3.7.2.2. Working with Cipher Suites in GnuTLS
GnuTLS is a communications library that implements the
SSL and TLS protocols and related technologies.
Note
The GnuTLS installation on Red Hat Enterprise Linux offers optimal default configuration values that provide sufficient security for the majority of use cases. Unless you need to satisfy special security requirements, it is recommended to use the supplied defaults.
Use the
gnutls-cli command with the -l (or --list) option to list all supported cipher suites:
~]$ gnutls-cli -l
To narrow the list of cipher suites displayed by the
-l option, pass one or more parameters (referred to as priority strings and keywords in GnuTLS documentation) to the --priority option. See the GnuTLS documentation at http://www.gnutls.org/manual/gnutls.html#Priority-Strings for a list of all available priority strings. For example, issue the following command to get a list of cipher suites that offer at least 128 bits of security:
~]$ gnutls-cli --priority SECURE128 -l
To obtain a list of cipher suites that satisfy the recommendations outlined in Section 3.7.1, “Choosing Algorithms to Enable”, use a command similar to the following:
~]$ gnutls-cli --priority SECURE256:+SECURE128:-VERS-TLS-ALL:+VERS-TLS1.2:-RSA:-DHE-DSS:-CAMELLIA-128-CBC:-CAMELLIA-256-CBC -l
Cipher suites for SECURE256:+SECURE128:-VERS-TLS-ALL:+VERS-TLS1.2:-RSA:-DHE-DSS:-CAMELLIA-128-CBC:-CAMELLIA-256-CBC
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384 0xc0, 0x2c TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA384 0xc0, 0x24 TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA1 0xc0, 0x0a SSL3.0
TLS_ECDHE_ECDSA_AES_128_GCM_SHA256 0xc0, 0x2b TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA256 0xc0, 0x23 TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA1 0xc0, 0x09 SSL3.0
TLS_ECDHE_RSA_AES_256_GCM_SHA384 0xc0, 0x30 TLS1.2
TLS_ECDHE_RSA_AES_256_CBC_SHA1 0xc0, 0x14 SSL3.0
TLS_ECDHE_RSA_AES_128_GCM_SHA256 0xc0, 0x2f TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA256 0xc0, 0x27 TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA1 0xc0, 0x13 SSL3.0
TLS_DHE_RSA_AES_256_CBC_SHA256 0x00, 0x6b TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA1 0x00, 0x39 SSL3.0
TLS_DHE_RSA_AES_128_GCM_SHA256 0x00, 0x9e TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA256 0x00, 0x67 TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA1 0x00, 0x33 SSL3.0
Certificate types: CTYPE-X.509
Protocols: VERS-TLS1.2
Compression: COMP-NULL
Elliptic curves: CURVE-SECP384R1, CURVE-SECP521R1, CURVE-SECP256R1
PK-signatures: SIGN-RSA-SHA384, SIGN-ECDSA-SHA384, SIGN-RSA-SHA512, SIGN-ECDSA-SHA512, SIGN-RSA-SHA256, SIGN-DSA-SHA256, SIGN-ECDSA-SHA256
The above command limits the output to ciphers with at least 128 bits of security while giving preference to the stronger ones. It also forbids
RSA key exchange and DSS authentication.
Note that this is a rather strict configuration, and it might be necessary to relax the conditions in real-world scenarios to allow for a compatibility with a broader range of clients.
