression_get_name(gnutls_compression_get(session));
printf("- Compression: %s\n", tmp);
/* print the name of the cipher used.
* ie 3DES.
*/
tmp = gnutls_cipher_get_name(gnutls_cipher_get(session));
printf("- Cipher: %s\n", tmp);
/* Print the MAC algorithms name.
* ie SHA1
*/
tmp = gnutls_mac_get_name(gnutls_mac_get(session));
printf("- MAC: %s\n", tmp);
return 0;
}
A TLS session is not secure just after the handshake procedure has finished. It must be considered secure, only after the peer's certificate and identity have been verified. That is, you have to verify the signature in peer's certificate, the hostname in the certificate, and expiration dates. Just after this step you should treat the connection as being a secure one.
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
/* This function will try to verify the peer's certificate, and
* also check if the hostname matches, and the activation, expiration dates.
*/
void verify_certificate( gnutls_session_t session, const char* hostname)
{
unsigned int status;
const gnutls_datum_t* cert_list;
int cert_list_size, ret;
gnutls_x509_crt_t cert;
/* This verification function uses the trusted CAs in the credentials
* structure. So you must have installed one or more CA certificates.
*/
ret = gnutls_certificate_verify_peers2(session, &status);
if (ret < 0) {
printf("Error\n");
return;
}
if (status & GNUTLS_CERT_INVALID)
printf("The certificate is not trusted.\n");
if (status & GNUTLS_CERT_SIGNER_NOT_FOUND)
printf("The certificate hasn't got a known issuer.\n");
if (status & GNUTLS_CERT_REVOKED)
printf("The certificate has been revoked.\n");
/* Up to here the process is the same for X.509 certificates and
* OpenPGP keys. From now on X.509 certificates are assumed. This can
* be easily extended to work with openpgp keys as well.
*/
if ( gnutls_certificate_type_get(session) != GNUTLS_CRT_X509)
return;
if ( gnutls_x509_crt_init( &cert) < 0) {
printf("error in initialization\n");
return;
}
cert_list = gnutls_certificate_get_peers( session, &cert_list_size);
if ( cert_list == NULL) {
printf("No certificate was found!\n");
return;
}
/* This is not a real world example, since we only check the first
* certificate in the given chain.
*/
if ( gnutls_x509_crt_import( cert, &cert_list[0], GNUTLS_X509_FMT_DER) < 0) {
printf("error parsing certificate\n");
return;
}
/* Beware here we do not check for errors.
*/
if ( gnutls_x509_crt_get_expiration( cert) < time(0)) {
printf("The certificate has expired\n");
return;
}
if ( gnutls_x509_crt_get_activation_time( cert) > time(0)) {
printf("The certificate is not yet activated\n");
return;
}
if ( !gnutls_x509_crt_check_hostname( cert, hostname)) {
printf("The certificate's owner does not match hostname '%s'\n", hostname);
return;
}
gnutls_x509_crt_deinit( cert);
return;
}
An other example is listed below which provides a more detailed
verification output.
#include <stdio.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
/* All the available CRLs
*/
extern gnutls_x509_crl_t *crl_list;
extern int crl_list_size;
/* All the available trusted CAs
*/
extern gnutls_x509_crt_t *ca_list;
extern int ca_list_size;
static void verify_cert2(gnutls_x509_crt_t crt,
gnutls_x509_crt_t issuer,
gnutls_x509_crl_t * crl_list, int crl_list_size);
static void verify_last_cert(gnutls_x509_crt_t crt,
gnutls_x509_crt_t * ca_list, int ca_list_size,
gnutls_x509_crl_t * crl_list,
int crl_list_size);
/* This function will try to verify the peer's certificate chain, and
* also check if the hostname matches, and the activation, expiration dates.
*/
void verify_certificate_chain(gnutls_session_t session,
const char *hostname,
const gnutls_datum_t * cert_chain,
int cert_chain_length)
{
int i, ret;
gnutls_x509_crt_t cert[cert_chain_length];
/* Import all the certificates in the chain to
* native certificate format.
*/
for (i = 0; i < cert_chain_length; i++) {
gnutls_x509_crt_init(&cert[i]);
gnutls_x509_crt_import(cert[i], &cert_chain[i],
GNUTLS_X509_FMT_DER);
}
/* Now verify the certificates against their issuers
* in the chain.
*/
for (i = 1; i < cert_chain_length; i++) {
verify_cert2(cert[i - 1], cert[i], crl_list, crl_list_size);
}
/* Here we must verify the last certificate in the chain against
* our trusted CA list.
*/
verify_last_cert(cert[cert_chain_length - 1],
ca_list, ca_list_size, crl_list, crl_list_size);
/* Check if the name in the first certificate matches our destination!
*/
if (!gnutls_x509_crt_check_hostname(cert[0], hostname)) {
printf("The certificate's owner does not match hostname '%s'\n",
hostname);
}
for (i = 0; i < cert_chain_length; i++)
gnutls_x509_crt_deinit(cert[i]);
return;
}
/* Verifies a certificate against an other certificate
* which is supposed to be it's issuer. Also checks the
* crl_list if the certificate is revoked.
*/
static void verify_cert2(gnutls_x509_crt crt_t,
gnutls_x509_crt_t issuer,
gnutls_x509_crl_t * crl_list, int crl_list_size)
{
unsigned int output;
int ret;
time_t now = time(0);
size_t name_size;
char name[64];
/* Print information about the certificates to
* be checked.
*/
name_size = sizeof(name);
gnutls_x509_crt_get_dn(crt, name, &name_size);
fprintf(stderr, "\nCertificate: %s\n", name);
name_size = sizeof(name);
gnutls_x509_crt_get_issuer_dn(crt, name, &name_size);
fprintf(stderr, "Issued by: %s\n", name);
/* Get the DN of the issuer cert.
*/
name_size = sizeof(name);
gnutls_x509_crt_get_dn(issuer, name, &name_size);
fprintf(stderr, "Checking against: %s\n", name);
/* Do the actual verification.
*/
gnutls_x509_crt_verify(crt, &issuer, 1, 0, &output);
if (output & GNUTLS_CERT_INVALID) {
fprintf(stderr, "Not trusted");
if (output & GNUTLS_CERT_SIGNER_NOT_FOUND)
fprintf(stderr, ": no issuer was found");
if (output & GNUTLS_CERT_SIGNER_NOT_CA)
fprintf(stderr, ": issuer is not a CA");
fprintf(stderr, "\n");
} else
fprintf(stderr, "Trusted\n");
/* Now check the expiration dates.
*/
if (gnutls_x509_crt_get_activation_time(crt) > now)
fprintf(stderr, "Not yet activated\n");
if (gnutls_x509_crt_get_expiration_time(crt) < now)
fprintf(stderr, "Expired\n");
/* Check if the certificate is revoked.
*/
ret = gnutls_x509_crt_check_revocation(crt, crl_list, crl_list_size);
if (ret == 1) { /* revoked */
fprintf(stderr, "Revoked\n");
}
}
/* Verifies a certificate against the trusted CA list.
* Also checks the crl_list if the certificate is revoked.
*/
static void verify_last_cert(gnutls_x509_crt_t crt,
gnutls_x509_crt_t * ca_list, int ca_list_size,
gnutls_x509_crl_t * crl_list,
int crl_list_size)
{
unsigned int output;
int ret;
time_t now = time(0);
size_t name_size;
char name[64];
/* Print information about the certificates to
* be checked.
*/
name_size = sizeof(name);
gnutls_x509_crt_get_dn(crt, name, &name_size);
fprintf(stderr, "\nCertificate: %s\n", name);
name_size = sizeof(name);
gnutls_x509_crt_get_issuer_dn(crt, name, &name_size);
fprintf(stderr, "Issued by: %s\n", name);
/* Do the actual verification.
*/
gnutls_x509_crt_verify(crt, ca_list, ca_list_size, 0, &output);
if (output & GNUTLS_CERT_INVALID) {
fprintf(stderr, "Not trusted");
if (output & GNUTLS_CERT_SIGNER_NOT_CA)
fprintf(stderr, ": Issuer is not a CA\n");
else
fprintf(stderr, "\n");
} else
fprintf(stderr, "Trusted\n");
/* Now check the expiration dates.
*/
if (gnutls_x509_crt_get_activation_time(crt) > now)
fprintf(stderr, "Not yet activated\n");
if (gnutls_x509_crt_get_expiration_time(crt) < now)
fprintf(stderr, "Expired\n");
/* Check if the certificate is revoked.
*/
ret = gnutls_x509_crt_check_revocation(crt, crl_list, crl_list_size);
if (ret == 1) { /* revoked */
fprintf(stderr, "Revoked\n");
}
}
There are cases where a client holds several certificate and key pairs, and may not want to load all of them in the credentials structure. The following example demonstrates the use of the certificate selection callback.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
/* A TLS client that loads the certificate and key.
*/
#define MAX_BUF 1024
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"
#define CERT_FILE "cert.pem"
#define KEY_FILE "key.pem"
#define CAFILE "ca.pem"
static int cert_callback(gnutls_session_t session,
const gnutls_datum_t * req_ca_rdn, int nreqs,
const gnutls_pk_algorithm_t * sign_algos,
int sign_algos_length, gnutls_retr_st * st);
gnutls_x509_crt_t crt;
gnutls_x509_privkey_t key;
/* Helper functions to load a certificate and key
* files into memory. They use mmap for simplicity.
*/
static gnutls_datum_t mmap_file(const char *file)
{
int fd;
gnutls_datum_t mmaped_file = { NULL, 0 };
struct stat stat_st;
void *ptr;
fd = open(file, 0);
if (fd == -1)
return mmaped_file;
fstat(fd, &stat_st);
if ((ptr =
mmap(NULL, stat_st.st_size, PROT_READ, MAP_SHARED, fd,
0)) == MAP_FAILED)
return mmaped_file;
mmaped_file.data = ptr;
mmaped_file.size = stat_st.st_size;
return mmaped_file;
}
static void munmap_file(gnutls_datum_t data)
{
munmap(data.data, data.size);
}
/* Load the certificate and the private key.
*/
static void load_keys(void)
{
int ret;
gnutls_datum_t data;
data = mmap_file(CERT_FILE);
if (data.data == NULL) {
fprintf(stderr, "*** Error loading cert file.\n");
exit(1);
}
gnutls_x509_crt_init(&crt);
ret = gnutls_x509_crt_import(crt, &data, GNUTLS_X509_FMT_PEM);
if (ret < 0) {
fprintf(stderr, "*** Error loading key file: %s\n",
gnutls_strerror(ret));
exit(1);
}
munmap_file(data);
data = mmap_file(KEY_FILE);
if (data.data == NULL) {
fprintf(stderr, "*** Error loading key file.\n");
exit(1);
}
gnutls_x509_privkey_init(&key);
ret = gnutls_x509_privkey_import(key, &data, GNUTLS_X509_FMT_PEM);
if (ret < 0) {
fprintf(stderr, "*** Error loading key file: %s\n",
gnutls_strerror(ret));
exit(1);
}
munmap_file(data);
}
int main()
{
int ret, sd, ii;
gnutls_session_t session;
char buffer[MAX_BUF + 1];
gnutls_certificate_credentials_t xcred;
/* Allow connections to servers that have OpenPGP keys as well.
*/
gnutls_global_init();
load_keys();
/* X509 stuff */
gnutls_certificate_allocate_credentials(&xcred);
/* sets the trusted cas file
*/
gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_client_set_retrieve_function(xcred, cert_callback);
/* Initialize TLS session
*/
gnutls_init(&session, GNUTLS_CLIENT);
/* Use default priorities */
gnutls_set_default_priority(session);
/* put the x509 credentials to the current session
*/
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);
/* connect to the peer
*/
sd = tcp_connect();
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake(session);
if (ret < 0) {
fprintf(stderr, "*** Handshake failed\n");
gnutls_perror(ret);
goto end;
} else {
printf("- Handshake was completed\n");
}
gnutls_record_send(session, MSG, strlen(MSG));
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("- Peer has closed the TLS connection\n");
goto end;
} else if (ret < 0) {
fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
goto end;
}
printf("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++) {
fputc(buffer[ii], stdout);
}
fputs("\n", stdout);
gnutls_bye(session, GNUTLS_SHUT_RDWR);
end:
tcp_close(sd);
gnutls_deinit(session);
gnutls_certificate_free_credentials(xcred);
gnutls_global_deinit();
return 0;
}
/* This callback should be associated with a session by calling
* gnutls_certificate_client_set_retrieve_function( session, cert_callback),
* before a handshake.
*/
static int cert_callback(gnutls_session_t session,
const gnutls_datum_t * req_ca_rdn, int nreqs,
const gnutls_pk_algorithm_t * sign_algos,
int sign_algos_length, gnutls_retr_st * st)
{
char issuer_dn[256];
int i, ret;
size_t len;
gnutls_certificate_type_t type;
/* Print the server's trusted CAs
*/
if (nreqs > 0)
printf("- Server's trusted authorities:\n");
else
printf
("- Server did not send us any trusted authorities names.\n");
/* print the names (if any) */
for (i = 0; i < nreqs; i++) {
len = sizeof(issuer_dn);
ret = gnutls_x509_rdn_get(&req_ca_rdn[i], issuer_dn, &len);
if (ret >= 0) {
printf(" [%d]: ", i);
printf("%s\n", issuer_dn);
}
}
/* Select a certificate and return it.
* The certificate must be of any of the "sign algorithms"
* supported by the server.
*/
type = gnutls_certificate_type_get(session);
if (type == GNUTLS_CRT_X509) {
st->type = type;
st->ncerts = 1;
st->cert.x509 = &crt;
st->key.x509 = key;
st->deinit_all = 0;
} else {
return -1;
}
return 0;
}
This is a modification of the simple client example. Here we demonstrate the use of session resumption. The client tries to connect once using TLS, close the connection and then try to establish a new connection using the previously negotiated data.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
/* Those functions are defined in other examples.
*/
extern void check_alert(gnutls_session_t session, int ret);
extern int tcp_connect(void);
extern void tcp_close(int sd);
#define MAX_BUF 1024
#define CRLFILE "crl.pem"
#define CAFILE "ca.pem"
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"
int main()
{
int ret;
int sd, ii, alert;
gnutls_session_t session;
char buffer[MAX_BUF + 1];
gnutls_certificate_credentials_t xcred;
/* variables used in session resuming
*/
int t;
char *session_data;
size_t session_data_size;
gnutls_global_init();
/* X509 stuff */
gnutls_certificate_allocate_credentials(&xcred);
gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
GNUTLS_X509_FMT_PEM);
for (t = 0; t < 2; t++) { /* connect 2 times to the server */
sd = tcp_connect();
gnutls_init(&session, GNUTLS_CLIENT);
gnutls_set_default_priority(session);
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);
if (t > 0) {
/* if this is not the first time we connect */
gnutls_session_set_data(session, session_data,
session_data_size);
free(session_data);
}
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake(session);
if (ret < 0) {
fprintf(stderr, "*** Handshake failed\n");
gnutls_perror(ret);
goto end;
} else {
printf("- Handshake was completed\n");
}
if (t == 0) { /* the first time we connect */
/* get the session data size */
gnutls_session_get_data(session, NULL, &session_data_size);
session_data = malloc(session_data_size);
/* put session data to the session variable */
gnutls_session_get_data(session, session_data,
&session_data_size);
} else { /* the second time we connect */
/* check if we actually resumed the previous session */
if (gnutls_session_is_resumed(session) != 0) {
printf("- Previous session was resumed\n");
} else {
fprintf(stderr, "*** Previous session was NOT resumed\n");
}
}
/* This function was defined in a previous example
*/
/* print_info(session); */
gnutls_record_send(session, MSG, strlen(MSG));
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("- Peer has closed the TLS connection\n");
goto end;
} else if (ret < 0) {
fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
goto end;
}
printf("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++) {
fputc(buffer[ii], stdout);
}
fputs("\n", stdout);
gnutls_bye(session, GNUTLS_SHUT_RDWR);
end:
tcp_close(sd);
gnutls_deinit(session);
} /* for() */
gnutls_certificate_free_credentials(xcred);
gnutls_global_deinit();
return 0;
}
The following client is a very simple SRP TLS client which connects to a server and authenticates using a username and a password. The server may authenticate itself using a certificate, and in that case it has to be verified.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/extra.h>
/* Those functions are defined in other examples.
