path: root/doc/cha-internals.texi

@node Internal architecture of GnuTLS
@chapter Internal Architecture of GnuTLS
@cindex Internal architecture

This chapter is to give a brief description of the
way @acronym{GnuTLS} works. The focus is to give an idea
to potential developers and those who want to know what
happens inside the black box.

@menu
* The TLS Protocol::
* TLS Handshake Protocol::
* TLS Authentication Methods::
* TLS Extension Handling::
* Certificate Handling::
* Cryptographic Backend::
@end menu

@node The TLS Protocol
@section The TLS Protocol
The main needs for the TLS protocol to be used are
shown in the image below.

@image{gnutls-client-server-use-case,9cm}

This is being accomplished by the following object diagram.
Note that since @acronym{GnuTLS} is being developed in C
object are just structures with attributes. The operations listed
are functions that require the first parameter to be that object.
@image{gnutls-objects,15cm}

@node TLS Handshake Protocol
@section TLS Handshake Protocol
The @acronym{GnuTLS} handshake protocol is implemented as a state
machine that waits for input or returns immediately when the non-blocking
transport layer functions are used. The main idea is shown in the following
figure.

@image{gnutls-handshake-state,9cm}

Also the way the input is processed varies per ciphersuite. Several 
implementations of the internal handlers are available and 
@ref{gnutls_handshake} only multiplexes the input to the appropriate 
handler. For example a @acronym{PSK} ciphersuite has a different 
implementation of the @code{process_client_key_exchange} than a
certificate ciphersuite.

@image{gnutls-handshake-sequence,12cm}

@node TLS Authentication Methods
@section TLS Authentication Methods
In @acronym{GnuTLS} authentication methods can be implemented quite
easily.  Since the required changes to add a new authentication method
affect only the handshake protocol, a simple interface is used. An
authentication method needs only to implement the functions as seen in
the figure below.

@image{gnutls-mod_auth_st,12cm}

The functions that need to be implemented are the ones responsible for
interpreting the handshake protocol messages. It is common for such
functions to read data from one or more @code{credentials_t}
structures@footnote{such as the
@code{gnutls_certificate_credentials_t} structures} and write data,
such as certificates, usernames etc. to @code{auth_info_t} structures.

Simple examples of existing authentication methods can be seen in
@code{auth_psk.c} for PSK ciphersuites and @code{auth_srp.c} for SRP
ciphersuites. After implementing these functions the structure holding
its pointers has to be registered in @code{gnutls_algorithms.c} in the
@code{_gnutls_kx_algorithms} structure.

@node TLS Extension Handling
@section TLS Extension Handling
As with authentication methods, the TLS extensions handlers can be
implemented using the following interface.

@image{gnutls-extensions_st,12cm}

Here there are two functions, one for receiving the extension data
and one for sending. These functions have to check internally whether
they operate in client or server side. 

A simple example of an extension handler can be seen in
@code{ext_srp.c} After implementing these functions, together with the
extension number they handle, they have to be registered in
@code{gnutls_extensions.c} in the @code{_gnutls_extensions} structure.

@subsection Adding a New TLS Extension

Adding support for a new TLS extension is done from time to time, and
the process to do so is not difficult.  Here are the steps you need to
follow if you wish to do this yourself.  For sake of discussion, let's
consider adding support for the hypothetical TLS extension
@code{foobar}.

@enumerate

@item Add @code{configure} option like @code{--enable-foobar} or @code{--disable-foobar}.

Which to chose depends on whether you intend to make the extension be
enabled by default.  Look at existing checks (i.e., SRP, authz) for
how to model the code.  For example:

@example
AC_MSG_CHECKING([whether to disable foobar support])
AC_ARG_ENABLE(foobar,
	AS_HELP_STRING([--disable-foobar],
		[disable foobar support]),
	ac_enable_foobar=no)
if test x$ac_enable_foobar != xno; then
 AC_MSG_RESULT(no)
 AC_DEFINE(ENABLE_FOOBAR, 1, [enable foobar])
else
 ac_full=0
 AC_MSG_RESULT(yes)
fi
AM_CONDITIONAL(ENABLE_FOOBAR, test "$ac_enable_foobar" != "no")
@end example

These lines should go in @code{lib/m4/hooks.m4}.

@item Add IANA extension value to @code{extensions_t} in @code{gnutls_int.h}.

A good name for the value would be GNUTLS_EXTENSION_FOOBAR.  Check
with @url{http://www.iana.org/assignments/tls-extensiontype-values}
for allocated values.  For experiments, you could pick a number but
remember that some consider it a bad idea to deploy such modified
version since it will lead to interoperability problems in the future
when the IANA allocates that number to someone else, or when the
foobar protocol is allocated another number.

@item Add an entry to @code{_gnutls_extensions} in @code{gnutls_extensions.c}.

A typical entry would be:

@example
  int ret;

  /* ...
   */

#if ENABLE_FOOBAR
  ret = gnutls_ext_register (GNUTLS_EXTENSION_FOOBAR,
                             "FOOBAR",
                             GNUTLS_EXT_TLS,
                             _gnutls_foobar_recv_params,
                             _gnutls_foobar_send_params);
  if (ret != GNUTLS_E_SUCCESS)
    return ret;
#endif
@end example

The GNUTLS_EXTENSION_FOOBAR is the integer value you added to
@code{gnutls_int.h} earlier.  The two functions are new functions that
you will need to implement, most likely you'll need to add an
@code{#include "ext_foobar.h"} as well.

