\input texinfo   @c -*-texinfo-*-
@comment %**start of header
@setfilename gnutls-guile.info
@include version-guile.texi
@include abs-top-srcdir.texi
@settitle Guile-GnuTLS @value{VERSION}

@c don't indent the paragraphs.
@paragraphindent 0

@c Unify some of the indices.
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@comment %**end of header
@finalout
@copying
This manual is last updated @value{UPDATED} for version
@value{VERSION} of Guile-GnuTLS.

Copyright @copyright{} 2001--2025 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 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''.
@end quotation
@end copying

@dircategory Software libraries
@direntry
* Guile-GnuTLS: (gnutls-guile).		Guile bindings for GNU Transport Layer Security Library.
@end direntry

@titlepage
@title Guile-GnuTLS
@subtitle Guile binding for GnuTLS
@subtitle for version @value{VERSION}, @value{UPDATED}
@author Ludovic Courtès
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@macro xcite{ref}
[\ref\] (@pxref{Bibliography})
@end macro

@contents

@node Top
@top Guile-GnuTLS

@insertcopying

@menu
* Preface::                     Preface.
* Guile Preparations::          Note on installation and environment.
* Guile API Conventions::       Naming conventions and other idiosyncrasies.
* Guile Examples::              Quick start.
* Guile Reference::             The Scheme GnuTLS programming interface.

* Copying Information::         You can copy and modify this manual.
* Procedure Index::
* Concept Index::
@end menu

@node Preface
@chapter Preface

This manual describes @code{Guile-GnuTLS}, the
@uref{https://www.gnu.org/software/guile/, GNU Guile} Scheme
programming interface to @uref{https://gnutls.org,GnuTLS}.  The reader
is assumed to have basic knowledge of the TLS protocol and GnuTLS
library (@pxref{Introduction to GnuTLS, GnuTLS intro,, gnutls, GnuTLS
Manual}).

At this stage, not all the C functions are available from Scheme, but
a large subset thereof is available.

@c *********************************************************************
@node Guile Preparations
@chapter Guile Preparations

The Guile bindings for GnuTLS are available for the Guile 3.0 and 2.2
series, as well as the legacy 2.0 series.

By default they are installed into
@file{/usr/local/share/guile/site/}).  Normally Guile will not find
the module there without help.  You may experience something like
this:

@example
$ guile
@dots{}
scheme@@(guile-user)> (use-modules (gnutls))
ERROR: no code for module (gnutls)
@end example

There are two ways to solve this.  The first is to make sure that when
building Guile-GnuTLS, the bindings will be installed in the same
place where Guile looks.  You may do this by using the
@code{--with-guile-site-dir} parameter as follows:

@example
$ ./configure --with-guile-site-dir=no
@end example

This will instruct Guile-GnuTLS to attempt to install the bindings
where Guile will look for them.  It will use @code{guile-config info
pkgdatadir} to learn the path to use.

If Guile was installed into @code{/usr}, you may also install
Guile-GnuTLS using the same prefix:

@example
$ ./configure --prefix=/usr
@end example

If you want to specify the path to install the Guile bindings you can
also specify the path directly:

@example
$ ./configure --with-guile-site-dir=/opt/guile/share/guile/site
@end example

The second solution requires some more work but may be easier to use
if you do not have system administrator rights to your machine.  You
need to instruct Guile so that it finds the Guile-GnuTLS bindings.
Either use the @code{GUILE_LOAD_PATH} environment variable as follows:

@example
$ GUILE_LOAD_PATH="/usr/local/share/guile/site:$GUILE_LOAD_PATH" guile
scheme@@(guile-user)> (use-modules (gnutls))
scheme@@(guile-user)>
@end example

Alternatively, you can modify Guile's @code{%load-path} variable
(@pxref{Build Config, Guile's run-time options,, guile, The GNU Guile
Reference Manual}).

At this point, you might get an error regarding
@file{guile-gnutls-v-2} similar to:

@example
gnutls.scm:361:1: In procedure dynamic-link in expression (load-extension "guile-gnutls-v-2" "scm_init_gnutls"):
gnutls.scm:361:1: file: "guile-gnutls-v-2", message: "guile-gnutls-v-2.so: cannot open shared object file: No such file or directory"
@end example

In this case, you will need to modify the run-time linker path, for
example as follows:

@example
$ LD_LIBRARY_PATH=/usr/local/lib GUILE_LOAD_PATH=/usr/local/share/guile/site guile
scheme@@(guile-user)> (use-modules (gnutls))
scheme@@(guile-user)>
@end example

To check that you got the intended GnuTLS library version, you may
print the version number of the loaded library as follows:

@example
$ guile
scheme@@(guile-user)> (use-modules (gnutls))
scheme@@(guile-user)> (gnutls-version)
"@value{VERSION}"
scheme@@(guile-user)>
@end example


@c *********************************************************************
@node Guile API Conventions
@chapter Guile API Conventions

This chapter details the conventions used by Guile API, as well as
specificities of the mapping of the C API to Scheme.

