This manual is last updated 9 February 2023 for version 3.7.9 of GnuTLS.
Copyright © 2001-2012, 2014, 2016, 2019, 2022 Free Software Foundation, Inc.
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”.
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This manual describes the GNU Guile Scheme programming interface to GnuTLS, which is distributed as part of GnuTLS. The reader is assumed to have basic knowledge of the protocol and library. Details missing from this chapter may be found in Function reference, of the C API reference.
At this stage, not all the C functions are available from Scheme, but a large subset thereof is available.
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The GnuTLS Guile bindings are available for the Guile 3.0 and 2.2 series, as well as the legacy 2.0 series.
By default they are installed under the GnuTLS installation directory, typically /usr/local/share/guile/site/). Normally Guile will not find the module there without help. You may experience something like this:
$ guile … scheme@(guile-user)> (use-modules (gnutls)) ERROR: no code for module (gnutls)
There are two ways to solve this. The first is to make sure that when
building GnuTLS, the Guile bindings will be installed in the same
place where Guile looks. You may do this by using the
--with-guile-site-dir parameter as follows:
$ ./configure --with-guile-site-dir=no
This will instruct GnuTLS to attempt to install the Guile bindings
where Guile will look for them. It will use guile-config info
pkgdatadir to learn the path to use.
If Guile was installed into /usr, you may also install GnuTLS
using the same prefix:
$ ./configure --prefix=/usr
If you want to specify the path to install the Guile bindings you can also specify the path directly:
$ ./configure --with-guile-site-dir=/opt/guile/share/guile/site
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 GnuTLS Guile bindings.
Either use the GUILE_LOAD_PATH environment variable as follows:
$ GUILE_LOAD_PATH="/usr/local/share/guile/site:$GUILE_LOAD_PATH" guile scheme@(guile-user)> (use-modules (gnutls)) scheme@(guile-user)>
Alternatively, you can modify Guile’s %load-path variable
(see Guile’s run-time options in The GNU Guile
Reference Manual).
At this point, you might get an error regarding guile-gnutls-v-2 similar to:
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"
In this case, you will need to modify the run-time linker path, for example as follows:
$ LD_LIBRARY_PATH=/usr/local/lib GUILE_LOAD_PATH=/usr/local/share/guile/site guile scheme@(guile-user)> (use-modules (gnutls)) scheme@(guile-user)>
To check that you got the intended GnuTLS library version, you may print the version number of the loaded library as follows:
$ guile scheme@(guile-user)> (use-modules (gnutls)) scheme@(guile-user)> (gnutls-version) "3.7.9" scheme@(guile-user)>
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This chapter details the conventions used by Guile API, as well as specificities of the mapping of the C API to Scheme.
Next: Procedure Names, Up: Guile API Conventions [Contents][Index]
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
(gnutls) module. These variables
are named according to the following convention:
_
character used in the C API is replaced by hyphen -.
/ character.
Consider for instance this C-side enumerate:
typedef enum
{
GNUTLS_CRD_CERTIFICATE = 1,
GNUTLS_CRD_ANON,
GNUTLS_CRD_SRP,
GNUTLS_CRD_PSK
} gnutls_credentials_type_t;
The corresponding Scheme values are bound to the following variables
exported by the (gnutls) module:
credentials/certificate credentials/anonymous credentials/srp credentials/psk
Hopefully, most variable names can be deduced from this convention.
Scheme-side “enumerate” values can be compared using eq?
(see equality predicates in The GNU Guile Reference
Manual). Consider the following 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")))
In addition, all enumerate values can be converted to a human-readable
string, in a type-specific way. For instance, (cipher->string
cipher/arcfour) yields "ARCFOUR 128", while
(key-usage->string key-usage/digital-signature) yields
"digital-signature". Note that these strings may not be
sufficient for use in a user interface since they are fairly concise
and not internationalized.
Next: Representation of Binary Data, Previous: Enumerates and Constants, Up: Guile API Conventions [Contents][Index]
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
gnutls_certificate_set_dh_params is named
set-certificate-credentials-dh-parameters!. The 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 (see Using Guile Modules in 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 set-session-transport-port! corresponds
to gnutls_transport_set_ptr, making it clear that this
procedure applies to session.
Next: Input and Output, Previous: Procedure Names, Up: Guile API Conventions [Contents][Index]
Many procedures operate on binary data. For instance,
pkcs3-import-dh-parameters expects binary data as input.
Binary data is represented on the Scheme side using bytevectors
(see Bytevectors in The GNU Guile Reference Manual).
Homogeneous vectors such as SRFI-4 u8vectors can also be
used1.
As an example, generating and then exporting Diffie-Hellman parameters in the PEM format can be done as follows:
(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))))
Next: Exception Handling, Previous: Representation of Binary Data, Up: Guile API Conventions [Contents][Index]
The underlying transport of a TLS session can be any Scheme
input/output port (see Ports and File Descriptors in The GNU
Guile Reference Manual). This has to be specified using
set-session-transport-port!.
However, for better performance, a raw file descriptor can be
specified, using set-session-transport-fd!. For instance, if
the transport layer is a socket port over an OS-provided socket, you
can use the port->fdes or fileno procedure to obtain the
underlying file descriptor and pass it to
set-session-transport-fd! (see port->fdes and fileno in The GNU Guile Reference
Manual). This would work as follows:
(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)))
Once a TLS session is established, data can be communicated through it
(i.e., via the TLS record layer) using the port returned by
session-record-port:
(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)))
;; …
(bye session close-request/rdwr))))
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 write system call), so we recommend that
applications do their own buffering.
A lower-level I/O API is provided by record-send and
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.
Previous: Input and Output, Up: Guile API Conventions [Contents][Index]
GnuTLS errors are implemented as Scheme exceptions (see exceptions in Guile in The GNU Guile Reference Manual). Each
time a GnuTLS function returns an error, an exception with key
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 error/ variables exported by the
(gnutls) module (see Enumerates and Constants). Exceptions
can be turned into error messages using the error->string
procedure.
The following examples illustrates how GnuTLS exceptions can be handled:
(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)))))
Again, error values can be compared using eq?:
;; `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))))
Note that the 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.
Next: Guile Reference, Previous: Guile API Conventions, Up: GnuTLS-Guile [Contents][Index]
This chapter provides examples that illustrate common use cases.
Up: Guile Examples [Contents][Index]
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 some-socket is bound to an open socket port):
;; Client-side.
(let ((client (make-session connection-end/client)))
;; Use the default settings.
(set-session-default-priority! client)
;; Don't use certificate-based authentication.
(set-session-certificate-type-priority! client '())
;; Request the "anonymous Diffie-Hellman" key exchange method.
(set-session-kx-priority! client (list kx/anon-dh))
;; Specify the underlying socket.
(set-session-transport-fd! client (fileno some-socket))
;; Create anonymous credentials.
(set-session-credentials! client
(make-anonymous-client-credentials))
;; Perform the TLS handshake with the server.
(handshake client)
;; Send data over the TLS record layer.
(write "hello, world!" (session-record-port client))
;; Terminate the TLS session.
(bye client close-request/rdwr))
The corresponding server would look like this (again, assuming some-socket is bound to a socket port):
;; Server-side.
(let ((server (make-session connection-end/server)))
(set-session-default-priority! server)
(set-session-certificate-type-priority! server '())
(set-session-kx-priority! server (list kx/anon-dh))
;; Specify the underlying transport socket.
(set-session-transport-fd! server (fileno some-socket))
;; Create anonymous credentials.
(let ((cred (make-anonymous-server-credentials))
(dh-params (make-dh-parameters 1024)))
;; Note: DH parameter generation can take some time.
(set-anonymous-server-dh-parameters! cred dh-params)
(set-session-credentials! server cred))
;; Perform the TLS handshake with the client.
(handshake server)
;; Receive data over the TLS record layer.
(let ((message (read (session-record-port server))))
(format #t "received the following message: ~a~%"
message)
(bye server close-request/rdwr)))
This is it!
Next: Copying Information, Previous: Guile Examples, Up: GnuTLS-Guile [Contents][Index]
This chapter lists the GnuTLS Scheme procedures exported by the
(gnutls) module (see The Guile module system in The
GNU Guile Reference Manual).
Enable GnuTLS logging up to level (an integer).
Use proc (a two-argument procedure) as the global GnuTLS log procedure.
Use certificate pub and secret key sec in certificate credentials cred.
Return #f if key ID id is in keyring, #f otherwise.
Import data (a u8vector) according to format and return the imported keyring.
Return a list of values denoting the key usage of key.
Return the version of the OpenPGP message format (RFC2440) honored by key.
Return two values: the certificate algorithm used by key and the number of bits used.
Return the list of names for key.