*/
extern void check_alert(gnutls_session_t session, int ret);
extern int tcp_connect(void);
extern void tcp_close(int sd);
#define MAX_BUF 1024
#define USERNAME "user"
#define PASSWORD "pass"
#define CAFILE "ca.pem"
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"
const int kx_priority[] = { GNUTLS_KX_SRP, GNUTLS_KX_SRP_DSS,
GNUTLS_KX_SRP_RSA, 0
};
int main()
{
int ret;
int sd, ii;
gnutls_session_t session;
char buffer[MAX_BUF + 1];
gnutls_srp_client_credentials_t srp_cred;
gnutls_certificate_client_credentials_t cert_cred;
gnutls_global_init();
/* now enable the gnutls-extra library which contains the
* SRP stuff.
*/
gnutls_global_init_extra();
gnutls_srp_allocate_client_credentials(&srp_cred);
gnutls_certificate_allocate_client_credentials(&cert_cred);
gnutls_certificate_set_x509_trust_file(cert_cred, CAFILE,
GNUTLS_X509_FMT_PEM);
gnutls_srp_set_client_credentials(srp_cred, USERNAME, PASSWORD);
/* connects to server
*/
sd = tcp_connect();
/* Initialize TLS session
*/
gnutls_init(&session, GNUTLS_CLIENT);
/* Set the priorities.
*/
gnutls_set_default_priority(session);
gnutls_kx_set_priority(session, kx_priority);
/* put the SRP credentials to the current session
*/
gnutls_credentials_set(session, GNUTLS_CRD_SRP, srp_cred);
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, cert_cred);
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake(session);
if (ret < 0) {
fprintf(stderr, "*** Handshake failed\n");
gnutls_perror(ret);
goto end;
} else {
printf("- Handshake was completed\n");
}
gnutls_record_send(session, MSG, strlen(MSG));
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (gnutls_error_is_fatal(ret) == 1 || ret == 0) {
if (ret == 0) {
printf("- Peer has closed the GNUTLS connection\n");
goto end;
} else {
fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
goto end;
}
} else
check_alert(session, ret);
if (ret > 0) {
printf("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++) {
fputc(buffer[ii], stdout);
}
fputs("\n", stdout);
}
gnutls_bye(session, 0);
end:
tcp_close(sd);
gnutls_deinit(session);
gnutls_srp_free_client_credentials(srp_cred);
gnutls_certificate_free_credentials(cert_cred);
gnutls_global_deinit();
return 0;
}
This section contains examples of TLS and SSL servers, using GnuTLS.
This example is a very simple echo server which supports X.509 authentication, using the RSA ciphersuites.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "ca.pem"
#define CRLFILE "crl.pem"
/* This is a sample TLS 1.0 echo server, using X.509 authentication.
*/
#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556 /* listen to 5556 port */
#define DH_BITS 1024
/* These are global */
gnutls_certificate_credentials_t x509_cred;
gnutls_session_t initialize_tls_session()
{
gnutls_session_t session;
gnutls_init(&session, GNUTLS_SERVER);
/* avoid calling all the priority functions, since the defaults
* are adequate.
*/
gnutls_set_default_priority(session);
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, x509_cred);
/* request client certificate if any.
*/
gnutls_certificate_server_set_request(session, GNUTLS_CERT_REQUEST);
gnutls_dh_set_prime_bits(session, DH_BITS);
return session;
}
static gnutls_dh_params_t dh_params;
static int generate_dh_params(void)
{
/* Generate Diffie Hellman parameters - for use with DHE
* kx algorithms. These should be discarded and regenerated
* once a day, once a week or once a month. Depending on the
* security requirements.
*/
gnutls_dh_params_init(&dh_params);
gnutls_dh_params_generate2(dh_params, DH_BITS);
return 0;
}
int main()
{
int err, listen_sd, i;
int sd, ret;
struct sockaddr_in sa_serv;
struct sockaddr_in sa_cli;
int client_len;
char topbuf[512];
gnutls_session_t session;
char buffer[MAX_BUF + 1];
int optval = 1;
/* this must be called once in the program
*/
gnutls_global_init();
gnutls_certificate_allocate_credentials(&x509_cred);
gnutls_certificate_set_x509_trust_file(x509_cred, CAFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_x509_crl_file(x509_cred, CRLFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_x509_key_file(x509_cred, CERTFILE, KEYFILE,
GNUTLS_X509_FMT_PEM);
generate_dh_params();
gnutls_certificate_set_dh_params(x509_cred, dh_params);
/* Socket operations
*/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
SOCKET_ERR(listen_sd, "socket");
memset(&sa_serv, '\0', sizeof(sa_serv));
sa_serv.sin_family = AF_INET;
sa_serv.sin_addr.s_addr = INADDR_ANY;
sa_serv.sin_port = htons(PORT); /* Server Port number */
setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));
err = bind(listen_sd, (SA *) & sa_serv, sizeof(sa_serv));
SOCKET_ERR(err, "bind");
err = listen(listen_sd, 1024);
SOCKET_ERR(err, "listen");
printf("Server ready. Listening to port '%d'.\n\n", PORT);
client_len = sizeof(sa_cli);
for (;;) {
session = initialize_tls_session();
sd = accept(listen_sd, (SA *) & sa_cli, &client_len);
printf("- connection from %s, port %d\n",
inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
sizeof(topbuf)), ntohs(sa_cli.sin_port));
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
ret = gnutls_handshake(session);
if (ret < 0) {
close(sd);
gnutls_deinit(session);
fprintf(stderr, "*** Handshake has failed (%s)\n\n",
gnutls_strerror(ret));
continue;
}
printf("- Handshake was completed\n");
/* see the Getting peer's information example */
/* print_info(session); */
i = 0;
for (;;) {
bzero(buffer, MAX_BUF + 1);
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("\n- Peer has closed the GNUTLS connection\n");
break;
} else if (ret < 0) {
fprintf(stderr, "\n*** Received corrupted "
"data(%d). Closing the connection.\n\n", ret);
break;
} else if (ret > 0) {
/* echo data back to the client
*/
gnutls_record_send(session, buffer, strlen(buffer));
}
}
printf("\n");
/* do not wait for the peer to close the connection.
*/
gnutls_bye(session, GNUTLS_SHUT_WR);
close(sd);
gnutls_deinit(session);
}
close(listen_sd);
gnutls_certificate_free_credentials(x509_cred);
gnutls_global_deinit();
return 0;
}
The following example is a server which supports X.509 authentication. This server supports the export-grade cipher suites, the DHE ciphersuites and session resuming.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "ca.pem"
#define CRLFILE "crl.pem"
/* This is a sample TLS 1.0 echo server.
* Export-grade ciphersuites and session resuming are supported.
*/
#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556 /* listen to 5556 port */
#define DH_BITS 1024
/* These are global */
gnutls_certificate_credentials_t cert_cred;
static void wrap_db_init(void);
static void wrap_db_deinit(void);
static int wrap_db_store(void *dbf, gnutls_datum_t key,
gnutls_datum_t data);
static gnutls_datum_t wrap_db_fetch(void *dbf, gnutls_datum_t key);
static int wrap_db_delete(void *dbf, gnutls_datum_t key);
#define TLS_SESSION_CACHE 50
gnutls_session_t initialize_tls_session()
{
gnutls_session_t session;
gnutls_init(&session, GNUTLS_SERVER);
/* Use the default priorities, plus, export cipher suites.
*/
gnutls_set_default_export_priority(session);
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, cert_cred);
/* request client certificate if any.
*/
gnutls_certificate_server_set_request(session, GNUTLS_CERT_REQUEST);
gnutls_dh_set_prime_bits(session, DH_BITS);
if (TLS_SESSION_CACHE != 0) {
gnutls_db_set_retrieve_function(session, wrap_db_fetch);
gnutls_db_set_remove_function(session, wrap_db_delete);
gnutls_db_set_store_function(session, wrap_db_store);
gnutls_db_set_ptr(session, NULL);
}
return session;
}
gnutls_dh_params_t dh_params;
/* Export-grade cipher suites require temporary RSA
* keys.
*/
gnutls_rsa_params_t rsa_params;
int generate_dh_params(void)
{
/* Generate Diffie Hellman parameters - for use with DHE
* kx algorithms. These should be discarded and regenerated
* once a day, once a week or once a month. Depends on the
* security requirements.
*/
gnutls_dh_params_init(&dh_params);
gnutls_dh_params_generate2(dh_params, DH_BITS);
return 0;
}
static int generate_rsa_params(void)
{
gnutls_rsa_params_init(&rsa_params);
/* Generate RSA parameters - for use with RSA-export
* cipher suites. These should be discarded and regenerated
* once a day, once every 500 transactions etc. Depends on the
* security requirements.
*/
gnutls_rsa_params_generate2(rsa_params, 512);
return 0;
}
int main()
{
int err, listen_sd, i;
int sd, ret;
struct sockaddr_in sa_serv;
struct sockaddr_in sa_cli;
int client_len;
char topbuf[512];
gnutls_session_t session;
char buffer[MAX_BUF + 1];
int optval = 1;
char name[256];
strcpy(name, "Echo Server");
/* this must be called once in the program
*/
gnutls_global_init();
gnutls_certificate_allocate_credentials(&cert_cred);
gnutls_certificate_set_x509_trust_file(cert_cred, CAFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_x509_crl_file(cert_cred, CRLFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_x509_key_file(cert_cred, CERTFILE, KEYFILE,
GNUTLS_X509_FMT_PEM);
generate_dh_params();
generate_rsa_params();
if (TLS_SESSION_CACHE != 0) {
wrap_db_init();
}
gnutls_certificate_set_dh_params(cert_cred, dh_params);
gnutls_certificate_set_rsa_export_params(cert_cred, rsa_params);
/* Socket operations
*/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
SOCKET_ERR(listen_sd, "socket");
memset(&sa_serv, '\0', sizeof(sa_serv));
sa_serv.sin_family = AF_INET;
sa_serv.sin_addr.s_addr = INADDR_ANY;
sa_serv.sin_port = htons(PORT); /* Server Port number */
setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));
err = bind(listen_sd, (SA *) & sa_serv, sizeof(sa_serv));
SOCKET_ERR(err, "bind");
err = listen(listen_sd, 1024);
SOCKET_ERR(err, "listen");
printf("%s ready. Listening to port '%d'.\n\n", name, PORT);
client_len = sizeof(sa_cli);
for (;;) {
session = initialize_tls_session();
sd = accept(listen_sd, (SA *) & sa_cli, &client_len);
printf("- connection from %s, port %d\n",
inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
sizeof(topbuf)), ntohs(sa_cli.sin_port));
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
ret = gnutls_handshake(session);
if (ret < 0) {
close(sd);
gnutls_deinit(session);
fprintf(stderr, "*** Handshake has failed (%s)\n\n",
gnutls_strerror(ret));
continue;
}
printf("- Handshake was completed\n");
/* print_info(session); */
i = 0;
for (;;) {
bzero(buffer, MAX_BUF + 1);
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("\n- Peer has closed the TLS connection\n");
break;
} else if (ret < 0) {
fprintf(stderr, "\n*** Received corrupted "
"data(%d). Closing the connection.\n\n", ret);
break;
} else if (ret > 0) {
/* echo data back to the client
*/
gnutls_record_send(session, buffer, strlen(buffer));
}
}
printf("\n");
/* do not wait for the peer to close the connection.
*/
gnutls_bye(session, GNUTLS_SHUT_WR);
close(sd);
gnutls_deinit(session);
}
close(listen_sd);
gnutls_certificate_free_credentials(cert_cred);
gnutls_global_deinit();
return 0;
}
/* Functions and other stuff needed for session resuming.
* This is done using a very simple list which holds session ids
* and session data.
*/
#define MAX_SESSION_ID_SIZE 32
#define MAX_SESSION_DATA_SIZE 512
typedef struct {
char session_id[MAX_SESSION_ID_SIZE];
int session_id_size;
char session_data[MAX_SESSION_DATA_SIZE];
int session_data_size;
} CACHE;
static CACHE *cache_db;
static int cache_db_ptr = 0;
static void wrap_db_init(void)
{
/* allocate cache_db */
cache_db = calloc(1, TLS_SESSION_CACHE * sizeof(CACHE));
}
static void wrap_db_deinit(void)
{
return;
}
static int wrap_db_store(void *dbf, gnutls_datum_t key,
gnutls_datum_t data)
{
if (cache_db == NULL)
return -1;
if (key.size > MAX_SESSION_ID_SIZE)
return -1;
if (data.size > MAX_SESSION_DATA_SIZE)
return -1;
memcpy(cache_db[cache_db_ptr].session_id, key.data, key.size);
cache_db[cache_db_ptr].session_id_size = key.size;
memcpy(cache_db[cache_db_ptr].session_data, data.data, data.size);
cache_db[cache_db_ptr].session_data_size = data.size;
cache_db_ptr++;
cache_db_ptr %= TLS_SESSION_CACHE;
return 0;
}
static gnutls_datum_t wrap_db_fetch(void *dbf, gnutls_datum_t key)
{
gnutls_datum_t res = { NULL, 0 };
int i;
if (cache_db == NULL)
return res;
for (i = 0; i < TLS_SESSION_CACHE; i++) {
if (key.size == cache_db[i].session_id_size &&
memcmp(key.data, cache_db[i].session_id, key.size) == 0) {
res.size = cache_db[i].session_data_size;
res.data = gnutls_malloc(res.size);
if (res.data == NULL)
return res;
memcpy(res.data, cache_db[i].session_data, res.size);
return res;
}
}
return res;
}
static int wrap_db_delete(void *dbf, gnutls_datum_t key)
{
int i;
if (cache_db == NULL)
return -1;
for (i = 0; i < TLS_SESSION_CACHE; i++) {
if (key.size == cache_db[i].session_id_size &&
memcmp(key.data, cache_db[i].session_id, key.size) == 0) {
cache_db[i].session_id_size = 0;
cache_db[i].session_data_size = 0;
return 0;
}
}
return -1;
}
The following example is an echo server which supports OpenPGP key authentication. You can easily combine this functionality –that is have a server that supports both X.509 and OpenPGP certificates– but we separated them to keep these examples as simple as possible.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
/* Must be linked against gnutls-extra.
*/
#include <gnutls/extra.h>
#define KEYFILE "secret.asc"
#define CERTFILE "public.asc"
#define RINGFILE "ring.gpg"
/* This is a sample TLS 1.0-OpenPGP echo server.
*/
#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556 /* listen to 5556 port */
#define DH_BITS 1024
/* These are global */
gnutls_certificate_credentials_t cred;
const int cert_type_priority[2] = { GNUTLS_CRT_OPENPGP, 0 };
gnutls_dh_params_t dh_params;
/* Defined in a previous example */
extern int generate_dh_params(void);
extern gnutls_session_t initialize_tls_session(void);
int main()
{
int err, listen_sd, i;
int sd, ret;
struct sockaddr_in sa_serv;
struct sockaddr_in sa_cli;
int client_len;
char topbuf[512];
gnutls_session_t session;
char buffer[MAX_BUF + 1];
int optval = 1;
char name[256];
strcpy(name, "Echo Server");
/* this must be called once in the program
*/
gnutls_global_init();
gnutls_certificate_allocate_credentials(&cred);
gnutls_certificate_set_openpgp_keyring_file(cred, RINGFILE);
gnutls_certificate_set_openpgp_key_file(cred, CERTFILE, KEYFILE);
generate_dh_params();
gnutls_certificate_set_dh_params(cred, dh_params);
/* Socket operations
*/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
SOCKET_ERR(listen_sd, "socket");
memset(&sa_serv, '\0', sizeof(sa_serv));
sa_serv.sin_family = AF_INET;
sa_serv.sin_addr.s_addr = INADDR_ANY;
sa_serv.sin_port = htons(PORT); /* Server Port number */
setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));
err = bind(listen_sd, (SA *) & sa_serv, sizeof(sa_serv));
SOCKET_ERR(err, "bind");
err = listen(listen_sd, 1024);
SOCKET_ERR(err, "listen");
printf("%s ready. Listening to port '%d'.\n\n", name, PORT);
client_len = sizeof(sa_cli);
for (;;) {
session = initialize_tls_session();
gnutls_certificate_type_set_priority(session, cert_type_priority);
sd = accept(listen_sd, (SA *) & sa_cli, &client_len);
printf("- connection from %s, port %d\n",
inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
sizeof(topbuf)), ntohs(sa_cli.sin_port));
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
ret = gnutls_handshake(session);
if (ret < 0) {
close(sd);
gnutls_deinit(session);
fprintf(stderr, "*** Handshake has failed (%s)\n\n",
gnutls_strerror(ret));
continue;
}
printf("- Handshake was completed\n");
/* see the Getting peer's information example */
/* print_info(session); */
i = 0;
for (;;) {
bzero(buffer, MAX_BUF + 1);
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("\n- Peer has closed the GNUTLS connection\n");
break;
} else if (ret < 0) {
fprintf(stderr, "\n*** Received corrupted "
"data(%d). Closing the connection.\n\n", ret);
break;
} else if (ret > 0) {
/* echo data back to the client
*/
gnutls_record_send(session, buffer, strlen(buffer));
}
}
printf("\n");
/* do not wait for the peer to close the connection.
