This complex directive uses a colon-separated cipher-spec string consisting of OpenSSL cipher specifications to configure the Cipher Suite the client is permitted to negotiate in the SSL handshake phase. Notice that this directive can be used both in per-server and per-directory context. In per-server context it applies to the standard SSL handshake when a connection is established. In per-directory context it forces a SSL renegotiation with the reconfigured Cipher Suite after the HTTP request was read but before the HTTP response is sent.

An SSL cipher specification in cipher-spec is composed of 4 major attributes plus a few extra minor ones:

• Key Exchange Algorithm:
  RSA, Diffie-Hellman, Elliptic Curve Diffie-Hellman, Secure Remote Password
• Authentication Algorithm:
  RSA, Diffie-Hellman, DSS, ECDSA, or none.
• Cipher/Encryption Algorithm:
  AES, DES, Triple-DES, RC4, RC2, IDEA, etc.
• MAC Digest Algorithm:
  MD5, SHA or SHA1, SHA256, SHA384.

An SSL cipher can also be an export cipher. SSLv2 ciphers are no longer supported. To specify which ciphers to use, one can either specify all the Ciphers, one at a time, or use aliases to specify the preference and order for the ciphers (see Table 1). The actually available ciphers and aliases depends on the used openssl version. Newer openssl versions may include additional ciphers.

Now where this becomes interesting is that these can be put together to specify the order and ciphers you wish to use. To speed this up there are also aliases ( SSLv3, TLSv1, EXP, LOW, MEDIUM, HIGH ) for certain groups of ciphers. These tags can be joined together with prefixes to form the cipher-spec. Available prefixes are:

• none: add cipher to list
• + : move matching ciphers to the current location in list
• - : remove cipher from list (can be added later again)
• ! : kill cipher from list completely (can not be added later again)

aNULL , eNULL and EXP ciphers are always disabled

Beginning with version 2.4.7, null and export-grade ciphers are always disabled, as mod_ssl unconditionally adds !aNULL:!eNULL:!EXP to any cipher string at initialization.

A simpler way to look at all of this is to use the `` openssl ciphers -v '' command which provides a nice way to successively create the correct cipher-spec string. The default cipher-spec string depends on the version of the OpenSSL libraries used. Let's suppose it is `` RC4-SHA:AES128-SHA:HIGH:MEDIUM:!aNULL:!MD5 '' which means the following: Put RC4-SHA and AES128-SHA at the beginning. We do this, because these ciphers offer a good compromise between speed and security. Next, include high and medium security ciphers. Finally, remove all ciphers which do not authenticate, i.e. for SSL the Anonymous Diffie-Hellman ciphers, as well as all ciphers which use MD5 as hash algorithm, because it has been proven insufficient.

$ openssl ciphers -v 'RC4-SHA:AES128-SHA:HIGH:MEDIUM:!aNULL:!MD5'
RC4-SHA SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=SHA1
AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1
DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1
... ... ... ... ...
SEED-SHA SSLv3 Kx=RSA Au=RSA Enc=SEED(128) Mac=SHA1
PSK-RC4-SHA SSLv3 Kx=PSK Au=PSK Enc=RC4(128) Mac=SHA1
KRB5-RC4-SHA SSLv3 Kx=KRB5 Au=KRB5 Enc=RC4(128) Mac=SHA1

The complete list of particular RSA & DH ciphers for SSL is given in Table 2.

Example

SSLCipherSuite RSA:!EXP:!NULL:+HIGH:+MEDIUM:-LOW