@node How to use GnuTLS in applications
@chapter How To Use @acronym{GnuTLS} in Applications
@anchor{examples}
@cindex Example programs

@menu
* Preparation::
* Client examples::
* Server examples::
* Miscellaneous examples::
* Compatibility with the OpenSSL library::
* Keying Material Exporters::
* Channel Bindings::
@end menu

@node Preparation
@section Preparation

To use @acronym{GnuTLS}, you have to perform some changes to your
sources and your build system. The necessary changes are explained in
the following subsections.

@menu
* Headers::
* Initialization::
* Version check::
* Debugging and auditing::
* Building the source::
@end menu

@node Headers
@subsection Headers

All the data types and functions of the @acronym{GnuTLS} library are
defined in the header file @file{gnutls/gnutls.h}.  This must be
included in all programs that make use of the @acronym{GnuTLS}
library.

The extra functionality of the @acronym{GnuTLS-extra} library is
available by including the header file @file{gnutls/extra.h} in your
programs.

@node Initialization
@subsection Initialization

GnuTLS must be initialized before it can be used.  The library is
initialized by calling @ref{gnutls_global_init}.  The resources
allocated by the initialization process can be released if the
application no longer has a need to call GnuTLS functions, this is
done by calling @ref{gnutls_global_deinit}.

The extra functionality of the @acronym{GnuTLS-extra} library is
available after calling @ref{gnutls_global_init_extra}.

In order to take advantage of the internationalisation features in
GnuTLS, such as translated error messages, the application must set
the current locale using @code{setlocale} before initializing GnuTLS.

@node Version check
@subsection Version Check

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 @ref{gnutls_check_version}.

@node Debugging and auditing
@subsection Debugging and auditing

In many cases things may not go as expected and further information,
to assist debugging, from @acronym{GnuTLS} is desired. 
Those are the cases where the @ref{gnutls_global_set_log_level} and
@ref{gnutls_global_set_log_function} are to be used. Those will print
verbose information on the @acronym{GnuTLS} functions internal flow.


When debugging is not required, important issues, such as detected
attacks on the protocol still need to be logged. This is provided
by @ref{gnutls_global_set_audit_log_function}, that uses a logging
function that accepts the detected error message and the corresponding
TLS session. The session information might be used to derive IP addresses
or other information about the peer involved.

@node Building the source
@subsection Building the Source

If you want to compile a source file including the
@file{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 @option{-I}
option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, the library uses the
external package @command{pkg-config} that knows 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 @option{--cflags} option to @command{pkg-config gnutls}.  The
following example shows how it can be used at the command line:

@example
gcc -c foo.c `pkg-config gnutls --cflags`
@end example

Adding the output of @samp{pkg-config gnutls --cflags} to the
compilers command line will ensure that the compiler can find the
@file{gnutls/gnutls.h} 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 @option{-L} option).  For this, the option
@option{--libs} to @command{pkg-config gnutls} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with the libarary (for instance, the
@samp{-ltasn1} option).  The example shows how to link @file{foo.o}
with the library to a program @command{foo}.

@example
gcc -o foo foo.o `pkg-config gnutls --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{pkg-config}:

@example
gcc -o foo foo.c `pkg-config gnutls --cflags --libs`
@end example


@node Client examples
@section Client Examples

This section contains examples of @acronym{TLS} and @acronym{SSL}
clients, using @acronym{GnuTLS}.  Note that these examples contain
little or no error checking.  Some of the examples require functions
implemented by another example.

@menu
* Simple client example with anonymous authentication::
* Simple client example with X.509 certificate support::
* Simple Datagram TLS client example::
* Obtaining session information::
* Verifying peer's certificate::
* Using a callback to select the certificate to use::
* Verifying a certificate::
* Client using a PKCS #11 token with TLS::
* Client with Resume capability example::
* Simple client example with SRP authentication::
* Simple client example in C++::
* Helper function for TCP connections::
@end menu

@node Simple client example with anonymous authentication
@subsection Simple Client Example with Anonymous Authentication

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.

@verbatiminclude examples/ex-client1.c

@node Simple client example with X.509 certificate support
@subsection Simple Client Example with @acronym{X.509} Certificate Support

Let's assume now that we want to create a TCP client which
communicates with servers that use @acronym{X.509} or
@acronym{OpenPGP} certificate authentication. The following client is
a very simple @acronym{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.

@verbatiminclude examples/ex-client2.c

@node Simple Datagram TLS client example
@subsection Simple Datagram @acronym{TLS} client example

This is a client that uses @acronym{UDP} to connect to a
server. This is the @acronym{DTLS} equivalent to 
@ref{Simple client example with X.509 certificate support} above.

@verbatiminclude examples/ex-client-udp.c

@node Obtaining session information
@subsection Obtaining Session Information

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 @ref{gnutls_handshake}.

@verbatiminclude examples/ex-session-info.c

@node Verifying peer's certificate
@subsection Verifying Peer's Certificate
@anchor{ex:verify}

A @acronym{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.

@verbatiminclude examples/ex-rfc2818.c

@node Using a callback to select the certificate to use
@subsection Using a Callback to Select the Certificate to Use

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.

@verbatiminclude examples/ex-cert-select.c

@node Verifying a certificate
@subsection Verifying a Certificate
@anchor{ex:verify2}

An example is listed below which uses the high level verification
functions to verify a given certificate list.

@verbatiminclude examples/ex-verify.c

@node Client using a PKCS #11 token with TLS
@subsection Using a @acronym{PKCS #11} token with TLS
@anchor{ex:pkcs11-client}

This example will demonstrate how to load keys and certificates
from a @acronym{PKCS} #11 token, and use it with a TLS connection.

@verbatiminclude examples/ex-cert-select-pkcs11.c


@node Client with Resume capability example
@subsection Client with Resume Capability Example
@anchor{ex:resume-client}

This is a modification of the simple client example. Here we
demonstrate the use of session resumption. The client tries to connect
once using @acronym{TLS}, close the connection and then try to
establish a new connection using the previously negotiated data.

@verbatiminclude examples/ex-client-resume.c


@node Simple client example with SRP authentication
@subsection Simple Client Example with @acronym{SRP} Authentication

The following client is a very simple @acronym{SRP} @acronym{TLS}
client which connects to a server and authenticates using a
@emph{username} and a @emph{password}. The server may authenticate
itself using a certificate, and in that case it has to be verified.

@verbatiminclude examples/ex-client-srp.c

@node Simple client example in C++
@subsection Simple Client Example using the C++ API

The following client is a simple example of a client client utilizing
the GnuTLS C++ API.

@verbatiminclude examples/ex-cxx.cpp

@node Helper function for TCP connections
@subsection Helper Function for TCP Connections

This helper function abstracts away TCP connection handling from the
other examples.  It is required to build some examples.

@verbatiminclude examples/tcp.c

@node Server examples
@section Server Examples

This section contains examples of @acronym{TLS} and @acronym{SSL}
servers, using @acronym{GnuTLS}.

@menu
* Echo Server with X.509 authentication::
* Echo Server with OpenPGP authentication::
* Echo Server with SRP authentication::
* Echo Server with anonymous authentication::
@end menu

@node Echo Server with X.509 authentication
@subsection Echo Server with @acronym{X.509} Authentication

This example is a very simple echo server which supports
@acronym{X.509} authentication, using the RSA ciphersuites.

@verbatiminclude examples/ex-serv1.c

@node Echo Server with OpenPGP authentication
@subsection Echo Server with @acronym{OpenPGP} Authentication
@cindex @acronym{OpenPGP} Server

The following example is an echo server which supports
@acronym{@acronym{OpenPGP}} key authentication. You can easily combine
this functionality ---that is have a server that supports both
@acronym{X.509} and @acronym{OpenPGP} certificates--- but we separated
them to keep these examples as simple as possible.

@verbatiminclude examples/ex-serv-pgp.c

@node Echo Server with SRP authentication
@subsection Echo Server with @acronym{SRP} Authentication

This is a server which supports @acronym{SRP} authentication. It is
also possible to combine this functionality with a certificate
server. Here it is separate for simplicity.