@menu
* Living on the cutting edge::    Checking for newer functions.
* Enumerates and Constants::      Representation of C-side constants.
* Procedure Names::               Naming conventions.
* Representation of Binary Data:: Binary data buffers.
* Input and Output::              Input and output.
* Exception Handling::            Exceptions.
@end menu

@node Living on the cutting edge
@section Living on the cutting edge
Some GnuTLS features have been introduced recently. To keep
compatibility with older GnuTLS versions, they may not all be
available. If a GnuTLS feature is not available, then its
corresponding variable will not be exported by the @code{(gnutls)}
module.

@node Enumerates and Constants
@section Enumerates and Constants

@cindex enumerate
@cindex constant

Lots of enumerates and constants are used in the GnuTLS C API.  For
each C enumerate type, a disjoint Scheme type is used---thus,
enumerate values and constants are not represented by Scheme symbols
nor by integers.  This makes it impossible to use an enumerate value
of the wrong type on the Scheme side: such errors are automatically
detected by type-checking.

The enumerate values are bound to variables exported by the
@code{(gnutls)} module.  These variables
are named according to the following convention:

@itemize
@item All variable names are lower-case; the underscore @code{_}
character used in the C API is replaced by hyphen @code{-}.
@item All variable names are prepended by the name of the enumerate
type and the slash @code{/} character.
@item In some cases, the variable name is made more explicit than the
one of the C API, e.g., by avoid abbreviations.
@end itemize

Consider for instance this C-side enumerate:

@example
typedef enum
@{
  GNUTLS_CRD_CERTIFICATE = 1,
  GNUTLS_CRD_ANON,
  GNUTLS_CRD_SRP,
  GNUTLS_CRD_PSK
@} gnutls_credentials_type_t;
@end example

The corresponding Scheme values are bound to the following variables
exported by the @code{(gnutls)} module:

@example
credentials/certificate
credentials/anonymous
credentials/srp
credentials/psk
@end example

Hopefully, most variable names can be deduced from this convention.

Scheme-side ``enumerate'' values can be compared using @code{eq?}
(@pxref{Equality, equality predicates,, guile, The GNU Guile Reference
Manual}).  Consider the following example:

@findex session-cipher

@example
(let ((session (make-session connection-end/client)))

  ;;
  ;; ...
  ;;

  ;; Check the ciphering algorithm currently used by SESSION.
  (if (eq? cipher/arcfour (session-cipher session))
      (format #t "We're using the ARCFOUR algorithm")))
@end example

In addition, all enumerate values can be converted to a human-readable
string, in a type-specific way.  For instance, @code{(cipher->string
cipher/arcfour)} yields @code{"ARCFOUR 128"}, while
@code{(key-usage->string key-usage/digital-signature)} yields
@code{"digital-signature"}.  Note that these strings may not be
sufficient for use in a user interface since they are fairly concise
and not internationalized.


@node Procedure Names
@section Procedure Names

Unlike C functions in GnuTLS, the corresponding Scheme procedures are
named in a way that is close to natural English.  Abbreviations are
also avoided.  For instance, the Scheme procedure corresponding to
@code{gnutls_certificate_set_dh_params} is named
@code{set-certificate-credentials-dh-parameters!}.  The @code{gnutls_}
prefix is always omitted from variable names since a similar effect
can be achieved using Guile's nifty binding renaming facilities,
should it be needed (@pxref{Using Guile Modules,,, guile, The GNU
Guile Reference Manual}).

Often Scheme procedure names differ from C function names in a way
that makes it clearer what objects they operate on.  For example, the
Scheme procedure named @code{set-session-transport-port!} corresponds
to @code{gnutls_transport_set_ptr}, making it clear that this
procedure applies to session.

@node Representation of Binary Data
@section Representation of Binary Data

Many procedures operate on binary data.  For instance,
@code{pkcs3-import-dh-parameters} expects binary data as input.

@cindex bytevectors
@cindex SRFI-4
@cindex homogeneous vector
Binary data is represented on the Scheme side using bytevectors
(@pxref{Bytevectors,,, guile, The GNU Guile Reference Manual}).
Homogeneous vectors such as SRFI-4 @code{u8vector}s can also be
used@footnote{Historically, SRFI-4 @code{u8vector}s are the closest
thing to bytevectors that Guile 1.8 and earlier supported.}.