*/
gnutls_bye(session, GNUTLS_SHUT_WR);
close(sd);
gnutls_deinit(session);
}
close(listen_sd);
gnutls_certificate_free_credentials(cred);
gnutls_global_deinit();
return 0;
}
This is a server which supports SRP authentication. It is also possible to combine this functionality with a certificate server. Here it is separate for simplicity.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/extra.h>
#define SRP_PASSWD "tpasswd"
#define SRP_PASSWD_CONF "tpasswd.conf"
#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "ca.pem"
/* This is a sample TLS-SRP echo server.
*/
#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556 /* listen to 5556 port */
/* These are global */
gnutls_srp_server_credentials_t srp_cred;
gnutls_certificate_credentials_t cert_cred;
gnutls_session_t initialize_tls_session()
{
gnutls_session_t session;
const int kx_priority[] = { GNUTLS_KX_SRP, GNUTLS_KX_SRP_DSS,
GNUTLS_KX_SRP_RSA, 0
};
gnutls_init(&session, GNUTLS_SERVER);
gnutls_set_default_priority(session);
gnutls_kx_set_priority(session, kx_priority);
gnutls_credentials_set(session, GNUTLS_CRD_SRP, srp_cred);
/* for the certificate authenticated ciphersuites.
*/
gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, cert_cred);
/* request client certificate if any.
*/
gnutls_certificate_server_set_request(session, GNUTLS_CERT_IGNORE);
return session;
}
int main()
{
int err, listen_sd, i;
int sd, ret;
struct sockaddr_in sa_serv;
struct sockaddr_in sa_cli;
int client_len;
char topbuf[512];
gnutls_session_t session;
char buffer[MAX_BUF + 1];
int optval = 1;
char name[256];
strcpy(name, "Echo Server");
/* these must be called once in the program
*/
gnutls_global_init();
gnutls_global_init_extra(); /* for SRP */
/* SRP_PASSWD a password file (created with the included srptool utility)
*/
gnutls_srp_allocate_server_credentials(&srp_cred);
gnutls_srp_set_server_credentials_file(srp_cred, SRP_PASSWD,
SRP_PASSWD_CONF);
gnutls_certificate_allocate_credentials(&cert_cred);
gnutls_certificate_set_x509_trust_file(cert_cred, CAFILE,
GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_x509_key_file(cert_cred, CERTFILE, KEYFILE,
GNUTLS_X509_FMT_PEM);
/* TCP socket operations
*/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
SOCKET_ERR(listen_sd, "socket");
memset(&sa_serv, '\0', sizeof(sa_serv));
sa_serv.sin_family = AF_INET;
sa_serv.sin_addr.s_addr = INADDR_ANY;
sa_serv.sin_port = htons(PORT); /* Server Port number */
setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));
err = bind(listen_sd, (SA *) & sa_serv, sizeof(sa_serv));
SOCKET_ERR(err, "bind");
err = listen(listen_sd, 1024);
SOCKET_ERR(err, "listen");
printf("%s ready. Listening to port '%d'.\n\n", name, PORT);
client_len = sizeof(sa_cli);
for (;;) {
session = initialize_tls_session();
sd = accept(listen_sd, (SA *) & sa_cli, &client_len);
printf("- connection from %s, port %d\n",
inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
sizeof(topbuf)), ntohs(sa_cli.sin_port));
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
ret = gnutls_handshake(session);
if (ret < 0) {
close(sd);
gnutls_deinit(session);
fprintf(stderr, "*** Handshake has failed (%s)\n\n",
gnutls_strerror(ret));
continue;
}
printf("- Handshake was completed\n");
/* print_info(session); */
i = 0;
for (;;) {
bzero(buffer, MAX_BUF + 1);
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("\n- Peer has closed the GNUTLS connection\n");
break;
} else if (ret < 0) {
fprintf(stderr, "\n*** Received corrupted "
"data(%d). Closing the connection.\n\n", ret);
break;
} else if (ret > 0) {
/* echo data back to the client
*/
gnutls_record_send(session, buffer, strlen(buffer));
}
}
printf("\n");
/* do not wait for the peer to close the connection. */
gnutls_bye(session, GNUTLS_SHUT_WR);
close(sd);
gnutls_deinit(session);
}
close(listen_sd);
gnutls_srp_free_server_credentials(srp_cred);
gnutls_certificate_free_credentials(cert_cred);
gnutls_global_deinit();
return 0;
}
This example server support anonymous authentication, and could be used to serve the example client for anonymous authentication.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
/* This is a sample TLS 1.0 echo server, for anonymous authentication only.
*/
#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556 /* listen to 5556 port */
#define DH_BITS 1024
/* These are global */
gnutls_anon_server_credentials_t anoncred;
gnutls_session_t initialize_tls_session()
{
gnutls_session_t session;
const int kx_prio[] = { GNUTLS_KX_ANON_DH, 0 };
gnutls_init(&session, GNUTLS_SERVER);
/* avoid calling all the priority functions, since the defaults
* are adequate.
*/
gnutls_set_default_priority(session);
gnutls_kx_set_priority (session, kx_prio);
gnutls_credentials_set(session, GNUTLS_CRD_ANON, anoncred);
gnutls_dh_set_prime_bits(session, DH_BITS);
return session;
}
static gnutls_dh_params_t dh_params;
static int generate_dh_params(void)
{
/* Generate Diffie Hellman parameters - for use with DHE
* kx algorithms. These should be discarded and regenerated
* once a day, once a week or once a month. Depending on the
* security requirements.
*/
gnutls_dh_params_init(&dh_params);
gnutls_dh_params_generate2(dh_params, DH_BITS);
return 0;
}
int main()
{
int err, listen_sd, i;
int sd, ret;
struct sockaddr_in sa_serv;
struct sockaddr_in sa_cli;
int client_len;
char topbuf[512];
gnutls_session_t session;
char buffer[MAX_BUF + 1];
int optval = 1;
/* this must be called once in the program
*/
gnutls_global_init();
gnutls_anon_allocate_server_credentials (&anoncred);
generate_dh_params();
gnutls_anon_set_server_dh_params (anoncred, dh_params);
/* Socket operations
*/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
SOCKET_ERR(listen_sd, "socket");
memset(&sa_serv, '\0', sizeof(sa_serv));
sa_serv.sin_family = AF_INET;
sa_serv.sin_addr.s_addr = INADDR_ANY;
sa_serv.sin_port = htons(PORT); /* Server Port number */
setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));
err = bind(listen_sd, (SA *) & sa_serv, sizeof(sa_serv));
SOCKET_ERR(err, "bind");
err = listen(listen_sd, 1024);
SOCKET_ERR(err, "listen");
printf("Server ready. Listening to port '%d'.\n\n", PORT);
client_len = sizeof(sa_cli);
for (;;) {
session = initialize_tls_session();
sd = accept(listen_sd, (SA *) & sa_cli, &client_len);
printf("- connection from %s, port %d\n",
inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
sizeof(topbuf)), ntohs(sa_cli.sin_port));
gnutls_transport_set_ptr(session, (gnutls_transport_ptr_t) sd);
ret = gnutls_handshake(session);
if (ret < 0) {
close(sd);
gnutls_deinit(session);
fprintf(stderr, "*** Handshake has failed (%s)\n\n",
gnutls_strerror(ret));
continue;
}
printf("- Handshake was completed\n");
/* see the Getting peer's information example */
/* print_info(session); */
i = 0;
for (;;) {
bzero(buffer, MAX_BUF + 1);
ret = gnutls_record_recv(session, buffer, MAX_BUF);
if (ret == 0) {
printf("\n- Peer has closed the GNUTLS connection\n");
break;
} else if (ret < 0) {
fprintf(stderr, "\n*** Received corrupted "
"data(%d). Closing the connection.\n\n", ret);
break;
} else if (ret > 0) {
/* echo data back to the client
*/
gnutls_record_send(session, buffer, strlen(buffer));
}
}
printf("\n");
/* do not wait for the peer to close the connection.
*/
gnutls_bye(session, GNUTLS_SHUT_WR);
close(sd);
gnutls_deinit(session);
}
close(listen_sd);
gnutls_anon_free_client_credentials (anoncred);
gnutls_global_deinit();
return 0;
}
This is a function that checks if an alert has been received in the current session.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
/* This function will check whether the given return code from
* a gnutls function (recv/send), is an alert, and will print
* that alert.
*/
void check_alert(gnutls_session_t session, int ret)
{
int last_alert;
if (ret == GNUTLS_E_WARNING_ALERT_RECEIVED
|| ret == GNUTLS_E_FATAL_ALERT_RECEIVED) {
last_alert = gnutls_alert_get(session);
/* The check for renegotiation is only useful if we are
* a server, and we had requested a rehandshake.
*/
if (last_alert == GNUTLS_A_NO_RENEGOTIATION &&
ret == GNUTLS_E_WARNING_ALERT_RECEIVED)
printf("* Received NO_RENEGOTIATION alert. "
"Client Does not support renegotiation.\n");
else
printf("* Received alert '%d': %s.\n", last_alert,
gnutls_alert_get_name(last_alert));
}
}
To demonstrate the X.509 parsing capabilities an example program is listed below. That program reads the peer's certificate, and prints information about it.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
static const char *bin2hex(const void *bin, size_t bin_size)
{
static char printable[110];
unsigned char *_bin = bin;
char *print;
if (bin_size > 50)
bin_size = 50;
print = printable;
for (i = 0; i < bin_size; i++) {
sprintf(print, "%.2x ", _bin[i]);
print += 2;
}
return printable;
}
/* This function will print information about this session's peer
* certificate.
*/
static void print_x509_certificate_info(gnutls_session_t session)
{
char serial[40];
char dn[128];
int i;
size_t size;
unsigned int algo, bits;
time_t expiration_time, activation_time;
const gnutls_datum_t *cert_list;
int cert_list_size = 0;
gnutls_x509_crt_t cert;
/* This function only works for X.509 certificates.
*/
if (gnutls_certificate_type_get(session) != GNUTLS_CRT_X509)
return;
cert_list = gnutls_certificate_get_peers(session, &cert_list_size);
printf("Peer provided %d certificates.\n", cert_list_size);
if (cert_list_size > 0) {
/* we only print information about the first certificate.
*/
gnutls_x509_crt_init(&cert);
gnutls_x509_crt_import(cert, &cert_list[0]);
printf("Certificate info:\n");
expiration_time = gnutls_x509_crt_get_expiration_time(cert);
activation_time = gnutls_x509_crt_get_activation_time(cert);
printf("\tCertificate is valid since: %s",
ctime(&activation_time));
printf("\tCertificate expires: %s", ctime(&expiration_time));
/* Print the serial number of the certificate.
*/
size = sizeof(serial);
gnutls_x509_crt_get_serial(cert, serial, &size);
size = sizeof(serial);
printf("\tCertificate serial number: %s\n", bin2hex(serial, size));
/* Extract some of the public key algorithm's parameters
*/
algo = gnutls_x509_crt_get_pk_algorithm(cert, &bits);
printf("Certificate public key: %s",
gnutls_pk_algorithm_get_name(algo));
/* Print the version of the X.509
* certificate.
*/
printf("\tCertificate version: #%d\n",
gnutls_x509_crt_get_version(cert));
size = sizeof(dn);
gnutls_x509_crt_get_dn(cert, dn, &size);
printf("\tDN: %s\n", dn);
size = sizeof(dn);
gnutls_x509_crt_get_issuer_dn(cert, dn, &size);
printf("\tIssuer's DN: %s\n", dn);
gnutls_x509_crt_deinit(cert);
}
}
The following example is about generating a certificate request, and a private key. A certificate request can be later be processed by a CA, which should return a signed certificate.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <time.h>
/* This example will generate a private key and a certificate
* request.
*/
int main()
{
gnutls_x509_crq_t crq;
gnutls_x509_privkey_t key;
unsigned char buffer[10 * 1024];
int buffer_size = sizeof(buffer);
int ret;
gnutls_global_init();
/* Initialize an empty certificate request, and
* an empty private key.
*/
gnutls_x509_crq_init(&crq);
gnutls_x509_privkey_init(&key);
/* Generate a 1024 bit RSA private key.
*/
gnutls_x509_privkey_generate(key, GNUTLS_PK_RSA, 1024, 0);
/* Add stuff to the distinguished name
*/
gnutls_x509_crq_set_dn_by_oid(crq, GNUTLS_OID_X520_COUNTRY_NAME,
0, "GR", 2);
gnutls_x509_crq_set_dn_by_oid(crq, GNUTLS_OID_X520_COMMON_NAME,
0, "Nikos", strlen("Nikos"));
/* Set the request version.
*/
gnutls_x509_crq_set_version(crq, 1);
/* Set a challenge password.
*/
gnutls_x509_crq_set_challenge_password(crq,
"something to remember here");
/* Associate the request with the private key
*/
gnutls_x509_crq_set_key(crq, key);
/* Self sign the certificate request.
*/
gnutls_x509_crq_sign(crq, key);
/* Export the PEM encoded certificate request, and
* display it.
*/
gnutls_x509_crq_export(crq, GNUTLS_X509_FMT_PEM, buffer, &buffer_size);
printf("Certificate Request: \n%s", buffer);
/* Export the PEM encoded private key, and
* display it.
*/
buffer_size = sizeof(buffer);
gnutls_x509_privkey_export(key, GNUTLS_X509_FMT_PEM, buffer,
&buffer_size);
printf("\n\nPrivate key: \n%s", buffer);
gnutls_x509_crq_deinit(crq);
gnutls_x509_privkey_deinit(key);
return 0;
}
The following example is about generating a PKCS #12 structure.
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/pkcs12.h>
#define OUTFILE "out.p12"
/* This function will write a pkcs12 structure into a file.
* cert: is a DER encoded certificate
* pkcs8_key: is a PKCS #8 encrypted key (note that this must be
* encrypted using a PKCS #12 cipher, or some browsers will crash)
* password: is the password used to encrypt the PKCS #12 packet.
*/
int write_pkcs12(const gnutls_datum_t * cert,
const gnutls_datum_t * pkcs8_key, const char *password)
{
gnutls_pkcs12_t pkcs12;
int ret, bag_index;
gnutls_pkcs12_bag_t bag, key_bag;
char pkcs12_struct[10 * 1024];
int pkcs12_struct_size;
FILE *fd;
/* A good idea might be to use gnutls_x509_privkey_get_key_id()
* to obtain a unique ID.
*/
gnutls_datum_t key_id = { "\x00\x00\x07", 3 };
gnutls_global_init();
/* Firstly we create two helper bags, which hold the certificate,
* and the (encrypted) key.
*/
gnutls_pkcs12_bag_init(&bag);
gnutls_pkcs12_bag_init(&key_bag);
ret = gnutls_pkcs12_bag_set_data(bag, GNUTLS_BAG_CERTIFICATE, cert);
if (ret < 0) {
fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
exit(1);
}
/* ret now holds the bag's index.
*/
bag_index = ret;
/* Associate a friendly name with the given certificate. Used
* by browsers.
*/
gnutls_pkcs12_bag_set_friendly_name(bag, bag_index, "My name");
/* Associate the certificate with the key using a unique key
* ID.
*/
gnutls_pkcs12_bag_set_key_id(bag, bag_index, &key_id);
/* use weak encryption for the certificate.
*/
gnutls_pkcs12_bag_encrypt(bag, password,
GNUTLS_PKCS_USE_PKCS12_RC2_40);
/* Now the key.
*/
ret = gnutls_pkcs12_bag_set_data(key_bag,
GNUTLS_BAG_PKCS8_ENCRYPTED_KEY,
pkcs8_key);
if (ret < 0) {
fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
exit(1);
}
/* Note that since the PKCS #8 key is already encrypted we don't
* bother encrypting that bag.
*/
bag_index = ret;
gnutls_pkcs12_bag_set_friendly_name(key_bag, bag_index, "My name");
gnutls_pkcs12_bag_set_key_id(key_bag, bag_index, &key_id);
/* The bags were filled. Now create the PKCS #12 structure.
*/
gnutls_pkcs12_init(&pkcs12);
/* Insert the two bags in the PKCS #12 structure.
