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
http://sourceforge.net/projects/ezwinports/files/ thanks to Eli
Zaretskii) in the same directory as Emacs, you should be OK.
This function returns
t if GnuTLS is available in this instance of Emacs.
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.
gnutls-log-level variable 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
gnutls-algorithm-priority variable sets the GnuTLS priority
string. This is global, not per host name (although
gnutls-negotiate supports a priority string per connection so
it could be done if needed). The priority string syntax is in the
gnutls-trustfiles variable is a list of trustfiles
(certificates for the issuing authorities). This is global, not per
host name (although
gnutls-negotiate supports 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 four 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. You can easily
gnutls-trustfiles to be something else, but let us
know if you do, so we can make the change to benefit the other users
of that platform.
gnutls-verify-error variable allows you to verify SSL/TLS
server certificates for all connections or by host name. It defaults
nil for now but will likely be changed to
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
t enables 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.
gnutls-min-prime-bits variable 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