As an example, generating and then exporting Diffie-Hellman parameters
in the PEM format can be done as follows:

@findex make-dh-parameters
@findex pkcs3-export-dh-parameters
@vindex x509-certificate-format/pem

@example
(let* ((dh  (make-dh-parameters 1024))
       (pem (pkcs3-export-dh-parameters dh
                                        x509-certificate-format/pem)))
  (call-with-output-file "some-file.pem"
    (lambda (port)
      (uniform-vector-write pem port))))
@end example


@node Input and Output
@section Input and Output

@findex set-session-transport-port!
@findex set-session-transport-fd!

The underlying transport of a TLS session can be any Scheme
input/output port (@pxref{Ports and File Descriptors,,, guile, The GNU
Guile Reference Manual}).  This has to be specified using
@code{set-session-transport-port!}.

However, for better performance, a raw file descriptor can be
specified, using @code{set-session-transport-fd!}.  For instance, if
the transport layer is a socket port over an OS-provided socket, you
can use the @code{port->fdes} or @code{fileno} procedure to obtain the
underlying file descriptor and pass it to
@code{set-session-transport-fd!}  (@pxref{Ports and File Descriptors,
@code{port->fdes} and @code{fileno},, guile, The GNU Guile Reference
Manual}).  This would work as follows:

@example
(let ((socket (socket PF_INET SOCK_STREAM 0))
      (session (make-session connection-end/client)))

  ;;
  ;; Establish a TCP connection...
  ;;

  ;; Use the file descriptor that underlies SOCKET.
  (set-session-transport-fd! session (fileno socket)))
@end example

@findex session-record-port

Once a TLS session is established, data can be communicated through it
(i.e., @emph{via} the TLS record layer) using the port returned by
@code{session-record-port}:

@example
(let ((session (make-session connection-end/client)))

  ;;
  ;; Initialize the various parameters of SESSION, set up
  ;; a network connection, etc.
  ;;

  (let ((i/o (session-record-port session)))
    (display "Hello peer!" i/o)
    (let ((greetings (read i/o)))

      ;; @dots{}

      (bye session close-request/rdwr))))
@end example

@c See <https://bugs.gnu.org/22966> for details.
@cindex buffering
Note that each write to the session record port leads to the
transmission of an encrypted TLS ``Application Data'' packet.  In the
above example, we create an Application Data packet for the 11 bytes for
the string that we write.  This is not efficient both in terms of CPU
usage and bandwidth (each packet adds at least 5 bytes of overhead and
can lead to one @code{write} system call), so we recommend that
applications do their own buffering.

@findex record-send
@findex record-receive!

A lower-level I/O API is provided by @code{record-send} and
@code{record-receive!} which take a bytevector (or a SRFI-4 vector) to
represent the data sent or received.  While it might improve
performance, it is much less convenient than the session record port and
should rarely be needed.


@node Exception Handling
@section Exception Handling

@cindex exceptions
@cindex errors
@cindex @code{gnutls-error}
@findex error->string

GnuTLS errors are implemented as Scheme exceptions (@pxref{Exceptions,
exceptions in Guile,, guile, The GNU Guile Reference Manual}).  Each
time a GnuTLS function returns an error, an exception with key
@code{gnutls-error} is raised.  The additional arguments that are
thrown include an error code and the name of the GnuTLS procedure that
raised the exception.  The error code is pretty much like an enumerate
value: it is one of the @code{error/} variables exported by the
@code{(gnutls)} module (@pxref{Enumerates and Constants}).  Exceptions
can be turned into error messages using the @code{error->string}
procedure.

The following examples illustrates how GnuTLS exceptions can be
handled:

@example
(let ((session (make-session connection-end/server)))

  ;;
  ;; ...
  ;;

  (catch 'gnutls-error
    (lambda ()
      (handshake session))
    (lambda (key err function . currently-unused)
      (format (current-error-port)
              "a GnuTLS error was raised by `~a': ~a~%"
              function (error->string err)))))
@end example

Again, error values can be compared using @code{eq?}:

@example
    ;; `gnutls-error' handler.
    (lambda (key err function . currently-unused)
      (if (eq? err error/fatal-alert-received)
          (format (current-error-port)
                  "a fatal alert was caught!~%")
          (format (current-error-port)
                  "something bad happened: ~a~%"
                  (error->string err))))
@end example

Note that the @code{catch} handler is currently passed only 3
arguments but future versions might provide it with additional
arguments.  Thus, it must be prepared to handle more than 3 arguments,
as in this example.


@c *********************************************************************
@node Guile Examples
@chapter Guile Examples

This chapter provides examples that illustrate common use cases.

@menu
* Anonymous Authentication Guile Example::    Simplest client and server.
* Using GnuTLS as a cryptography library::    Use the lower-level API.
@end menu

@node Anonymous Authentication Guile Example
@section Anonymous Authentication Guile Example

@dfn{Anonymous authentication} is very easy to use.  No certificates
are needed by the communicating parties.  Yet, it allows them to
benefit from end-to-end encryption and integrity checks.