Return the indexth name of key.
Return a new u8vector denoting the fingerprint of key.
Store in fpr (a u8vector) the fingerprint of key. Return the number of bytes stored in fpr.
Store the ID (an 8 byte sequence) of certificate key in id (a u8vector).
Return the ID (an 8-element u8vector) of certificate key.
Return a new OpenPGP private key object resulting from the import of data (a uniform array) according to format. Optionally, a passphrase may be provided.
Return a new OpenPGP certificate object resulting from the import of data (a uniform array) according to format.
Return the fingerprint (a u8vector) of the certificate cert, computed using the digest algorithm algo.
Return two values: the alternative name type for cert (i.e., one of the x509-subject-alternative-name/ values) and the actual subject alternative name (a string) at index. Both values are #f if no alternative name is available at index.
Return the subject key ID (a u8vector) for cert.
Return the key ID (a u8vector) of the X.509 certificate authority of cert.
Return a statistically unique ID (a u8vector) for cert that depends on its public key parameters. This is normally a 20-byte SHA-1 hash.
Return the version of cert.
Return the key usage of cert (i.e., a list of key-usage/ values), or the empty list if cert does not contain such information.
Return two values: the public key algorithm (i.e., one of the pk-algorithm/ values) of cert and the number of bits used.
Return the signature algorithm used by cert (i.e., one of the sign-algorithm/ values).
Return true if cert matches hostname, a string denoting a DNS host name. This is the basic implementation of RFC 2818 (aka. HTTPS).
Return the OID (a string) at index from cert’s issuer DN. Return #f if no OID is available at index.
Return OID (a string) at index from cert. Return #f if no OID is available at index.
Return the distinguished name (DN) of X.509 certificate cert.
Return the distinguished name (DN) of X.509 certificate cert. The form of the DN is as described in RFC 2253.
Return a new X.509 private key object resulting from the import of data (a uniform array) according to format. Optionally, if pass is not #f, it should be a string denoting a passphrase. encrypted tells whether the private key is encrypted (#t by default).
Return a new X.509 private key object resulting from the import of data (a uniform array) according to format.
Return a new X.509 certificate object resulting from the import of data (a uniform array) according to format.
Return the username associated with PSK server session session.
Set the client credentials for cred, a PSK client credentials object.
Return a new PSK client credentials object.
Use file as the password file for PSK server credentials cred.
Return new PSK server credentials.
Verify the peer certificate for session and return a list of certificate-status values (such as certificate-status/revoked), or the empty list if the certificate is valid.
Set the certificate verification flags to flags, a series of certificate-verify values.
Set the verification limits of peer-certificate-status for certificate credentials cred to max_bits bits for an acceptable certificate and max_depth as the maximum depth of a certificate chain.
Have certificate credentials cred use the X.509 certificates listed in certs and X.509 private key privkey.
Use X.509 certificate cert and private key key, both uniform arrays containing the X.509 certificate and key in format format, for certificate credentials cred.
Use data (a uniform array) as the X.509 CRL (certificate revocation list) database for cred. On success, return the number of CRLs processed.
Use data (a uniform array) as the X.509 trust database for cred. On success, return the number of certificates processed.
Use file as the X.509 CRL (certificate revocation list) file for certificate credentials cred. On success, return the number of CRLs processed.
Use file as the X.509 trust file for certificate credentials cred. On success, return the number of certificates processed.
Use file as the password file for PSK server credentials cred.
Use Diffie-Hellman parameters dh_params for certificate credentials cred.
Return new certificate credentials (i.e., for use with either X.509 or OpenPGP certificates.
Set the Diffie-Hellman parameters of anonymous server credentials cred.
Return anonymous client credentials.
Return anonymous server credentials.
Use bits DH prime bits for session.
Export Diffie-Hellman parameters dh_params in PKCS3 format according for format (an x509-certificate-format value). Return a u8vector containing the result.
Import Diffie-Hellman parameters in PKCS3 format (further specified by format, an x509-certificate-format value) from array (a homogeneous array) and return a new dh-params object.
Return new Diffie-Hellman parameters.
Use port as the input/output port for session.
Use file descriptor fd as the underlying transport for session.
Set close, a one-argument procedure, as the procedure called when port is closed. close will be passed port. It may be called when close-port is called on port, or when port is garbage-collected. It is a useful way to free resources associated with port such as the session’s transport file descriptor or port.