@verbatiminclude examples/ex-serv-srp.c

@node Echo Server with anonymous authentication
@subsection Echo Server with Anonymous Authentication

This example server support anonymous authentication, and could be
used to serve the example client for anonymous authentication.

@verbatiminclude examples/ex-serv-anon.c

@node Miscellaneous examples
@section Miscellaneous Examples

@menu
* Checking for an alert::
* X.509 certificate parsing example::
* Certificate request generation::
* PKCS #12 structure generation::
@end menu

@node Checking for an alert
@subsection Checking for an Alert

This is a function that checks if an alert has been received in the
current session.

@verbatiminclude examples/ex-alert.c

@node X.509 certificate parsing example
@subsection @acronym{X.509} Certificate Parsing Example
@anchor{ex:x509-info}

To demonstrate the @acronym{X.509} parsing capabilities an example program is
listed below.  That program reads the peer's certificate, and prints
information about it.

@verbatiminclude examples/ex-x509-info.c

@node Certificate request generation
@subsection Certificate Request Generation
@anchor{ex:crq}

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.

@verbatiminclude examples/ex-crq.c

@node PKCS #12 structure generation
@subsection @acronym{PKCS} #12 Structure Generation
@anchor{ex:pkcs12}

The following example is about generating a @acronym{PKCS} #12
structure.

@verbatiminclude examples/ex-pkcs12.c

@node Compatibility with the OpenSSL library
@section Compatibility with the OpenSSL Library
@cindex OpenSSL

To ease @acronym{GnuTLS}' integration with existing applications, a
compatibility layer with the widely used OpenSSL library is included
in the @code{gnutls-openssl} library. This compatibility layer is not
complete and it is not intended to completely reimplement the OpenSSL
API with @acronym{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
@file{gnutls/openssl.h} header file.

Current limitations imposed by the compatibility layer include:

@itemize

@item Error handling is not thread safe.

@end itemize

@node Keying Material Exporters
@section Keying Material Exporters
@cindex Keying Material Exporters
@cindex Exporting Keying Material

The TLS PRF can be used by other protocols to derive data.  The API to
use is @ref{gnutls_prf}.  The function needs to be provided with the
label in the parameter @code{label}, and the extra data to mix in the
@code{extra} parameter.  Depending on whether you want to mix in the
client or server random data first, you can set the
@code{server_random_first} parameter.

For example, after establishing a TLS session using
@ref{gnutls_handshake}, you can invoke the TLS PRF with this call:

@smallexample
#define MYLABEL "EXPORTER-FOO"
#define MYCONTEXT "some context data"
char out[32];
rc = gnutls_prf (session, strlen (MYLABEL), MYLABEL, 0,
                 strlen (MYCONTEXT), MYCONTEXT, 32, out);
@end smallexample

If you don't want to mix in the client/server random, there is a more
low-level TLS PRF interface called @ref{gnutls_prf_raw}.

@node Channel Bindings
@section Channel Bindings
@cindex Channel Bindings

In user authentication protocols (e.g., EAP or SASL mechanisms) it is
useful to have a unique string that identifies the secure channel that
is used, to bind together the user authentication with the secure
channel.  This can protect against man-in-the-middle attacks in some
situations.  The unique strings is a ``channel bindings''.  For
background and more discussion see @xcite{RFC5056}.

You can extract a channel bindings using the
@ref{gnutls_session_channel_binding} function.  Currently only the
@code{GNUTLS_CB_TLS_UNIQUE} type is supported, which corresponds to
the @code{tls-unique} channel bindings for TLS defined in
@xcite{RFC5929}.

The following example describes how to print the channel binding data.
Note that it must be run after a successful TLS handshake.

@smallexample
@{
  gnutls_datum cb;
  int rc;

  rc = gnutls_session_channel_binding (session,
                                       GNUTLS_CB_TLS_UNIQUE,
                                       &cb);
  if (rc)
    fprintf (stderr, "Channel binding error: %s\n",
             gnutls_strerror (rc));
  else
    @{
      size_t i;
      printf ("- Channel binding 'tls-unique': ");
      for (i = 0; i < cb.size; i++)
        printf ("%02x", cb.data[i]);
      printf ("\n");
    @}
@}
@end smallexample