This manual describes the Emacs GnuTLS integration.
GnuTLS is a library that establishes encrypted SSL or TLS connections. Emacs supports it through the gnutls.c and gnutls.h C files and the gnutls.el Emacs Lisp library.
This file describes the Emacs GnuTLS integration.
Copyright © 2012–2022 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, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”.
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Table of Contents
The GnuTLS library is an optional add-on for Emacs. Through it, any Emacs Lisp program can establish encrypted network connections that use Secure Socket Layer (SSL) and Transport Layer Security (TLS) protocols. The process of using SSL and TLS in establishing connections is as automated and transparent as possible.
The user has only a few customization options currently: the log level, priority string, trustfile list, and the minimum number of bits to be used in Diffie-Hellman key exchange. Rumors that every Emacs library requires at least 83 customizable variables are thus proven false.
2 Help For Users
From the user’s perspective, there’s nothing to the GnuTLS
integration. It Just Works for any Emacs Lisp code that uses
(see Network Connections in The Emacs Lisp Reference
Manual). The two functions are equivalent, the first one being an
alias of the second.
There’s one way to find out if GnuTLS is available, by calling
gnutls-available-p. This is a little bit trickier on the W32
(Windows) platform, but if you have the GnuTLS DLLs (available from
https://sourceforge.net/projects/ezwinports/files/ thanks to Eli
Zaretskii) in the same directory as Emacs, you should be OK.
- Function: gnutls-available-p ¶
This function returns non-
nilif GnuTLS is available in this instance of Emacs,
nilotherwise. If GnuTLS is available, the value is a list of GnuTLS capabilities supported by the installed GnuTLS library, which depends on the library version. The meaning of the capabilities is documented in the doc string of this function.
Oh, but sometimes things go wrong. Budgets aren’t balanced, television ads lie, and even TLS and SSL connections can fail to work properly. Well, there’s something to be done in the last case.
- Variable: gnutls-log-level ¶
gnutls-log-levelvariable sets the log level. 1 is verbose. 2 is very verbose. 5 is crazy. Crazy! Set it to 1 or 2 and look in the *Messages* buffer for the debugging information.
- Variable: gnutls-algorithm-priority ¶
gnutls-algorithm-priorityvariable sets the GnuTLS priority string. This is global, not per host name (although
gnutls-negotiatesupports a priority string per connection so it could be done if needed). For details see the GnuTLS documentation and the GnuTLS priority string syntax and description.
- Variable: gnutls-trustfiles ¶
gnutls-trustfilesvariable is a list of trustfiles (certificates for the issuing authorities). This is global, not per host name (although
gnutls-negotiatesupports a trustfile per connection so it could be done if needed). The trustfiles can be in PEM or DER format and examples can be found in most Unix distributions. By default the following locations are tried in this order: /etc/ssl/certs/ca-certificates.crt for Debian, Ubuntu, Gentoo and Arch Linux; /etc/pki/tls/certs/ca-bundle.crt for Fedora and RHEL; /etc/ssl/ca-bundle.pem for Suse; /usr/ssl/certs/ca-bundle.crt for Cygwin; /usr/local/share/certs/ca-root-nss.crt for FreeBSD. You can easily customize
gnutls-trustfilesto be something else, but let us know if you do, so we can make the change to benefit the other users of that platform.
- Variable: gnutls-verify-error ¶
gnutls-verify-errorvariable allows you to verify SSL/TLS server certificates for all connections or by host name. It defaults to
nilfor now but will likely be changed to
tlater, meaning that all certificates will be verified.
There are two checks available currently, that the certificate has been issued by a trusted authority as defined by
gnutls-trustfiles, and that the hostname matches the certificate.
tenables both checks, but you can enable them individually as well with
Because of the low-level interactions with the GnuTLS library, there is no way currently to ask if a certificate can be accepted. You have to look in the *Messages* buffer.
- Variable: gnutls-min-prime-bits ¶
gnutls-min-prime-bitsvariable is a pretty exotic customization for cases where you want to refuse handshakes with keys under a specific size. If you don’t know for sure that you need it, you don’t. Leave it
3 Help For Developers
The GnuTLS library is detected automatically at compile time. You
should see that it’s enabled in the
configure output. If not,
follow the standard procedure for finding out why a system library is
not picked up by the Emacs compilation. On the W32 (Windows)
platform, installing the DLLs with a recent build should be enough.
(the two are equivalent, the first one being an alias to the second).
You should not have to use the gnutls.el functions directly.
But you can test them with
- Function: open-gnutls-stream name buffer host service &optional parameters ¶
This function creates a buffer connected to a specific host and service (port number or service name). The mandatory arguments and their syntax are the same as those given to
open-network-stream(see Network Connections in The Emacs Lisp Reference Manual). The connection process is called name (made unique if necessary). This function returns the connection process.
The optional parameters argument is a list of keywords and values. The only keywords which currently have any effect are
:client certificate ttriggers looking up of client certificates matching host and service using the auth-source library. Any resulting client certificates are passed down to the lower TLS layers. The format used by .authinfo to specify the per-server keys is described in auth-source in Emacs auth-source Library.
:nowait tmeans that the socket should be asynchronous, and the connection process will be returned to the caller before TLS negotiation has happened.
For historical reasons parameters can also be a symbol, which is interpreted the same as passing a list containing
:nowaitand the value of that symbol.
;; open a HTTPS connection (open-gnutls-stream "tls" "tls-buffer" "yourserver.com" "https") ;; open a IMAPS connection (open-gnutls-stream "tls" "tls-buffer" "imap.gmail.com" "imaps")
If called with nowait, the process is returned immediately
(before connecting to the server). In that case, the process object
is told what parameters to use when negotiating the connection
by using the
gnutls-negotiate is not generally useful and it
may change as needed, so please see gnutls.el for the details.
- Function: gnutls-negotiate spec ¶
Please see gnutls.el for the spec details and for usage, but do not rely on this function’s interface if possible.
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