Return a read-write port that may be used to communicate over session. All invocations of session-port on a given session return the same object (in the sense of eq?).
If close is provided, it must be a one-argument procedure, and it will be called when the returned port is closed. This is equivalent to setting it by calling set-session-record-port-close!.
Receive data from session into array, a uniform homogeneous array. Return the number of bytes actually received.
Send the record constituted by array through session.
For a client, this procedure provides a way to inform the server that it is known under name, via the SERVER NAME TLS extension. type must be a server-name-type value, server-name-type/dns for DNS names.
Use cred as session’s credentials.
Return the name of the given cipher suite.
Have session use the given priorities for the ciphers, key exchange methods, MACs and compression methods. priorities must be a string (see Priority Strings in GnuTLS, Transport Layer Security Library for the GNU system). When priorities cannot be parsed, an error/invalid-request error is raised, with an extra argument indication the position of the error.
Have session use the default priorities.
Tell how session, a server-side session, should deal with certificate requests. request should be either certificate-request/request or certificate-request/require.
Return our certificate chain for session (as sent to the peer) in raw format (a u8vector). In the case of OpenPGP there is exactly one certificate. Return the empty list if no certificate was used.
Return the a list of certificates in raw format (u8vectors) where the first one is the peer’s certificate. In the case of OpenPGP, there is always exactly one certificate. In the case of X.509, subsequent certificates indicate form a certificate chain. Return the empty list if no certificate was sent.
Return the client authentication type (a credential-type value) used in session.
Return the server authentication type (a credential-type value) used in session.
Return the authentication type (a credential-type value) used by session.
Return the protocol used by session.
Return session’s certificate type.
Return session’s compression method.
Return session’s MAC.
Return session’s kx.
Return session’s cipher.
Send alert via session.
Get an aleter from session.
Perform a re-authentication step for session.
Perform a re-handshaking for session.
Perform a handshake for session.
Close session according to how.
Return a new session for connection end end, either connection-end/server or connection-end/client. The optional flags arguments are connection-flag values such as connection-flag/auto-reauth.
Return a string denoting the version number of the underlying GnuTLS library, e.g., "1.7.2".
Return true if obj is of type openpgp-keyring.
Return true if obj is of type openpgp-private-key.
Return true if obj is of type openpgp-certificate.
Return true if obj is of type x509-private-key.
Return true if obj is of type x509-certificate.
Return true if obj is of type psk-client-credentials.
Return true if obj is of type psk-server-credentials.
Return true if obj is of type srp-client-credentials.
Return true if obj is of type srp-server-credentials.
Return true if obj is of type certificate-credentials.
Return true if obj is of type dh-parameters.
Return true if obj is of type anonymous-server-credentials.
Return true if obj is of type anonymous-client-credentials.
Return true if obj is of type session.
Return a string describing enumval, a openpgp-certificate-format value.
Return a string describing enumval, a error value.
Return a string describing enumval, a certificate-verify value.
Return a string describing enumval, a key-usage value.
Return a string describing enumval, a psk-key-format value.
Return a string describing enumval, a server-name-type value.
Return a string describing enumval, a sign-algorithm value.
Return a string describing enumval, a pk-algorithm value.
Return a string describing enumval, a x509-subject-alternative-name value.
Return a string describing enumval, a x509-certificate-format value.
Return a string describing enumval, a certificate-type value.
Return a string describing enumval, a protocol value.
Return a string describing enumval, a close-request value.
Return a string describing enumval, a certificate-request value.
Return a string describing enumval, a certificate-status value.
Return a string describing enumval, a handshake-description value.
Return a string describing enumval, a alert-description value.
Return a string describing enumval, a alert-level value.
Return a string describing enumval, a connection-flag value.
Return a string describing enumval, a connection-end value.
Return a string describing enumval, a compression-method value.
Return a string describing enumval, a digest value.
Return a string describing enumval, a mac value.
Return a string describing enumval, a credentials value.
Return a string describing enumval, a params value.
Return a string describing enumval, a kx value.
Return a string describing enumval, a cipher value.
Next: Procedure Index, Previous: Guile Reference, Up: GnuTLS-Guile [Contents][Index]
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Next: Concept Index, Previous: Copying Information, Up: GnuTLS-Guile [Contents][Index]
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Previous: Procedure Index, Up: GnuTLS-Guile [Contents][Index]
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Historically, SRFI-4 u8vectors are the closest
thing to bytevectors that Guile 1.8 and earlier supported.