*/
gnutls_pkcs12_set_bag(pkcs12, bag);
gnutls_pkcs12_set_bag(pkcs12, key_bag);
/* Generate a message authentication code for the PKCS #12
* structure.
*/
gnutls_pkcs12_generate_mac(pkcs12, password);
pkcs12_struct_size = sizeof(pkcs12_struct);
ret =
gnutls_pkcs12_export(pkcs12, GNUTLS_X509_FMT_DER, pkcs12_struct,
&pkcs12_struct_size);
if (ret < 0) {
fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
exit(1);
}
fd = fopen(OUTFILE, "w");
if (fd == NULL) {
fprintf(stderr, "cannot open file\n");
exit(1);
}
fwrite(pkcs12_struct, 1, pkcs12_struct_size, fd);
fclose(fd);
gnutls_pkcs12_bag_deinit(bag);
gnutls_pkcs12_bag_deinit(key_bag);
gnutls_pkcs12_deinit(pkcs12);
}
To ease GnuTLS' integration with existing applications, a
compatibility layer with the widely used OpenSSL library is included
in the gnutls-openssl library. This compatibility layer is not
complete and it is not intended to completely reimplement the OpenSSL
API with GnuTLS. It only provides source-level
compatibility. There is currently no attempt to make it
binary-compatible with OpenSSL.
The prototypes for the compatibility functions are in the gnutls/openssl.h header file.
Current limitations imposed by the compatibility layer include:
Included with GnuTLS are also a few command line tools that let you use the library for common tasks without writing an application. The applications are discussed in this chapter.
The srptool is a very simple program that emulates the programs in the Stanford SRP libraries. It is intended for use in places where you don't expect SRP authentication to be the used for system users. Traditionally libsrp used two files. One called 'tpasswd' which holds usernames and verifiers, and 'tpasswd.conf' which holds generators and primes.
How to use srptool:
$ srptool --create-conf /etc/tpasswd.conf
$ srptool --passwd /etc/tpasswd \
--passwd-conf /etc/tpasswd.conf -u test
$ srptool --passwd /etc/tpasswd \
--passwd-conf /etc/tpasswd.conf --verify -u test
Simple client program to set up a TLS connection to some other computer. It sets up a TLS connection and forwards data from the standard input to the secured socket and vice versa.
GNU TLS test client
Usage: gnutls-cli [options] hostname
-d, --debug integer Enable debugging
-r, --resume Connect, establish a session. Connect
again and resume this session.
-s, --starttls Connect, establish a plain session and
start TLS when EOF or a SIGALRM is
received.
--crlf Send CR LF instead of LF.
--x509fmtder Use DER format for certificates to read
from.
-f, --fingerprint Send the openpgp fingerprint, instead
of the key.
--disable-extensions Disable all the TLS extensions.
--xml Print the certificate information in
XML format.
--print-cert Print the certificate in PEM format.
-p, --port integer The port to connect to.
--recordsize integer The maximum record size to advertize.
-V, --verbose More verbose output.
--ciphers cipher1 cipher2...
Ciphers to enable.
--protocols protocol1 protocol2...
Protocols to enable.
--comp comp1 comp2... Compression methods to enable.
--macs mac1 mac2... MACs to enable.
--kx kx1 kx2... Key exchange methods to enable.
--ctypes certType1 certType2...
Certificate types to enable.
--x509cafile FILE Certificate file to use.
--x509crlfile FILE CRL file to use.
--pgpkeyfile FILE PGP Key file to use.
--pgpkeyring FILE PGP Key ring file to use.
--pgptrustdb FILE PGP trustdb file to use.
--pgpcertfile FILE PGP Public Key (certificate) file to
use.
--x509keyfile FILE X.509 key file to use.
--x509certfile FILE X.509 Certificate file to use.
--srpusername NAME SRP username to use.
--srppasswd PASSWD SRP password to use.
-l, --list Print a list of the supported
algorithms and modes.
-h, --help prints this help
-v, --version prints the program's version number
--copyright prints the program's license
This program was created to assist in debugging GnuTLS, but it might be useful to extract a TLS server's capabilities. It's purpose is to connect onto a TLS server, perform some tests and print the server's capabilities. If called with the `-v' parameter a more checks will be performed. An example output is:
crystal:/cvs/gnutls/src$ ./gnutls-cli-debug localhost -p 5556
Resolving 'localhost'...
Connecting to '127.0.0.1:5556'...
Checking for TLS 1.1 support... yes
Checking fallback from TLS 1.1 to... N/A
Checking for TLS 1.0 support... yes
Checking for SSL 3.0 support... yes
Checking for version rollback bug in RSA PMS... no
Checking for version rollback bug in Client Hello... no
Checking whether we need to disable TLS 1.0... N/A
Checking whether the server ignores the RSA PMS version... no
Checking whether the server can accept Hello Extensions... yes
Checking whether the server can accept cipher suites not in SSL 3.0 spec... yes
Checking whether the server can accept a bogus TLS record version in the client hello... yes
Checking for certificate information... N/A
Checking for trusted CAs... N/A
Checking whether the server understands TLS closure alerts... yes
Checking whether the server supports session resumption... yes
Checking for export-grade ciphersuite support... no
Checking RSA-export ciphersuite info... N/A
Checking for anonymous authentication support... no
Checking anonymous Diffie Hellman group info... N/A
Checking for ephemeral Diffie Hellman support... no
Checking ephemeral Diffie Hellman group info... N/A
Checking for AES cipher support (TLS extension)... yes
Checking for 3DES cipher support... yes
Checking for ARCFOUR 128 cipher support... yes
Checking for ARCFOUR 40 cipher support... no
Checking for MD5 MAC support... yes
Checking for SHA1 MAC support... yes
Checking for RIPEMD160 MAC support (TLS extension)... yes
Checking for ZLIB compression support (TLS extension)... yes
Checking for LZO compression support (GnuTLS extension)... yes
Checking for max record size (TLS extension)... yes
Checking for SRP authentication support (TLS extension)... yes
Checking for OpenPGP authentication support (TLS extension)... no
Simple server program that listens to incoming TLS connections.
GNU TLS test server
Usage: gnutls-serv [options]
-d, --debug integer Enable debugging
-g, --generate Generate Diffie Hellman Parameters.
-p, --port integer The port to connect to.
-q, --quiet Suppress some messages.
--nodb Does not use the resume database.
--http Act as an HTTP Server.
--echo Act as an Echo Server.
--dhparams FILE DH params file to use.
--x509fmtder Use DER format for certificates
--x509cafile FILE Certificate file to use.
--x509crlfile FILE CRL file to use.
--pgpkeyring FILE PGP Key ring file to use.
--pgptrustdb FILE PGP trustdb file to use.
--pgpkeyfile FILE PGP Key file to use.
--pgpcertfile FILE PGP Public Key (certificate) file to
use.
--x509keyfile FILE X.509 key file to use.
--x509certfile FILE X.509 Certificate file to use.
--x509dsakeyfile FILE Alternative X.509 key file to use.
--x509dsacertfile FILE Alternative X.509 certificate file to
use.
--srppasswd FILE SRP password file to use.
--srppasswdconf FILE SRP password conf file to use.
--ciphers cipher1 cipher2...
Ciphers to enable.
--protocols protocol1 protocol2...
Protocols to enable.
--comp comp1 comp2... Compression methods to enable.
--macs mac1 mac2... MACs to enable.
--kx kx1 kx2... Key exchange methods to enable.
--ctypes certType1 certType2...
Certificate types to enable.
-l, --list Print a list of the supported
algorithms and modes.
-h, --help prints this help
-v, --version prints the program's version number
--copyright prints the program's license
This is a program to generate X.509 certificates, certificate
requests, CRLs and private keys. The program can be used interactively
or non interactively by specifying the --template command line
option. See doc/certtool.cfg, in the distribution, for an
example of a template file.
How to use certtool interactively:
$ certtool --generate-dh-params --outfile dh.pem
$ certtool --generate-privkey --bits 512 --outfile rsa.pem
$ certtool --generate-privkey --outfile ca-key.pem
$ certtool --generate-self-signed --load-privkey ca-key.pem \
--outfile ca-cert.pem
Note that a self-signed certificate usually belongs to a certificate authority, that signs other certificates.
$ certtool --generate-privkey --outfile key.pem
$ certtool --generate-request --load-privkey key.pem \
--outfile request.pem
$ certtool --generate-certificate --load-request request.pem \
--outfile cert.pem \
--load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem
$ certtool --certificate-info --infile cert.pem
$ certtool --load-certificate cert.pem --load-privkey key.pem \
--to-p12 --outder --outfile key.p12
Certtool's template file format:
$ certtool --generate-certificate cert.pem --load-privkey key.pem \
--template cert.cfg \
--load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem
An example certtool template file:
# X.509 Certificate options
#
# DN options
# The organization of the subject.
organization = "Koko inc."
# The organizational unit of the subject.
unit = "sleeping dept."
# The locality of the subject.
# locality =
# The state of the certificate owner.
state = "Attiki"
# The country of the subject. Two letter code.
country = GR
# The common name of the certificate owner.
cn = "Cindy Lauper"
# A user id of the certificate owner.
#uid = "clauper"
# If the supported DN OIDs are not adequate you can set
# any OID here.
# For example set the X.520 Title and the X.520 Pseudonym
# by using OID and string pairs.
#dn_oid = "2.5.4.12" "Dr." "2.5.4.65" "jackal"
# This is deprecated and should not be used in new
# certificates.
# pkcs9_email = "none@none.org"
# The serial number of the certificate
serial = 007
# In how many days, counting from today, this certificate will expire.
expiration_days = 700
# X.509 v3 extensions
# A dnsname in case of a WWW server.
#dns_name = "www.none.org"
# An IP address in case of a server.
#ip_address = "192.168.1.1"
# An email in case of a person
email = "none@none.org"
# An URL that has CRLs (certificate revocation lists)
# available. Needed in CA certificates.
#crl_dist_points = "http://www.getcrl.crl/getcrl/"
# Whether this is a CA certificate or not
#ca
# Whether this certificate will be used for a TLS client
#tls_www_client
# Whether this certificate will be used for a TLS server
#tls_www_server
# Whether this certificate will be used to sign data (needed
# in TLS DHE ciphersuites).
signing_key
# Whether this certificate will be used to encrypt data (needed
# in TLS RSA ciphersuites). Note that it is prefered to use different
# keys for encryption and signing.
#encryption_key
# Whether this key will be used to sign other certificates.
#cert_signing_key
# Whether this key will be used to sign CRLs.
#crl_signing_key
# Whether this key will be used to sign code.
#code_signing_key
# Whether this key will be used to sign OCSP data.
#ocsp_signing_key
# Whether this key will be used for time stamping.
#time_stamping_key
The prototypes for the following functions lie in gnutls/gnutls.h.
alert: is an alert number
gnutls_session_tstructure.Returns a string that describes the given alert number or NULL. See
gnutls_alert_get().
session: is a
gnutls_session_tstructure.Returns the last alert number received. This function should be called if GNUTLS_E_WARNING_ALERT_RECEIVED or GNUTLS_E_FATAL_ALERT_RECEIVED has been returned by a gnutls function. The peer may send alerts if he thinks some things were not right. Check gnutls.h for the available alert descriptions.
session: is a
gnutls_session_tstructure.err: is an integer
This function is DEPRECATED, and may be removed.
Sends an alert to the peer depending on the error code returned by a gnutls function. This function will call
gnutls_error_to_alert()to determine the appropriate alert to send.This function may also return GNUTLS_E_AGAIN, or GNUTLS_E_INTERRUPTED.
If the return value is GNUTLS_E_INVALID_REQUEST, then no alert has been sent to the peer.
session: is a
gnutls_session_tstructure.level: is the level of the alert
desc: is the alert description
This function will send an alert to the peer in order to inform him of something important (eg. his Certificate could not be verified). If the alert level is Fatal then the peer is expected to close the connection, otherwise he may ignore the alert and continue.
The error code of the underlying record send function will be returned, so you may also receive GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN as well.
Returns 0 on success.
sc: is a pointer to an
gnutls_anon_client_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.
sc: is a pointer to an
gnutls_anon_server_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.
sc: is an
gnutls_anon_client_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.
sc: is an
gnutls_anon_server_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.
res: is a gnutls_certificate_credentials_t structure
func: is the function to be called
This function will set a callback in order for the server to get the diffie hellman parameters for anonymous authentication. The callback should return zero on success.
res: is a gnutls_anon_server_credentials_t structure
dh_params: is a structure that holds diffie hellman parameters.
This function will set the diffie hellman parameters for an anonymous server to use. These parameters will be used in Anonymous Diffie Hellman cipher suites.
session: is a
gnutls_session_tstructure.Returns the type of credentials that were used for client authentication. The returned information is to be used to distinguish the function used to access authentication data.
session: is a
gnutls_session_tstructure.Returns type of credentials for the current authentication schema. The returned information is to be used to distinguish the function used to access authentication data.
Eg. for CERTIFICATE ciphersuites (key exchange algorithms: KX_RSA, KX_DHE_RSA), the same function are to be used to access the authentication data.
session: is a
gnutls_session_tstructure.Returns the type of credentials that were used for server authentication. The returned information is to be used to distinguish the function used to access authentication data.
session: is a
gnutls_session_tstructure.how: is an integer
Terminates the current TLS/SSL connection. The connection should have been initiated using
gnutls_handshake().howshould be one of GNUTLS_SHUT_RDWR, GNUTLS_SHUT_WR.In case of GNUTLS_SHUT_RDWR then the TLS connection gets terminated and further receives and sends will be disallowed. If the return value is zero you may continue using the connection. GNUTLS_SHUT_RDWR actually sends an alert containing a close request and waits for the peer to reply with the same message.
In case of GNUTLS_SHUT_WR then the TLS connection gets terminated and further sends will be disallowed. In order to reuse the connection you should wait for an EOF from the peer. GNUTLS_SHUT_WR sends an alert containing a close request.
This function may also return GNUTLS_E_AGAIN or GNUTLS_E_INTERRUPTED; cf.
gnutls_record_get_direction().
session: is a gnutls session
This function will return the peer's certificate activation time. This is the creation time for openpgp keys.
Returns (time_t) -1 on error.
res: is a pointer to an
gnutls_certificate_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.
Returns 0 on success.
session: is a gnutls session
This function will return 0 if the peer (server) did not request client authentication or 1 otherwise. Returns a negative value in case of an error.
cred: is a
gnutls_certificate_credentials_tstructure.func: is the callback function
This function sets a callback to be called in order to retrieve the certificate to be used in the handshake. The callback's function prototype is: int (*callback)(gnutls_session_t, const gnutls_datum_t* req_ca_dn, int nreqs, gnutls_pk_algorithm_t* pk_algos, int pk_algos_length, gnutls_retr_st* st);
stshould contain the certificates and private keys.
req_ca_cert, is only used in X.509 certificates. Contains a list with the CA names that the server considers trusted. Normally we should send a certificate that is signed by one of these CAs. These names are DER encoded. To get a more meaningful value use the functiongnutls_x509_rdn_get().
pk_algos, contains a list with server's acceptable signature algorithms. The certificate returned should support the server's given algorithms.If the callback function is provided then gnutls will call it, in the handshake, after the certificate request message has been received.
The callback function should set the certificate list to be sent, and return 0 on success. If no certificate was selected then the number of certificates should be set to zero. The value (-1) indicates error and the handshake will be terminated.
session: is a gnutls session
This function will return the peer's certificate expiration time.
Returns (time_t) -1 on error.
sc: is an
gnutls_certificate_credentials_tstructure.This function will delete all the CA name in the given credentials. Clients may call this to save some memory since in client side the CA names are not used.
CA names are used by servers to advertize the CAs they support to clients.
sc: is an
gnutls_certificate_credentials_tstructure.This function will delete all the CAs associated with the given credentials. Servers that do not use
gnutls_certificate_verify_peers2()may call this to save some memory.
sc: is an
gnutls_certificate_credentials_tstructure.This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.
This function does not free any temporary parameters associated with this structure (ie RSA and DH parameters are not freed by this function).
sc: is an
gnutls_certificate_credentials_tstructure.This function will delete all the CRLs associated with the given credentials.
sc: is an
gnutls_certificate_credentials_tstructure.This function will delete all the keys and the certificates associated with the given credentials. This function must not be called when a TLS negotiation that uses the credentials is in progress.
session: is a gnutls session
This function will return the certificate as sent to the peer, in the last handshake. These certificates are in raw format. In X.509 this is a certificate list. In OpenPGP this is a single certificate. Returns NULL in case of an error, or if no certificate was used.
session: is a gnutls session
list_size: is the length of the certificate list
This function will return the peer's raw certificate (chain) as sent by the peer. These certificates are in raw format (DER encoded for X.509). In case of a X.509 then a certificate list may be present. The first certificate in the list is the peer's certificate, following the issuer's certificate, then the issuer's issuer etc.