The client-side code would look like this (assuming @var{some-socket}
is bound to an open socket port):

@vindex connection-end/client
@vindex kx/anon-dh
@vindex close-request/rdwr

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/anonymous-client.scm
@end example

The corresponding server would look like this (again, assuming
@var{some-socket} is bound to a socket port):

@vindex connection-end/server

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/anonymous-server.scm
@end example

This is it!

@node Using GnuTLS as a cryptography library
@section Using GnuTLS as a cryptography library

The low-level functions in GnuTLS can be accessed for various tasks.

@menu
* Hash Message Authentication Code:: Hashing with a secret.
* Hash Digest Algorithms:: Hashes.
* Authenticated Encryption:: The AEAD API.
* Low-lovel encryption API:: Block-by-block encryption of data.
* Public key cryptography:: Using private and public keys.
* Generating random numbers:: Pseudo-random number generation.
* Encoding binary data:: Base16, base64.
@end menu

@node Hash Message Authentication Code
@subsection Hash Message Authentication Code

The library provides support for @dfn{Hash Message Authentication
Code} (@emph{hmac}). This API provides a way to hash a message in a
way that is only reproducible with the knowledge of a secret.

This first example demonstrates how to use @code{hmac-fast} to hash a
bytevector in memory.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/hmac-example-1.scm
@end example

The next example shows how to hash a whole file that might not fit in
memory.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/hmac-example-2.scm
@end example

The final example shows how you can reuse a state to continue hashing
different inputs. This requires the @code{hmac-copy} function, which
is not always available (@pxref{Living on the cutting edge}).

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/hmac-example-3.scm
@end example

@node Hash Digest Algorithms
@subsection Hash Digest Algorithms

The API for hash algorithm is similar to that for hmac, except that
there is no secret data to reproduce the hash. So for instance, the
second hmac example becomes (@pxref{Hash Message Authentication
Code}):

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/hash-example.scm
@end example

@node Authenticated Encryption
@subsection Authenticated Encryption
The goal of authenticated encryption is to make sure that the data has
been encrypted by a party that knows a shared secret. The encryption
and decryption procedures are very similar. Both parties must know a
shared secret key and a nonce. The nonce is a value that must only be
used once to encrypt data. The nonce may be as long as you want, but
the secret key must be the exact size expected by the cipher
algorithm.

The API can also use extra authentication data, that can change on a
packet-by-packet basis, whatever your definition for a packet is.

To use the API, the cipher algorithm must be compatible with
AEAD. When it is, it defines a default tag size. You can override the
tag size, or use the 0 value to use the default tag size.

This example encrypts a file:

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/aead-encrypt-example.scm
@end example

And this example decrypts the same file. Please note that the
decrypted file is written to disk, which is acceptable for this simple
example:

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/aead-decrypt-example.scm
@end example

@node Low-lovel encryption API
@subsection Low-lovel encryption API

In some cases, you may want to use a lower-level encryption API. In
this example, the data to encrypt spans an integer number of
blocks. You need to specify the initialization vector, to seed the
encryption, and a private key.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/low-level-cipher-example.scm
@end example

@node Public key cryptography
@subsection Public key cryptography
The following example shows how to use elliptic curve cryptography
with a private key.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/use-private-key.scm
@end example

The next example shows how to generate a private key.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/generate-private-key.scm
@end example

In addition, abstract public or private keys can be obtained from X509
certificate and private key, with @code{x509-certificate->public-key}
and @code{x509-private-key->private-key}.

@node Generating random numbers
@subsection Generating random numbers

Gnutls lets you generate different kinds of pseudo-random numbers,
depending on the implications if it is guessed. Here is how you
generate a random number with the lowest security requirements:

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/random-example.scm
@end example

@node Encoding binary data
@subsection Encoding binary data
When working with gnutls, you may come across a lot of binary data, in
the form of guile bytevectors. Base16 and base64 are popular encoding
schemes for binary data.

This example shows how to encode and decode binary data to and from
base16.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/base16-example.scm
@end example

This example shows how to encode and decode binary data to and from
base64.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/base64-example.scm
@end example

Unfortunately, gnutls does not provide an API to encode data to the
popular base64-url encoding. However, it is possible to convert from
base64 to base64-url and back.

@example
@verbatiminclude @value{abs_top_srcdir}/guile/examples/base64url-example.scm
@end example

@c *********************************************************************
@node Guile Reference
@chapter Guile Reference

This chapter lists the Scheme procedures exported by the
@code{(gnutls)} module (@pxref{Using the Guile Module System,,, guile,
The GNU Guile Reference Manual}).

@include core.c.texi

@c Local Variables:
@c ispell-local-dictionary: "american"
@c End:

@include cha-copying.texi

@node Procedure Index
@unnumbered Procedure Index

@printindex fn

@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye
