int gnutls_priority_init(gnutls_priority_t * priority_cache, const char * priorities, const char ** err_pos);
The priorities option allows you to specify a semi-colon separated list of the cipher priorities to enable.
Unless the first keyword is "NONE" the defaults (in preference order) are for TLS protocols TLS1.1, TLS1.0, SSL3.0; for compression NULL; for certificate types X.509, OpenPGP.
For key exchange algorithms when in NORMAL or SECURE levels the perfect forward secrecy algorithms take precendence of the other protocols. In all cases all the supported key exchange algorithms are enabled (except for the RSA-EXPORT which is only enabled in EXPORT level).
Note that although one can select very long key sizes (such as 256 bits) for symmetric algorithms, to actually increase security the public key algorithms have to use longer key sizes as well.
For all the current available algorithms and protocols use "gnutls-cli -l" to get a listing.
"PERFORMANCE" means all the "secure" ciphersuites are enabled, limited to 128 bit ciphers and sorted by terms of speed performance.
"NORMAL" means all "secure" ciphersuites. The 256-bit ciphers are included as a fallback only. The ciphers are sorted by security margin.
"SECURE128" means all "secure" ciphersuites with ciphers up to 128 bits, sorted by security margin.
"SECURE256" means all "secure" ciphersuites including the 256 bit ciphers, sorted by security margin.
"EXPORT" means all ciphersuites are enabled, including the low-security 40 bit ciphers.
"NONE" means nothing is enabled. This disables even protocols and compression methods.
"+" appended with an algorithm will add this algorithm.
"%COMPAT" will enable compatibility features for a server.
"%SSL3_RECORD_VERSION" will use SSL3.0 record version in client hello.
"%VERIFY_ALLOW_SIGN_RSA_MD5" will allow RSA-MD5 signatures in certificate chains.
"%VERIFY_ALLOW_X509_V1_CA_CRT" will allow V1 CAs in chains.
"EXPORT:!VERS-TLS1.0:+COMP-DEFLATE" means that export ciphers are enabled, TLS 1.0 is disabled, and libz compression enabled.
"NONE:+VERS-TLS1.0:+AES-128-CBC:+RSA:+SHA1:+COMP-NULL", "NORMAL", "%COMPAT".
should give you access to the complete manual.