In case of OpenPGP keys a single key will be returned in raw format.
Returns NULL in case of an error, or if no certificate was sent.
session: is a pointer to a
gnutls_session_tstructure.status: is 0 or 1
If status is non zero, this function will order gnutls not to send the rdnSequence in the certificate request message. That is the server will not advertize it's trusted CAs to the peer. If status is zero then the default behaviour will take effect, which is to advertize the server's trusted CAs.
This function has no effect in clients, and in authentication methods other than certificate with X.509 certificates.
session: is an
gnutls_session_tstructure.req: is one of GNUTLS_CERT_REQUEST, GNUTLS_CERT_REQUIRE
This function specifies if we (in case of a server) are going to send a certificate request message to the client. If
reqis GNUTLS_CERT_REQUIRE then the server will return an error if the peer does not provide a certificate. If you do not call this function then the client will not be asked to send a certificate.
cred: is a
gnutls_certificate_credentials_tstructure.func: is the callback function
This function sets a callback to be called in order to retrieve the certificate to be used in the handshake. The callback's function prototype is: int (*callback)(gnutls_session_t, gnutls_retr_st* st);
stshould contain the certificates and private keys.If the callback function is provided then gnutls will call it, in the handshake, after the certificate request message has been received.
The callback function should set the certificate list to be sent, and return 0 on success. The value (-1) indicates error and the handshake will be terminated.
res: is a gnutls_certificate_credentials_t structure
dh_params: is a structure that holds diffie hellman parameters.
This function will set the diffie hellman parameters for a certificate server to use. These parameters will be used in Ephemeral Diffie Hellman cipher suites. Note that only a pointer to the parameters are stored in the certificate handle, so if you deallocate the parameters before the certificate is deallocated, you must change the parameters stored in the certificate first.
res: is a gnutls_certificate_credentials_t structure
func: is the function to be called
This function will set a callback in order for the server to get the diffie hellman or RSA parameters for certificate authentication. The callback should return zero on success.
res: is a gnutls_certificate_credentials_t structure
rsa_params: is a structure that holds temporary RSA parameters.
This function will set the temporary RSA parameters for a certificate server to use. These parameters will be used in RSA-EXPORT cipher suites.
res: is a gnutls_certificate_credentials_t structure
flags: are the flagsis a structure that holds diffie hellman parameters.
This function will set the flags to be used at verification of the certificates. Flags must be OR of the gnutls_certificate_verify_flags enumerations.
res: is a gnutls_certificate_credentials structure
max_bits: is the number of bits of an acceptable certificate (default 8200)
max_depth: is maximum depth of the verification of a certificate chain (default 5)
This function will set some upper limits for the default verification function (
gnutls_certificate_verify_peers()) to avoid denial of service attacks.
res: is an
gnutls_certificate_credentials_tstructure.crlfile: is a file containing the list of verified CRLs (DER or PEM list)
type: is PEM or DER
This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.Returns the number of CRLs processed or a negative value on error.
res: is an
gnutls_certificate_credentials_tstructure.CRL: is a list of trusted CRLs. They should have been verified before.
type: is DER or PEM
This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.Returns the number of CRLs processed or a negative value on error.
res: is an
gnutls_certificate_credentials_tstructure.crl_list: is a list of trusted CRLs. They should have been verified before.
crl_list_size: holds the size of the crl_list
This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.Returns 0 on success.
res: is an
gnutls_certificate_credentials_tstructure.CERTFILE: is a file that containing the certificate list (path) for the specified private key, in PKCS7 format, or a list of certificates
KEYFILE: is a file that contains the private key
type: is PEM or DER
This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).
Currently only PKCS-1 encoded RSA and DSA private keys are accepted by this function.
res: is an
gnutls_certificate_credentials_tstructure.cert: contains a certificate list (path) for the specified private key
key: is the private key
type: is PEM or DER
This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).
Currently are supported: RSA PKCS-1 encoded private keys, DSA private keys.
DSA private keys are encoded the OpenSSL way, which is an ASN.1 DER sequence of 6 INTEGERs - version, p, q, g, pub, priv.
Note that the keyUsage (2.5.29.15) PKIX extension in X.509 certificates is supported. This means that certificates intended for signing cannot be used for ciphersuites that require encryption.
If the certificate and the private key are given in PEM encoding then the strings that hold their values must be null terminated.
res: is an
gnutls_certificate_credentials_tstructure.cert_list: contains a certificate list (path) for the specified private key
cert_list_size: holds the size of the certificate list
key: is a gnutls_x509_privkey_t key
This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).
res: is an
gnutls_certificate_credentials_tstructure.cafile: is a file containing the list of trusted CAs (DER or PEM list)
type: is PEM or DER
This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.In case of a server the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using
gnutls_certificate_send_x509_rdn_sequence().Returns the number of certificates processed or a negative value on error.
res: is an
gnutls_certificate_credentials_tstructure.ca: is a list of trusted CAs or a DER certificate
type: is DER or PEM
This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.In case of a server the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using
gnutls_certificate_send_x509_rdn_sequence().Returns the number of certificates processed or a negative value on error.
res: is an
gnutls_certificate_credentials_tstructure.ca_list: is a list of trusted CAs
ca_list_size: holds the size of the CA list
This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using
gnutls_certificate_verify_peers(). This function may be called multiple times.In case of a server the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using
gnutls_certificate_send_x509_rdn_sequence().Returns 0 on success.
type: is a certificate type
Returns a string (or NULL) that contains the name of the specified certificate type.
session: is a
gnutls_session_tstructure.Returns the currently used certificate type. The certificate type is by default X.509, unless it is negotiated as a TLS extension.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_certificate_type_t elements.
Sets the priority on the certificate types supported by gnutls. Priority is higher for types specified before others. After specifying the types you want, you must append a 0. Note that the certificate type priority is set on the client. The server does not use the cert type priority except for disabling types that were not specified.
session: is a gnutls session
status: is the output of the verification
This function will try to verify the peer's certificate and return its status (trusted, invalid etc.). The value of
statusshould be one or more of the gnutls_certificate_status_t enumerated elements bitwise or'd. To avoid denial of service attacks some default upper limits regarding the certificate key size and chain size are set. To override them usegnutls_certificate_set_verify_limits().Note that you must also check the peer's name in order to check if the verified certificate belongs to the actual peer.
Returns a negative error code on error and zero on success.
This is the same as
gnutls_x509_verify_certificate()and uses the loaded CAs in the credentials as trusted CAs.
req_version: the version to check
Check that the version of the library is at minimum the requested one and return the version string; return NULL if the condition is not satisfied. If a NULL is passed to this function, no check is done, but the version string is simply returned.
algorithm: is an encryption algorithm
Returns the length (in bytes) of the given cipher's key size. Returns 0 if the given cipher is invalid.
algorithm: is an encryption algorithm
Returns a pointer to a string that contains the name of the specified cipher or NULL.
session: is a
gnutls_session_tstructure.Returns the currently used cipher.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_cipher_algorithm_t elements.
Sets the priority on the ciphers supported by gnutls. Priority is higher for ciphers specified before others. After specifying the ciphers you want, you must append a 0. Note that the priority is set on the client. The server does not use the algorithm's priority except for disabling algorithms that were not specified.
kx_algorithm: is a Key exchange algorithm
cipher_algorithm: is a cipher algorithm
mac_algorithm: is a MAC algorithm
Returns a string that contains the name of a TLS cipher suite, specified by the given algorithms, or NULL.
Note that the full cipher suite name must be prepended by TLS or SSL depending of the protocol in use.
algorithm: is a Compression algorithm
Returns a pointer to a string that contains the name of the specified compression algorithm or NULL.
session: is a
gnutls_session_tstructure.Returns the currently used compression method.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_compression_method_t elements.
Sets the priority on the compression algorithms supported by gnutls. Priority is higher for algorithms specified before others. After specifying the algorithms you want, you must append a 0. Note that the priority is set on the client. The server does not use the algorithm's priority except for disabling algorithms that were not specified.
TLS 1.0 does not define any compression algorithms except NULL. Other compression algorithms are to be considered as gnutls extensions.
session: is a
gnutls_session_tstructure.Clears all the credentials previously set in this session.
session: is a
gnutls_session_tstructure.type: is the type of the credentials
cred: is a pointer to a structure.
Sets the needed credentials for the specified type. Eg username, password - or public and private keys etc. The (void* cred) parameter is a structure that depends on the specified type and on the current session (client or server). [ In order to minimize memory usage, and share credentials between several threads gnutls keeps a pointer to cred, and not the whole cred structure. Thus you will have to keep the structure allocated until you call
gnutls_deinit(). ]For GNUTLS_CRD_ANON cred should be gnutls_anon_client_credentials_t in case of a client. In case of a server it should be gnutls_anon_server_credentials_t.
For GNUTLS_CRD_SRP cred should be gnutls_srp_client_credentials_t in case of a client, and gnutls_srp_server_credentials_t, in case of a server.
For GNUTLS_CRD_CERTIFICATE cred should be gnutls_certificate_credentials_t.
session: is a
gnutls_session_tstructure.session_entry: is the session data (not key)
This function returns GNUTLS_E_EXPIRED, if the database entry has expired or 0 otherwise. This function is to be used when you want to clear unnesessary session which occupy space in your backend.
session: is a
gnutls_session_tstructure.Returns the pointer that will be sent to db store, retrieve and delete functions, as the first argument.
session: is a
gnutls_session_tstructure.This function will remove the current session data from the session database. This will prevent future handshakes reusing these session data. This function should be called if a session was terminated abnormally, and before
gnutls_deinit()is called.Normally
gnutls_deinit()will remove abnormally terminated sessions.
session: is a
gnutls_session_tstructure.seconds: is the number of seconds.
Sets the expiration time for resumed sessions. The default is 3600 (one hour) at the time writing this.
session: is a
gnutls_session_tstructure.ptr: is the pointer
Sets the pointer that will be provided to db store, retrieve and delete functions, as the first argument.
session: is a
gnutls_session_tstructure.rem_func: is the function.
Sets the function that will be used to remove data from the resumed sessions database. This function must return 0 on success.
The first argument to
rem_function()will be null unlessgnutls_db_set_ptr()has been called.
session: is a
gnutls_session_tstructure.retr_func: is the function.
Sets the function that will be used to retrieve data from the resumed sessions database. This function must return a gnutls_datum_t containing the data on success, or a gnutls_datum_t containing null and 0 on failure.
The datum's data must be allocated using the function
gnutls_malloc().The first argument to
store_function()will be null unlessgnutls_db_set_ptr()has been called.
session: is a
gnutls_session_tstructure.store_func: is the function
Sets the function that will be used to store data from the resumed sessions database. This function must remove 0 on success.
The first argument to
store_function()will be null unlessgnutls_db_set_ptr()has been called.
session: is a
gnutls_session_tstructure.This function clears all buffers associated with the
session. This function will also remove session data from the session database if the session was terminated abnormally.
session: is a gnutls session
raw_gen: will hold the generator.
raw_prime: will hold the prime.
This function will return the group parameters used in the last Diffie Hellman authentication with the peer. These are the prime and the generator used. This function should be used for both anonymous and ephemeral diffie Hellman. The output parameters must be freed with
gnutls_free().Returns a negative value in case of an error.
session: is a gnutls session
This function will return the bits used in the last Diffie Hellman authentication with the peer. Should be used for both anonymous and ephemeral diffie Hellman. Returns a negative value in case of an error.
session: is a gnutls session
This function will return the bits of the prime used in the last Diffie Hellman authentication with the peer. Should be used for both anonymous and ephemeral diffie Hellman. Returns a negative value in case of an error.
session: is a gnutls session
raw_key: will hold the public key.
This function will return the peer's public key used in the last Diffie Hellman authentication. This function should be used for both anonymous and ephemeral diffie Hellman. The output parameters must be freed with
gnutls_free().Returns a negative value in case of an error.
session: is a gnutls session
This function will return the bits used in the last Diffie Hellman authentication with the peer. Should be used for both anonymous and ephemeral diffie Hellman. Returns a negative value in case of an error.
dst: Is the destination structure, which should be initialized.
src: Is the source structure
This function will copy the DH parameters structure from source to destination.
dh_params: Is a structure that holds the prime numbers
This function will deinitialize the DH parameters structure.
params: Holds the DH parameters
format: the format of output params. One of PEM or DER.
params_data: will contain a PKCS3 DHParams structure PEM or DER encoded
params_data_size: holds the size of params_data (and will be replaced by the actual size of parameters)
This function will export the given dh parameters to a PKCS3 DHParams structure. This is the format generated by "openssl dhparam" tool. If the buffer provided is not long enough to hold the output, then GNUTLS_E_SHORT_MEMORY_BUFFER will be returned.
If the structure is PEM encoded, it will have a header of "BEGIN DH PARAMETERS".
In case of failure a negative value will be returned, and 0 on success.
params: Holds the DH parameters
prime: will hold the new prime
generator: will hold the new generator
bits: if non null will hold is the prime's number of bits
This function will export the pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters will be allocated using
gnutls_malloc()and will be stored in the appropriate datum.
params: Is the structure that the DH parameters will be stored
bits: is the prime's number of bits
This function will generate a new pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters will be allocated using
gnutls_malloc()and will be stored in the appropriate datum. This function is normally slow.Note that the bits value should be one of 768, 1024, 2048, 3072 or 4096. Also note that the DH parameters are only useful to servers. Since clients use the parameters sent by the server, it's of no use to call this in client side.
params: A structure where the parameters will be copied to
pkcs3_params: should contain a PKCS3 DHParams structure PEM or DER encoded
format: the format of params. PEM or DER.
This function will extract the DHParams found in a PKCS3 formatted structure. This is the format generated by "openssl dhparam" tool.
If the structure is PEM encoded, it should have a header of "BEGIN DH PARAMETERS".
In case of failure a negative value will be returned, and 0 on success.
dh_params: Is a structure that will hold the prime numbers
prime: holds the new prime
generator: holds the new generator
This function will replace the pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters should be stored in the appropriate gnutls_datum.
dh_params: Is a structure that will hold the prime numbers
This function will initialize the DH parameters structure.
session: is a
gnutls_session_tstructure.bits: is the number of bits
This function sets the number of bits, for use in an Diffie Hellman key exchange. This is used both in DH ephemeral and DH anonymous cipher suites. This will set the minimum size of the prime that will be used for the handshake.
In the client side it sets the minimum accepted number of bits. If a server sends a prime with less bits than that GNUTLS_E_DH_PRIME_UNACCEPTABLE will be returned by the handshake.
error: is an error returned by a gnutls function. Error should be a negative value.
If a function returns a negative value you may feed that value to this function to see if it is fatal. Returns 1 for a fatal error 0 otherwise. However you may want to check the error code manually, since some non-fatal errors to the protocol may be fatal for you (your program).
This is only useful if you are dealing with errors from the record layer or the handshake layer.
err: is a negative integer
level: the alert level will be stored there
Returns an alert depending on the error code returned by a gnutls function. All alerts sent by this function should be considered fatal. The only exception is when err == GNUTLS_E_REHANDSHAKE, where a warning alert should be sent to the peer indicating that no renegotiation will be performed.
If the return value is GNUTLS_E_INVALID_REQUEST, then there was no mapping to an alert.
algo: is a digest algorithm
data: is the data
result: is the place where the result will be copied (may be null).
result_size: should hold the size of the result. The actual size of the returned result will also be copied there.
This function will calculate a fingerprint (actually a hash), of the given data. The result is not printable data. You should convert it to hex, or to something else printable.
This is the usual way to calculate a fingerprint of an X.509 DER encoded certificate. Note however that the fingerprint of an OpenPGP is not just a hash and cannot be calculated with this function.
Returns a negative value in case of an error.
This function will free data pointed by ptr.
The deallocation function used is the one set by
gnutls_global_set_mem_functions().
This function deinitializes the global data, that were initialized using
gnutls_global_init().
This function initializes the global data to defaults. Every gnutls application has a global data which holds common parameters shared by gnutls session structures. You must call
gnutls_global_deinit()when gnutls usage is no longer needed Returns zero on success.Note that this function will also initialize libgcrypt, if it has not been initialized before. Thus if you want to manually initialize libgcrypt you must do it before calling this function. This is useful in cases you want to disable libgcrypt's internal lockings etc.
log_func: it's a log function
This is the function where you set the logging function gnutls is going to use. This function only accepts a character array. Normally you may not use this function since it is only used for debugging purposes.
gnutls_log_func is of the form, void (*gnutls_log_func)( int level, const char*);
level: it's an integer from 0 to 9.
This is the function that allows you to set the log level. The level is an integer between 0 and 9. Higher values mean more verbosity. The default value is 0. Larger values should only be used with care, since they may reveal sensitive information.
Use a log level over 10 to enable all debugging options.
alloc_func: it's the default memory allocation function. Like
malloc().secure_alloc_func: This is the memory allocation function that will be used for sensitive data.
is_secure_func: a function that returns 0 if the memory given is not secure. May be NULL.
realloc_func: A realloc function
free_func: The function that frees allocated data. Must accept a NULL pointer.
This is the function were you set the memory allocation functions gnutls is going to use. By default the libc's allocation functions (
malloc(),free()), are used by gnutls, to allocate both sensitive and not sensitive data. This function is provided to set the memory allocation functions to something other than the defaults (ie the gcrypt allocation functions).This function must be called before
gnutls_global_init()is called.
session: is a
gnutls_session_tstructure.Returns the last handshake message received. This function is only useful to check where the last performed handshake failed. If the previous handshake succeed or was not performed at all then no meaningful value will be returned.
Check gnutls.h for the available handshake descriptions.
session: is a
gnutls_session_tstructure.Returns the last handshake message sent. This function is only useful to check where the last performed handshake failed. If the previous handshake succeed or was not performed at all then no meaningful value will be returned.
Check gnutls.h for the available handshake descriptions.
session: is a
gnutls_session_tstructure.max: is the maximum number.
This function will set the maximum size of a handshake message. Handshake messages over this size are rejected. The default value is 16kb which is large enough. Set this to 0 if you do not want to set an upper limit.
session: is a
gnutls_session_tstructure.allow: is an integer (0 or 1)
This function will enable or disable the use of private cipher suites (the ones that start with 0xFF). By default or if
allowis 0 then these cipher suites will not be advertized nor used.Unless this function is called with the option to allow (1), then no compression algorithms, like LZO. That is because these algorithms are not yet defined in any RFC or even internet draft.
Enabling the private ciphersuites when talking to other than gnutls servers and clients may cause interoperability problems.
session: is a
gnutls_session_tstructure.This function does the handshake of the TLS/SSL protocol, and initializes the TLS connection.
This function will fail if any problem is encountered, and will return a negative error code. In case of a client, if the client has asked to resume a session, but the server couldn't, then a full handshake will be performed.
The non-fatal errors such as GNUTLS_E_AGAIN and GNUTLS_E_INTERRUPTED interrupt the handshake procedure, which should be later be resumed. Call this function again, until it returns 0; cf.
gnutls_record_get_direction()andgnutls_error_is_fatal().If this function is called by a server after a rehandshake request then GNUTLS_E_GOT_APPLICATION_DATA or GNUTLS_E_WARNING_ALERT_RECEIVED may be returned. Note that these are non fatal errors, only in the specific case of a rehandshake. Their meaning is that the client rejected the rehandshake request.
session: is a pointer to a
gnutls_session_tstructure.con_end: is used to indicate if this session is to be used for server or client. Can be one of GNUTLS_CLIENT and GNUTLS_SERVER.
This function initializes the current session to null. Every session must be initialized before use, so internal structures can be allocated. This function allocates structures which can only be free'd by calling
gnutls_deinit(). Returns zero on success.
algorithm: is a key exchange algorithm
Returns a pointer to a string that contains the name of the specified key exchange algorithm or NULL.
session: is a
gnutls_session_tstructure.Returns the key exchange algorithm used in the last handshake.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_kx_algorithm_t elements.
Sets the priority on the key exchange algorithms supported by gnutls. Priority is higher for algorithms specified before others. After specifying the algorithms you want, you must append a 0. Note that the priority is set on the client. The server does not use the algorithm's priority except for disabling algorithms that were not specified.
algorithm: is a MAC algorithm
Returns a string that contains the name of the specified MAC algorithm or NULL.
session: is a
gnutls_session_tstructure.Returns the currently used mac algorithm.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_mac_algorithm_t elements.
Sets the priority on the mac algorithms supported by gnutls. Priority is higher for algorithms specified before others. After specifying the algorithms you want, you must append a 0. Note that the priority is set on the client. The server does not use the algorithm's priority except for disabling algorithms that were not specified.
This function will allocate 's' bytes data, and return a pointer to memory. This function is supposed to be used by callbacks.
The allocation function used is the one set by
gnutls_global_set_mem_functions().
session: is a pointer to a
gnutls_session_tstructure.status: is one of OPENPGP_KEY, or OPENPGP_KEY_FINGERPRINT
This function will order gnutls to send the key fingerprint instead of the key in the initial handshake procedure. This should be used with care and only when there is indication or knowledge that the server can obtain the client's key.
header: The PEM header (eg. CERTIFICATE)
b64_data: contains the encoded data
result: the place where decoded data lie
This function will decode the given encoded data. The decoded data will be allocated, and stored into result. If the header given is non null this function will search for "——BEGIN header" and decode only this part. Otherwise it will decode the first PEM packet found.
You should use
gnutls_free()to free the returned data.
header: A null terminated string with the PEM header (eg. CERTIFICATE)
b64_data: contain the encoded data
result: the place where decoded data will be copied
result_size: holds the size of the result
This function will decode the given encoded data. If the header given is non null this function will search for "——BEGIN header" and decode only this part. Otherwise it will decode the first PEM packet found.
Returns GNUTLS_E_SHORT_MEMORY_BUFFER if the buffer given is not long enough, or 0 on success.
msg: is a message to be put in the encoded header
data: contains the raw data
result: will hold the newly allocated encoded data
This function will convert the given data to printable data, using the base64 encoding. This is the encoding used in PEM messages. This function will allocate the required memory to hold the encoded data.
You should use
gnutls_free()to free the returned data.
msg: is a message to be put in the header
data: contain the raw data
result: the place where base64 data will be copied
result_size: holds the size of the result
This function will convert the given data to printable data, using the base64 encoding. This is the encoding used in PEM messages. If the provided buffer is not long enough GNUTLS_E_SHORT_MEMORY_BUFFER is returned.
The output string will be null terminated, although the size will not include the terminating null.
error: is an error returned by a gnutls function. Error is always a negative value.
This function is like
perror(). The only difference is that it accepts an error number returned by a gnutls function.
algorithm: is a pk algorithm
Returns a string that contains the name of the specified public key algorithm or NULL.
version: is a (gnutls) version number
Returns a string that contains the name of the specified TLS version or NULL.
session: is a
gnutls_session_tstructure.Returns the version of the currently used protocol.
session: is a
gnutls_session_tstructure.list: is a 0 terminated list of gnutls_protocol_t elements.
Sets the priority on the protocol versions supported by gnutls. This function actually enables or disables protocols. Newer protocol versions always have highest priority.
session: is a
gnutls_session_tstructure.This function checks if there are any data to receive in the gnutls buffers. Returns the size of that data or 0. Notice that you may also use
select()to check for data in a TCP connection, instead of this function. (gnutls leaves some data in the tcp buffer in order for select to work).
session: is a
gnutls_session_tstructure.This function provides information about the internals of the record protocol and is only useful if a prior gnutls function call (e.g.
gnutls_handshake()) was interrupted for some reason, that is, if a function returned GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN. In such a case, you might want to callselect()orpoll()before calling the interrupted gnutls function again. To tell you whether a file descriptor should be selected for either reading or writing,gnutls_record_get_direction()returns 0 if the interrupted function was trying to read data, and 1 if it was trying to write data.
session: is a
gnutls_session_tstructure.This function returns the maximum record packet size in this connection. The maximum record size is negotiated by the client after the first handshake message.
session: is a
gnutls_session_tstructure.data: contains the data to send
sizeofdata: is the length of the data
This function has the similar semantics to
send(). The only difference is that it accepts a GNUTLS session.If the server requests a renegotiation, the client may receive an error code of GNUTLS_E_REHANDSHAKE. This message may be simply ignored, replied with an alert containing NO_RENEGOTIATION, or replied with a new handshake.
A server may also receive GNUTLS_E_REHANDSHAKE when a client has initiated a handshake. In that case the server can only initiate a handshake or terminate the connection.
Returns the number of bytes received and zero on EOF. A negative error code is returned in case of an error.
session: is a
gnutls_session_tstructure.data: contains the data to send
sizeofdata: is the length of the data
This function has the similar semantics with
recv(). The only difference is that is accepts a GNUTLS session, and uses different error codes.If the EINTR is returned by the internal push function (the default is
recv()) then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again, with the same parameters; cf.gnutls_record_get_direction(). Alternatively you could provide a NULL pointer for data, and 0 for size. Otherwise the write operation will be corrupted and the connection will be terminated.Returns the number of bytes sent, or a negative error code. The number of bytes sent might be less than
sizeofdata. The maximum number of bytes this function can send in a single call depends on the negotiated maximum record size.
session: is a
gnutls_session_tstructure.size: is the new size
This function sets the maximum record packet size in this connection. This property can only be set to clients. The server may choose not to accept the requested size.
Acceptable values are 512(=2^9), 1024(=2^10), 2048(=2^11) and 4096(=2^12). Returns 0 on success. The requested record size does get in effect immediately only while sending data. The receive part will take effect after a successful handshake.
This function uses a TLS extension called 'max record size'. Not all TLS implementations use or even understand this extension.
session: is a
gnutls_session_tstructure.This function will renegotiate security parameters with the client. This should only be called in case of a server.
This message informs the peer that we want to renegotiate parameters (perform a handshake).
If this function succeeds (returns 0), you must call the
gnutls_handshake()function in order to negotiate the new parameters.If the client does not wish to renegotiate parameters he will should with an alert message, thus the return code will be GNUTLS_E_WARNING_ALERT_RECEIVED and the alert will be GNUTLS_A_NO_RENEGOTIATION. A client may also choose to ignore this message.
session: is a gnutls session
This function will return the bits used in the last RSA-EXPORT key exchange with the peer. Returns a negative value in case of an error.
session: is a gnutls session
exp: will hold the exponent.
mod: will hold the modulus.
This function will return the peer's public key exponent and modulus used in the last RSA-EXPORT authentication. The output parameters must be freed with
gnutls_free().Returns a negative value in case of an error.
dst: Is the destination structure, which should be initialized.
src: Is the source structure
This function will copy the RSA parameters structure from source to destination.
rsa_params: Is a structure that holds the parameters
This function will deinitialize the RSA parameters structure.
params: Holds the RSA parameters
format: the format of output params. One of PEM or DER.
params_data: will contain a PKCS1 RSAPublicKey structure PEM or DER encoded
params_data_size: holds the size of params_data (and will be replaced by the actual size of parameters)
This function will export the given RSA parameters to a PKCS1 RSAPublicKey structure. If the buffer provided is not long enough to hold the output, then GNUTLS_E_SHORT_MEMORY_BUFFER will be returned.
If the structure is PEM encoded, it will have a header of "BEGIN RSA PRIVATE KEY".
In case of failure a negative value will be returned, and 0 on success.
params: a structure that holds the rsa parameters
m: will hold the modulus
e: will hold the public exponent
d: will hold the private exponent
p: will hold the first prime (p)
q: will hold the second prime (q)
u: will hold the coefficient
bits: if non null will hold the prime's number of bits
This function will export the RSA parameters found in the given structure. The new parameters will be allocated using
gnutls_malloc()and will be stored in the appropriate datum.
params: The structure where the parameters will be stored
bits: is the prime's number of bits
This function will generate new temporary RSA parameters for use in RSA-EXPORT ciphersuites. This function is normally slow.
Note that if the parameters are to be used in export cipher suites the bits value should be 512 or less. Also note that the generation of new RSA parameters is only useful to servers. Clients use the parameters sent by the server, thus it's no use calling this in client side.
params: A structure where the parameters will be copied to
pkcs1_params: should contain a PKCS1 RSAPublicKey structure PEM or DER encoded
format: the format of params. PEM or DER.
This function will extract the RSAPublicKey found in a PKCS1 formatted structure.
If the structure is PEM encoded, it should have a header of "BEGIN RSA PRIVATE KEY".
In case of failure a negative value will be returned, and 0 on success.
rsa_params: Is a structure will hold the parameters
m: holds the modulus
e: holds the public exponent
d: holds the private exponent
p: holds the first prime (p)
q: holds the second prime (q)
u: holds the coefficient
This function will replace the parameters in the given structure. The new parameters should be stored in the appropriate gnutls_datum.
rsa_params: Is a structure that will hold the parameters
This function will initialize the temporary RSA parameters structure.
session: is a
gnutls_session_tstructure.data: will hold the data
data_length: will hold the data length. Must hold the maximum size of data.
type: will hold the server name indicator type
indx: is the index of the server_name
This function will allow you to get the name indication (if any), a client has sent. The name indication may be any of the enumeration gnutls_server_name_type_t.
If
typeis GNUTLS_NAME_DNS, then this function is to be used by servers that support virtual hosting, and the data will be a null terminated UTF-8 string.If
datahas not enough size to hold the server name GNUTLS_E_SHORT_MEMORY_BUFFER is returned, anddata_lengthwill hold the required size.
indexis used to retrieve more than one server names (if sent by the client). The first server name has an index of 0, the second 1 and so on. If no name with the given index exists GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE is returned.
session: is a
gnutls_session_tstructure.type: specifies the indicator type
name: is a string that contains the server name.
name_length: holds the length of name
This function is to be used by clients that want to inform (via a TLS extension mechanism) the server of the name they connected to. This should be used by clients that connect to servers that do virtual hosting.
The value of
namedepends on theindtype. In case of GNUTLS_NAME_DNS, an ASCII or UTF-8 null terminated string, without the trailing dot, is expected. IPv4 or IPv6 addresses are not permitted.
session: is a
gnutls_session_tstructure.session_data: is a pointer to space to hold the session.
session_data_size: is the session_data's size, or it will be set by the function.
Returns all session parameters, in order to support resuming. The client should call this, and keep the returned session, if he wants to resume that current version later by calling
gnutls_session_set_data()This function must be called after a successful handshake.Resuming sessions is really useful and speedups connections after a succesful one.
session: is a
gnutls_session_tstructure.session_id: is a pointer to space to hold the session id.
session_id_size: is the session id's size, or it will be set by the function.
Returns the current session id. This can be used if you want to check if the next session you tried to resume was actually resumed. This is because resumed sessions have the same sessionID with the original session.
Session id is some data set by the server, that identify the current session. In TLS 1.0 and SSL 3.0 session id is always less than 32 bytes.
session: is a
gnutls_session_tstructure.This function will return the user given pointer from the session structure. This is the pointer set with
gnutls_session_set_ptr().
session: is a
gnutls_session_tstructure.This function will return non zero if this session is a resumed one, or a zero if this is a new session.
session: is a
gnutls_session_tstructure.session_data: is a pointer to space to hold the session.
session_data_size: is the session's size
Sets all session parameters, in order to resume a previously established session. The session data given must be the one returned by
gnutls_session_get_data(). This function should be called beforegnutls_handshake().Keep in mind that session resuming is advisory. The server may choose not to resume the session, thus a full handshake will be performed.
Returns a negative value on error.
session: is a
gnutls_session_tstructure.ptr: is the user pointer
This function will set (associate) the user given pointer to the session structure. This is pointer can be accessed with
gnutls_session_get_ptr().
session: is a
gnutls_session_tstructure.Sets some default priority on the ciphers, key exchange methods, macs and compression methods. This is to avoid using the gnutls_*
_priority()functions, if these defaults are ok. This function also includes weak algorithms. The order is TLS1, SSL3 for protocols, RSA, DHE_DSS, DHE_RSA, RSA_EXPORT for key exchange algorithms. SHA, MD5, RIPEMD160 for MAC algorithms, AES_256_CBC, AES_128_CBC, and 3DES_CBC, ARCFOUR_128, ARCFOUR_40 for ciphers.
session: is a
gnutls_session_tstructure.Sets some default priority on the ciphers, key exchange methods, macs and compression methods. This is to avoid using the gnutls_*
_priority()functions, if these defaults are ok. You may override any of the following priorities by calling the appropriate functions.The order is TLS1, SSL3 for protocols. RSA, DHE_DSS, DHE_RSA for key exchange algorithms. SHA, MD5 and RIPEMD160 for MAC algorithms. AES_256_CBC, AES_128_CBC, 3DES_CBC, and ARCFOUR_128 for ciphers.
algorithm: is a sign algorithm
Returns a string that contains the name of the specified sign algorithm or NULL.
error: is an error returned by a gnutls function. Error is always a negative value.