@item Add new files @code{ext_foobar.c} and @code{ext_foobar.h} that implements the extension.

The functions you are responsible to add are those mentioned in the
previous step.  As a starter, you could add this:

@example
int
_gnutls_foobar_recv_params (gnutls_session_t session,
                            const opaque * data,
                            size_t data_size)
@{
  return 0;
@}

int
_gnutls_foobar_send_params (gnutls_session_t session,
                            opaque * data,
                            size_t _data_size)
@{
  return 0;
@}
@end example

The @code{_gnutls_foobar_recv_params} function is responsible for
parsing incoming extension data (both in the client and server).

The @code{_gnutls_foobar_send_params} function is responsible for
sending extension data (both in the client and server).

If you receive length fields that doesn't match, return
@code{GNUTLS_E_UNEXPECTED_PACKET_LENGTH}.  If you receive invalid
data, return @code{GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER}.  You can use
other error codes too.  Return 0 on success.

The function typically store some information in the @code{session}
variable for later usage.  If you need to add new fields there, check
@code{tls_ext_st} in @code{gnutls_int.h} and compare with existing TLS
extension specific variables.

Recall that both the client and server both send and receives
parameters, and your code most likely will need to do different things
depending on which mode it is in.  It may be useful to make this
distinction explicit in the code.  Thus, for example, a better
template than above would be:

@example
int
_gnutls_foobar_recv_params (gnutls_session_t session,
                            const opaque * data,
                            size_t data_size)
@{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_recv_client (session, data, data_size);
  else
    return foobar_recv_server (session, data, data_size);
@}

int
_gnutls_foobar_send_params (gnutls_session_t session,
                            opaque * data,
                            size_t data_size)
@{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_send_client (session, data, data_size);
  else
    return foobar_send_server (session, data, data_size);
@}
@end example

The functions used would be declared as @code{static} functions, of
the appropriate prototype, in the same file.

When adding the files, you'll need to add them to @code{Makefile.am}
as well, for example:

@example
if ENABLE_FOOBAR
COBJECTS += ext_foobar.c
HFILES += ext_foobar.h
endif
@end example

@item Add API functions to enable/disable the extension.

Normally the client will have one API to request use of the extension,
and setting some extension specific data.  The server will have one
API to let the library know that it is willing to accept the
extension, often this is implemented through a callback but it doesn't
have to.

The APIs need to be added to @code{includes/gnutls/gnutls.h} or
@code{includes/gnutls/extra.h} as appropriate.  It is recommended that
if you don't have a requirement to use the LGPLv2.1+ license for your
extension, that you place your work under the GPLv3+ license and thus
in the libgnutls-extra library.

You can implement the API function in the @code{ext_foobar.c} file, or
if that file ends up becoming rather larger, add a
@code{gnutls_foobar.c} file.

To make the API available in the shared library you need to add the
symbol in @code{lib/libgnutls.map} or
@code{libextra/libgnutls-extra.map} as appropriate, so that the symbol
is exported properly.

When writing GTK-DOC style documentation for your new APIs, don't
forget to add @code{Since:} tags to indicate the GnuTLS version the
API was introduced in.

@end enumerate

@node Certificate Handling
@section Certificate Handling
What is provided by the certificate handling functions
is summarized in the following diagram.

@image{gnutls-certificate-user-use-case,12cm}

@node Cryptographic Backend
@section Cryptographic Backend
Several new systems provide hardware assisted cryptographic algorithm
implementations that offer implementations some orders of magnitude
faster than the software. For this reason GnuTLS supports by default
the /dev/crypto device usually found in FreeBSD and OpenBSD system, to
take advantage of installed hardware. 

In addition it is possible to override parts of the crypto backend or the
whole. It is possible to override them both at runtime and compile
time, however here we will discuss the runtime possibility. The API
available for this functionality is in @code{gnutls/crypto.h} header
file.

@subsection Override specific algorithms
When an optimized implementation of a single algorithm is available,
say a hardware assisted version of @acronym{AES-CBC} then the
following functions can be used to register those algorithms.

@itemize

@item @ref{gnutls_crypto_single_cipher_register2}
To register a cipher algorithm.

@ref{gnutls_crypto_single_digest_register2}
To register a hash (digest) or MAC algorithm.

@end itemize

Those registration functions will only replace the specified algorithm
and leave the rest of subsystem intact.

@subsection Override parts of the backend
In some systems, such as embedded ones, it might be desirable to
override big parts of the cryptographic backend, or even all of
them. For this reason the following functions are provided.

@itemize

@item @ref{gnutls_crypto_cipher_register2}
To override the cryptographic algorithms backend.

@item @ref{gnutls_crypto_digest_register2}
To override the digest algorithms backend.

@item @ref{gnutls_crypto_rnd_register2}
To override the random number generator backend.

@item @ref{gnutls_crypto_bigint_register2}
To override the big number number operations backend.

@item @ref{gnutls_crypto_pk_register2}
To override the public key encryption backend. This is tight to the
big number operations so either both of them should be updated or care
must be taken to use the same format.

@end itemize

If all of them are used then GnuTLS will no longer use libgcrypt.