This function is similar to
strerror(). Differences: it accepts an error number returned by a gnutls function; In case of an unknown error a descriptive string is sent instead of NULL.
session: is a
gnutls_session_tstructure.recv_ptr: will hold the value for the pull function
send_ptr: will hold the value for the push function
Used to get the arguments of the transport functions (like PUSH and PULL). These should have been set using
gnutls_transport_set_ptr2().
session: is a
gnutls_session_tstructure.Used to get the first argument of the transport function (like PUSH and PULL). This must have been set using
gnutls_transport_set_ptr().
session: is a
gnutls_session_tstructure.num: is the low water value.
Used to set the lowat value in order for select to check if there are pending data to socket buffer. Used only if you have changed the default low water value (default is 1). Normally you will not need that function. This function is only useful if using berkeley style sockets. Otherwise it must be called and set lowat to zero.
session: is a
gnutls_session_tstructure.recv_ptr: is the value for the pull function
send_ptr: is the value for the push function
Used to set the first argument of the transport function (like PUSH and PULL). In berkeley style sockets this function will set the connection handle. With this function you can use two different pointers for receiving and sending.
session: is a
gnutls_session_tstructure.ptr: is the value.
Used to set the first argument of the transport function (like PUSH and PULL). In berkeley style sockets this function will set the connection handle.
session: gnutls session
pull_func: it's a function like read
This is the function where you set a function for gnutls to receive data. Normally, if you use berkeley style sockets, you may not use this function since the default (recv(2)) will probably be ok. This function should be called once and after
gnutls_global_init(). PULL_FUNC is of the form, ssize_t (*gnutls_pull_func)(gnutls_transport_ptr_t, const void*, size_t);
session: gnutls session
push_func: it's a function like write
This is the function where you set a push function for gnutls to use in order to send data. If you are going to use berkeley style sockets, you may not use this function since the default (send(2)) will probably be ok. Otherwise you should specify this function for gnutls to be able to send data.
This function should be called once and after
gnutls_global_init(). PUSH_FUNC is of the form, ssize_t (*gnutls_push_func)(gnutls_transport_ptr_t, const void*, size_t);
The following functions are to be used for X.509 certificate handling. Their prototypes lie in gnutls/x509.h.
These functions are only available in the GPL version of the library
called gnutls-extra. The prototypes for this library lie in
gnutls/extra.h.
The following functions are to be used for OpenPGP certificate handling. Their prototypes lie in gnutls/openpgp.h.
res: the destination context to save the data.
certfile: the file that contains the public key.
keyfile: the file that contains the secret key.
This funtion is used to load OpenPGP keys into the GnuTLS credentials structure. It doesn't matter whether the keys are armored or but, but the files should only contain one key which should not be encrypted.
res: the destination context to save the data.
cert: the datum that contains the public key.
key: the datum that contains the secret key.
This funtion is used to load OpenPGP keys into the GnuTLS credential structure. It doesn't matter whether the keys are armored or but, but the files should only contain one key which should not be encrypted.
c: A certificate credentials structure
file: filename of the keyring.
The function is used to set keyrings that will be used internally by various OpenPGP functions. For example to find a key when it is needed for an operations. The keyring will also be used at the verification functions.
c: A certificate credentials structure
data: buffer with keyring data.
dlen: length of data buffer.
The function is used to set keyrings that will be used internally by various OpenPGP functions. For example to find a key when it is needed for an operations. The keyring will also be used at the verification functions.
res: the destination context to save the data.
keyserver: is the key server address
port: is the key server port to connect to
This funtion will set a key server for use with openpgp keys. This key server will only be used if the peer sends a key fingerprint instead of a key in the handshake. Using a key server may delay the handshake process.
res: is an
gnutls_certificate_credentials_tstructure.key: contains an openpgp public key
pkey: is an openpgp private key
This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).
res: the destination context to save the data.
trustdb: is the trustdb filename
This funtion will set a GnuPG trustdb which will be used in key verification functions. Only version 3 trustdb files are supported.
key: should contain an gnutls_openpgp_key_t structure
hostname: A null terminated string that contains a DNS name
This function will check if the given key's owner matches the given hostname. This is a basic implementation of the matching described in RFC2818 (HTTPS), which takes into account wildcards.
Returns non zero on success, and zero on failure.
key: The structure to be initialized
This function will deinitialize a key structure.
key: Holds the key.
format: One of gnutls_openpgp_key_fmt_t elements.
output_data: will contain the key base64 encoded or raw
output_data_size: holds the size of output_data (and will be replaced by the actual size of parameters)
This function will convert the given key to RAW or Base64 format. If the buffer provided is not long enough to hold the output, then GNUTLS_E_SHORT_MEMORY_BUFFER will be returned.
Returns 0 on success.
key: the structure that contains the OpenPGP public key.
Returns the timestamp when the OpenPGP key was created.
key: the structure that contains the OpenPGP public key.
Returns the time when the OpenPGP key expires. A value of '0' means that the key doesn't expire at all.
key: the raw data that contains the OpenPGP public key.
fpr: the buffer to save the fingerprint.
fprlen: the integer to save the length of the fingerprint.
Returns the fingerprint of the OpenPGP key. Depends on the algorithm, the fingerprint can be 16 or 20 bytes.
key: the structure that contains the OpenPGP public key.
Returns the 64-bit keyID of the OpenPGP key.
key: should contain a gnutls_openpgp_key_t structure
key_usage: where the key usage bits will be stored
This function will return certificate's key usage, by checking the key algorithm. The key usage value will ORed values of the: GNUTLS_KEY_DIGITAL_SIGNATURE, GNUTLS_KEY_KEY_ENCIPHERMENT.
A negative value may be returned in case of parsing error.
key: the structure that contains the OpenPGP public key.
idx: the index of the ID to extract
buf: a pointer to a structure to hold the name
sizeof_buf: holds the size of 'buf'
Extracts the userID from the parsed OpenPGP key.
Returns 0 on success, and GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE if the index of the ID does not exist.
key: is an OpenPGP key
bits: if bits is non null it will hold the size of the parameters' in bits
This function will return the public key algorithm of an OpenPGP certificate.
If bits is non null, it should have enough size to hold the parameters size in bits. For RSA the bits returned is the modulus. For DSA the bits returned are of the public exponent.
Returns a member of the GNUTLS_PKAlgorithm enumeration on success, or a negative value on error.
key: the structure that contains the OpenPGP public key.
Extract the version of the OpenPGP key.
key: The structure to store the parsed key.
data: The RAW or BASE64 encoded key.
format: One of gnutls_openpgp_key_fmt_t elements.
This function will convert the given RAW or Base64 encoded key to the native gnutls_openpgp_key_t format. The output will be stored in 'key'.
Returns 0 on success.
key: The structure to be initialized
This function will initialize an OpenPGP key structure.
Returns 0 on success.
xmlkey: he datum struct to store the XML result.
ext: extension mode (1/0), 1 means include key signatures and key data.
This function will return the all OpenPGP key information encapsulated as a XML string.
key: the structure that holds the key.
keyring: holds the keyring to check against
flags: unused (should be 0)
verify: will hold the certificate verification output.
Verify all signatures in the key, using the given set of keys (keyring).
The key verification output will be put in
verifyand will be one or more of the gnutls_certificate_status_t enumerated elements bitwise or'd.GNUTLS_CERT_INVALID\: A signature on the key is invalid.
GNUTLS_CERT_REVOKED\: The key has been revoked.
NOTE: this function does not verify using any "web of trust". You may use GnuPG for that purpose, or any other external PGP application.
Returns 0 on success.
key: the structure that holds the key.
flags: unused (should be 0)
verify: will hold the key verification output.
Verifies the self signature in the key. The key verification output will be put in
verifyand will be one or more of the gnutls_certificate_status_t enumerated elements bitwise or'd.GNUTLS_CERT_INVALID\: The self signature on the key is invalid.
Returns 0 on success.
key: the structure that holds the key.
trustdb: holds the trustdb to check against
flags: unused (should be 0)
verify: will hold the certificate verification output.
Checks if the key is revoked or disabled, in the trustdb. The verification output will be put in
verifyand will be one or more of the gnutls_certificate_status_t enumerated elements bitwise or'd.GNUTLS_CERT_INVALID\: A signature on the key is invalid.
GNUTLS_CERT_REVOKED\: The key has been revoked.
NOTE: this function does not verify using any "web of trust". You may use GnuPG for that purpose, or any other external PGP application.
Returns 0 on success.
ring: holds the keyring to check against
flags: unused (should be 0)
Check if a given key ID exists in the keyring.
Returns 0 on success (if keyid exists) and a negative error code on failure.
keyring: The structure to be initialized
This function will deinitialize a CRL structure.
keyring: The structure to store the parsed key.
data: The RAW or BASE64 encoded keyring.
format: One of gnutls_openpgp_keyring_fmt elements.
This function will convert the given RAW or Base64 encoded keyring to the native gnutls_openpgp_keyring_t format. The output will be stored in 'keyring'.
Returns 0 on success.
keyring: The structure to be initialized
This function will initialize an OpenPGP keyring structure.
Returns 0 on success.
key: The structure to be initialized
This function will deinitialize a key structure.
key: is an OpenPGP key
bits: if bits is non null it will hold the size of the parameters' in bits
This function will return the public key algorithm of an OpenPGP certificate.
If bits is non null, it should have enough size to hold the parameters size in bits. For RSA the bits returned is the modulus. For DSA the bits returned are of the public exponent.
Returns a member of the GNUTLS_PKAlgorithm enumeration on success, or a negative value on error.
key: The structure to store the parsed key.
data: The RAW or BASE64 encoded key.
format: One of gnutls_openpgp_key_fmt_t elements.
pass: Unused for now
flags: should be zero
This function will convert the given RAW or Base64 encoded key to the native gnutls_openpgp_privkey_t format. The output will be stored in 'key'.
Returns 0 on success.
key: The structure to be initialized
This function will initialize an OpenPGP key structure.
Returns 0 on success.
session: a TLS session
func: the callback
This funtion will set a key retrieval function for OpenPGP keys. This callback is only useful in server side, and will be used if the peer sent a key fingerprint instead of a full key.
trustdb: The structure to be initialized
This function will deinitialize a CRL structure.
trustdb: The structure to store the parsed key.
file: The file that holds the trustdb.
This function will convert the given RAW or Base64 encoded trustdb to the native gnutls_openpgp_trustdb_t format. The output will be stored in 'trustdb'.
Returns 0 on success.
trustdb: The structure to be initialized
This function will initialize an OpenPGP trustdb structure.
Returns 0 on success.
The error codes used throughout the library are described below. The
return code GNUTLS_E_SUCCESS indicate successful operation, and
is guaranteed to have the value 0, so you can use it in logical
expressions.
GNUTLS_E_AGAIN:
Function was interrupted.
GNUTLS_E_ASN1_DER_ERROR:
ASN1 parser: Error in DER parsing.
GNUTLS_E_ASN1_DER_OVERFLOW:
ASN1 parser: Overflow in DER parsing.
GNUTLS_E_ASN1_ELEMENT_NOT_FOUND:
ASN1 parser: Element was not found.
GNUTLS_E_ASN1_GENERIC_ERROR:
ASN1 parser: Generic parsing error.
GNUTLS_E_ASN1_IDENTIFIER_NOT_FOUND:
ASN1 parser: Identifier was not found
GNUTLS_E_ASN1_SYNTAX_ERROR:
ASN1 parser: Syntax error.
GNUTLS_E_ASN1_TAG_ERROR:
ASN1 parser: Error in TAG.
GNUTLS_E_ASN1_TAG_IMPLICIT:
ASN1 parser: error in implicit tag
GNUTLS_E_ASN1_TYPE_ANY_ERROR:
ASN1 parser: Error in type 'ANY'.
GNUTLS_E_ASN1_VALUE_NOT_FOUND:
ASN1 parser: Value was not found.
GNUTLS_E_ASN1_VALUE_NOT_VALID:
ASN1 parser: Value is not valid.
GNUTLS_E_BASE64_DECODING_ERROR:
Base64 decoding error.
GNUTLS_E_BASE64_ENCODING_ERROR:
Base64 encoding error.
GNUTLS_E_CERTIFICATE_ERROR:
Error in the certificate.
GNUTLS_E_CERTIFICATE_KEY_MISMATCH:
The certificate and the given key do not match.
GNUTLS_E_COMPRESSION_FAILED:
Compression of the TLS record packet has failed.
GNUTLS_E_CONSTRAINT_ERROR:
Some constraint limits were reached.
GNUTLS_E_DB_ERROR:
Error in Database backend.
GNUTLS_E_DECOMPRESSION_FAILED:
Decompression of the TLS record packet has failed.
GNUTLS_E_DECRYPTION_FAILED:
Decryption has failed.
GNUTLS_E_DH_PRIME_UNACCEPTABLE:
The Diffie Hellman prime sent by the server is not acceptable (not long enough).
GNUTLS_E_ENCRYPTION_FAILED:
Encryption has failed.
GNUTLS_E_ERROR_IN_FINISHED_PACKET:
An error was encountered at the TLS Finished packet calculation.
GNUTLS_E_EXPIRED:
The requested session has expired.
GNUTLS_E_FATAL_ALERT_RECEIVED:
A TLS fatal alert has been received.
GNUTLS_E_FILE_ERROR:
Error while reading file.
GNUTLS_E_GOT_APPLICATION_DATA:
TLS Application data were received, while expecting handshake data.
GNUTLS_E_HASH_FAILED:
Hashing has failed.
GNUTLS_E_ILLEGAL_SRP_USERNAME:
The SRP username supplied is illegal.
GNUTLS_E_INCOMPATIBLE_GCRYPT_LIBRARY:
The gcrypt library version is too old.
GNUTLS_E_INCOMPATIBLE_LIBTASN1_LIBRARY:
The tasn1 library version is too old.
GNUTLS_E_INIT_LIBEXTRA:
The initialization of GnuTLS-extra has failed.
GNUTLS_E_INSUFFICIENT_CREDENTIALS:
Insufficient credentials for that request.
GNUTLS_E_INTERNAL_ERROR:
GnuTLS internal error.
GNUTLS_E_INTERRUPTED:
Function was interrupted.
GNUTLS_E_INVALID_PASSWORD:
The given password contains invalid characters.
GNUTLS_E_INVALID_REQUEST:
The request is invalid.
GNUTLS_E_INVALID_SESSION:
The specified session has been invalidated for some reason.
GNUTLS_E_KEY_USAGE_VIOLATION:
Key usage violation in certificate has been detected.
GNUTLS_E_LARGE_PACKET:
A large TLS record packet was received.
GNUTLS_E_LIBRARY_VERSION_MISMATCH:
The GnuTLS library version does not match the GnuTLS-extra library version.
GNUTLS_E_LZO_INIT_FAILED:
The initialization of LZO has failed.
GNUTLS_E_MAC_VERIFY_FAILED:
The Message Authentication Code verification failed.
GNUTLS_E_MEMORY_ERROR:
Internal error in memory allocation.
GNUTLS_E_MPI_PRINT_FAILED:
Could not export a large integer.
GNUTLS_E_MPI_SCAN_FAILED:
The scanning of a large integer has failed.
GNUTLS_E_NO_CERTIFICATE_FOUND:
The peer did not send any certificate.
GNUTLS_E_NO_CIPHER_SUITES:
No supported cipher suites have been found.
GNUTLS_E_NO_COMPRESSION_ALGORITHMS:
No supported compression algorithms have been found.
GNUTLS_E_NO_TEMPORARY_DH_PARAMS:
No temporary DH parameters were found.
GNUTLS_E_NO_TEMPORARY_RSA_PARAMS:
No temporary RSA parameters were found.
GNUTLS_E_OPENPGP_FINGERPRINT_UNSUPPORTED:
The OpenPGP fingerprint is not supported.
GNUTLS_E_OPENPGP_GETKEY_FAILED:
Could not get OpenPGP key.
GNUTLS_E_OPENPGP_KEYRING_ERROR:
Error loading the keyring.
GNUTLS_E_OPENPGP_TRUSTDB_VERSION_UNSUPPORTED:
The specified GnuPG TrustDB version is not supported. TrustDB v4 is supported.
GNUTLS_E_PKCS1_WRONG_PAD:
Wrong padding in PKCS1 packet.
GNUTLS_E_PK_DECRYPTION_FAILED:
Public key decryption has failed.
GNUTLS_E_PK_ENCRYPTION_FAILED:
Public key encryption has failed.
GNUTLS_E_PK_SIGN_FAILED:
Public key signing has failed.
GNUTLS_E_PK_SIG_VERIFY_FAILED:
Public key signature verification has failed.
GNUTLS_E_PULL_ERROR:
Error in the pull function.
GNUTLS_E_PUSH_ERROR:
Error in the push function.
GNUTLS_E_RANDOM_FAILED:
Failed to acquire random data.
GNUTLS_E_RECEIVED_ILLEGAL_EXTENSION:
An illegal TLS extension was received.
GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER:
An illegal parameter has been received.
GNUTLS_E_RECORD_LIMIT_REACHED:
The upper limit of record packet sequence numbers has been reached. Wow!
GNUTLS_E_REHANDSHAKE:
Rehandshake was requested by the peer.
GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE:
The requested data were not available.
GNUTLS_E_SHORT_MEMORY_BUFFER:
The given memory buffer is too short to hold parameters.
GNUTLS_E_SRP_PWD_ERROR:
Error in SRP password file.
GNUTLS_E_SRP_PWD_PARSING_ERROR:
Parsing error in SRP password file.
GNUTLS_E_SUCCESS:
Success.
GNUTLS_E_TOO_MANY_EMPTY_PACKETS:
Too many empty record packets have been received.
GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET:
An unexpected TLS handshake packet was received.
GNUTLS_E_UNEXPECTED_PACKET:
An unexpected TLS packet was received.
GNUTLS_E_UNEXPECTED_PACKET_LENGTH:
A TLS packet with unexpected length was received.
GNUTLS_E_UNKNOWN_CIPHER_SUITE:
Could not negotiate a supported cipher suite.
GNUTLS_E_UNKNOWN_CIPHER_TYPE:
The cipher type is unsupported.
GNUTLS_E_UNKNOWN_COMPRESSION_ALGORITHM:
Could not negotiate a supported compression method.
GNUTLS_E_UNKNOWN_HASH_ALGORITHM:
The hash algorithm is unknown.
GNUTLS_E_UNKNOWN_PKCS_BAG_TYPE:
The PKCS structure's bag type is unknown.
GNUTLS_E_UNKNOWN_PKCS_CONTENT_TYPE:
The PKCS structure's content type is unknown.
GNUTLS_E_UNKNOWN_PK_ALGORITHM:
An unknown public key algorithm was encountered.
GNUTLS_E_UNSUPPORTED_CERTIFICATE_TYPE:
The certificate type is not supported.
GNUTLS_E_UNSUPPORTED_VERSION_PACKET:
A record packet with illegal version was received.
GNUTLS_E_UNWANTED_ALGORITHM:
An algorithm that is not enabled was negotiated.
GNUTLS_E_WARNING_ALERT_RECEIVED:
A TLS warning alert has been received.
GNUTLS_E_X509_UNKNOWN_SAN:
Unknown Subject Alternative name in X.509 certificate.
GNUTLS_E_X509_UNSUPPORTED_ATTRIBUTE:
The certificate has unsupported attributes.
GNUTLS_E_X509_UNSUPPORTED_CRITICAL_EXTENSION:
Unsupported critical extension in X.509 certificate.
GNUTLS_E_X509_UNSUPPORTED_OID:
The OID is not supported.
This appendix contains some example output of the XML convertion functions:
gnutls_x509_crt_to_xml
gnutls_openpgp_key_to_xml
<?xml version="1.0" encoding="UTF-8"?>
<gnutls:x509:certificate version="1.1">
<certificate type="SEQUENCE">
<tbsCertificate type="SEQUENCE">
<version type="INTEGER" encoding="HEX">02</version>
<serialNumber type="INTEGER" encoding="HEX">01</serialNumber>
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<parameters type="ANY">
<md5WithRSAEncryption encoding="HEX">0500</md5WithRSAEncryption>
</parameters>
</signature>
<issuer type="CHOICE">
<rdnSequence type="SEQUENCE OF">
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<X520countryName>GR</X520countryName>
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<X520OrganizationalUnitName>GNUTLS dev.</X520OrganizationalUnitName>
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<X520CommonName>GNUTLS TEST CA</X520CommonName>
</value>
</unnamed1>
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</value>
</unnamed1>
</unnamed7>
</rdnSequence>
</issuer>
<validity type="SEQUENCE">
<notBefore type="CHOICE">
<utcTime type="TIME">010707101845Z</utcTime>
</notBefore>
<notAfter type="CHOICE">
<utcTime type="TIME">020707101845Z</utcTime>
</notAfter>
</validity>
<subject type="CHOICE">
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<X520CommonName>localhost</X520CommonName>
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<Pkcs9email>root@localhost</Pkcs9email>
</value>
</unnamed1>
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</rdnSequence>
</subject>
<subjectPublicKeyInfo type="SEQUENCE">
<algorithm type="SEQUENCE">
<algorithm type="OBJECT ID">1.2.840.113549.1.1.1</algorithm>
<parameters type="ANY">
<rsaEncryption encoding="HEX">0500</rsaEncryption>
</parameters>
</algorithm>
<subjectPublicKey type="BIT STRING" encoding="HEX" length="1120">
30818902818100D00B49EBB226D951F5CC57072199DDF287683D2DA1A0E
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1598E21E4BF721748C587F50CF46157D950203010001</subjectPublicKey>
</subjectPublicKeyInfo>
<extensions type="SEQUENCE OF">
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<critical type="BOOLEAN">FALSE</critical>
<extnValue type="SEQUENCE">
<keyIdentifier type="OCTET STRING" encoding="HEX">
EFEE94ABC8CA577F5313DB76DC1A950093BAF3C9</keyIdentifier>
</extnValue>
</unnamed1>
<unnamed2 type="SEQUENCE">
<extnID type="OBJECT ID">2.5.29.37</extnID>
<critical type="BOOLEAN">FALSE</critical>
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<unnamed1 type="OBJECT ID">1.3.6.1.5.5.7.3.1</unnamed1>
<unnamed2 type="OBJECT ID">1.3.6.1.5.5.7.3.2</unnamed2>
<unnamed3 type="OBJECT ID">1.3.6.1.4.1.311.10.3.3</unnamed3>
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</extnValue>
</unnamed2>
<unnamed3 type="SEQUENCE">
<extnID type="OBJECT ID">2.5.29.19</extnID>
<critical type="BOOLEAN">TRUE</critical>
<extnValue type="SEQUENCE">
<cA type="BOOLEAN">FALSE</cA>
</extnValue>
</unnamed3>
</extensions>
</tbsCertificate>
<signatureAlgorithm type="SEQUENCE">
<algorithm type="OBJECT ID">1.2.840.113549.1.1.4</algorithm>
<parameters type="ANY">
<md5WithRSAEncryption encoding="HEX">0500</md5WithRSAEncryption>
</parameters>
</signatureAlgorithm>
<signature type="BIT STRING" encoding="HEX" length="1024">
B73945273AF2A395EC54BF5DC669D953885A9D811A3B92909D24792D36A44EC
27E1C463AF8738BEFD29B311CCE8C6D9661BEC30911DAABB39B8813382B32D2
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23244A8F5997B41A1F44E61F4F22C94375775055A5E72F25D5E4557467A91BD
4251</signature>
</certificate>
</gnutls:x509:certificate>
<?xml version="1.0"?>
<gnutls:openpgp:key version="1.0">
<OPENPGPKEY>
<MAINKEY>
<KEYID>BD572CDCCCC07C3</KEYID>
<FINGERPRINT>BE615E88D6CFF27225B8A2E7BD572CDCCCC07C35</FINGERPRINT>
<PKALGO>DSA</PKALGO>
<KEYLEN>1024</KEYLEN>
<CREATED>1011533164</CREATED>
<REVOKED>0</REVOKED>
<KEY ENCODING="HEX"/>
<DSA-P>0400E72E76B62EEFA9A3BD594093292418050C02D7029D6CA2066E
FC34C86038627C643EB1A652A7AF1D37CF46FC505AC1E0C699B37895B4BCB
3E53541FFDA4766D6168C2B8AAFD6AB22466D06D18034D5DAC698E6993BA5
B350FF822E1CD8702A75114E8B73A6B09CB3B93CE44DBB516C9BB5F95BB66
6188602A0A1447236C0658F</DSA-P>
<DSA-Q>00A08F5B5E78D85F792CC2072F9474645726FB4D9373</DSA-Q>
<DSA-G>03FE3578D689D6606E9118E9F9A7042B963CF23F3D8F1377A273C0
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9EBD7EC76CE432441DF5D52FFFD06D39E5F61E36947B698A77CB62AB81E4A
4122BF9050671D9946C865E</DSA-G>
<DSA-Y>0400D061437A964DDE318818C2B24DE008E60096B60DB8A684B85A
838D119FC930311889AD57A3B927F448F84EB253C623EDA73B42FF78BCE63
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8765A8ECB9971F61A09FC73D509806106B5977D211CB0E1D04D0ED96BCE89
BAE8F73D800B052139CBF8D</DSA-Y>
</MAINKEY>
<USERID>
<NAME>OpenCDK test key (Only intended for test purposes!)</NAME>
<EMAIL>opencdk@foo-bar.org</EMAIL>
<PRIMARY>0</PRIMARY>
<REVOKED>0</REVOKED>
</USERID>
<SIGNATURE>
<VERSION>4</VERSION>
<SIGCLASS>19</SIGCLASS>
<EXPIRED>0</EXPIRED>
<PKALGO>DSA</PKALGO>
<MDALGO>SHA1</MDALGO>
<CREATED>1011533164</CREATED>
<KEYID>BD572CDCCCC07C3</KEYID>
</SIGNATURE>
<SUBKEY>
<KEYID>FCB0CF3A5261E06</KEYID>
<FINGERPRINT>297B48ACC09C0FF683CA1ED1FCB0CF3A5261E067</FINGERPRINT>
<PKALGO>ELG</PKALGO>
<KEYLEN>1024</KEYLEN>
<CREATED>1011533167</CREATED>
<REVOKED>0</REVOKED>
<KEY ENCODING="HEX"/>
<ELG-P>0400E20156526069D067D24F4D71E6D38658E08BE3BF246C1ADCE0
8DB69CD8D459C1ED335738410798755AFDB79F1797CF022E70C7960F12CA6
896D27CFD24A11CD316DDE1FBCC1EA615C5C31FEC656E467078C875FC509B
1ECB99C8B56C2D875C50E2018B5B0FA378606EB6425A2533830F55FD21D64
9015615D49A1D09E9510F5F</ELG-P>
<ELG-G>000305</ELG-G>
<ELG-Y>0400D0BDADE40432758675C87D0730C360981467BAE1BEB6CC105A
3C1F366BFDBEA12E378456513238B8AD414E52A2A9661D1DF1DB6BB5F33F6
906166107556C813224330B30932DB7C8CC8225672D7AE24AF2469750E539
B661EA6475D2E03CD8D3838DC4A8AC4AFD213536FE3E96EC9D0AEA65164B5
76E01B37A8DCA89F2B257D0</ELG-Y>
</SUBKEY>
<SIGNATURE>
<VERSION>4</VERSION>
<SIGCLASS>24</SIGCLASS>
<EXPIRED>0</EXPIRED>
<PKALGO>DSA</PKALGO>
<MDALGO>SHA1</MDALGO>
<CREATED>1011533167</CREATED>
<KEYID>BD572CDCCCC07C3</KEYID>
</SIGNATURE>
</OPENPGPKEY>
</gnutls:openpgp:key>
TLS_RSA_NULL_MD5 (0x00 0x01):
RFC 2246
TLS_ANON_DH_3DES_EDE_CBC_SHA (0x00 0x1B):
RFC 2246
TLS_ANON_DH_ARCFOUR_MD5 (0x00 0x18):
RFC 2246
TLS_ANON_DH_AES_128_CBC_SHA (0x00 0x34):
RFC 2246
TLS_ANON_DH_AES_256_CBC_SHA (0x00 0x3A):
RFC 2246
TLS_RSA_ARCFOUR_SHA (0x00 0x05):
RFC 2246
TLS_RSA_ARCFOUR_MD5 (0x00 0x04):
RFC 2246
TLS_RSA_3DES_EDE_CBC_SHA (0x00 0x0A):
RFC 2246
TLS_RSA_EXPORT_ARCFOUR_40_MD5 (0x00 0x03):
RFC 2246
TLS_DHE_DSS_3DES_EDE_CBC_SHA (0x00 0x13):
RFC 2246
TLS_DHE_RSA_3DES_EDE_CBC_SHA (0x00 0x16):
RFC 2246
TLS_RSA_AES_128_CBC_SHA (0x00 0x2F):
RFC 3268
TLS_RSA_AES_128_CBC_SHA (0x00 0x35):
RFC 3268
TLS_DHE_DSS_AES_256_CBC_SHA (0x00 0x38):
RFC 3268
TLS_DHE_DSS_AES_128_CBC_SHA (0x00 0x32):
RFC 3268
TLS_DHE_RSA_AES_256_CBC_SHA (0x00 0x39):
RFC 3268
TLS_DHE_RSA_AES_128_CBC_SHA (0x00 0x33):
RFC 3268
TLS_SRP_SHA_3DES_EDE_CBC_SHA (0x00 0x50):
draft-ietf-tls-srp
TLS_SRP_SHA_AES_128_CBC_SHA (0x00 0x53):
draft-ietf-tls-srp
TLS_SRP_SHA_AES_256_CBC_SHA (0x00 0x56):
draft-ietf-tls-srp
TLS_SRP_SHA_RSA_3DES_EDE_CBC_SHA (0x00 0x51):
draft-ietf-tls-srp
TLS_SRP_SHA_DSS_3DES_EDE_CBC_SHA (0x00 0x52):
draft-ietf-tls-srp
TLS_SRP_SHA_RSA_AES_128_CBC_SHA (0x00 0x54):
draft-ietf-tls-srp
TLS_SRP_SHA_DSS_AES_128_CBC_SHA (0x00 0x55):
draft-ietf-tls-srp
TLS_SRP_SHA_RSA_AES_256_CBC_SHA (0x00 0x57):
draft-ietf-tls-srp
TLS_SRP_SHA_DSS_AES_256_CBC_SHA (0x00 0x58):
draft-ietf-tls-srp
TLS_DHE_DSS_3DES_EDE_CBC_RMD (0x00 0x72):
draft-ietf-tls-openpgp-keys
TLS_DHE_RSA_3DES_EDE_CBC_RMD (0x00 0x77):
draft-ietf-tls-openpgp-keys
TLS_DHE_DSS_AES_256_CBC_RMD (0x00 0x73):
draft-ietf-tls-openpgp-keys
TLS_DHE_DSS_AES_128_CBC_RMD (0x00 0x74):
draft-ietf-tls-openpgp-keys
TLS_DHE_RSA_AES_128_CBC_RMD (0x00 0x78):
draft-ietf-tls-openpgp-keys
TLS_DHE_RSA_AES_256_CBC_RMD (0x00 0x79):
draft-ietf-tls-openpgp-keys
TLS_RSA_3DES_EDE_CBC_RMD (0x00 0x7C):
draft-ietf-tls-openpgp-keys
TLS_RSA_AES_128_CBC_RMD (0x00 0x7D):
draft-ietf-tls-openpgp-keys
TLS_RSA_AES_256_CBC_RMD (0x00 0x7E):
draft-ietf-tls-openpgp-keys
TLS_DHE_DSS_ARCFOUR_SHA (0x00 0x66):
draft-ietf-tls-56-bit-ciphersuites
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
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[1] Available from http://www.cs.auckland.ac.nz/~pgut001/pubs/pkitutorial.pdf
[2] A copy of the license is included in the distribution
[4] ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/libtasn1/
[5] ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/opencdk/
[6] ftp://ftp.gnupg.org/gcrypt/alpha/libgcrypt/
[7] The first message in a TLS handshake
[8] Described in SSL3
[9] IETF, or Internet Engineering Task Force, is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual.
[10] AES, or Advanced Encryption Standard, is actually the RIJNDAEL algorithm. This is the algorithm that replaced DES.
[11] ARCFOUR_128 is a compatible
algorithm with RSA's RC4 algorithm, which is considered to be a trade
secret.
[12] You should use
gnutls_handshake_set_private_extensions to enable private
extensions.
[13] MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.
[14] http://www.gnupg.org/gph/en/manual.html
[15] It really depends on the group used. Primes with lesser bits are always faster, but also easier to break. Values less than 768 should not be used today
[16] SRP stands for Secure Remote Password, and is described in RFC2945
[17] See also the Server Name Indication extension on serverind.
[18] See LDAP, IMAP etc.
[19] in SRP authentication