Linux, FreeBSD, Juniper, Cisco / Network security articles and troubleshooting guides

SSLCipherSuite Directive
Description: Cipher Suite available for negotiation in SSL handshake
Syntax: SSLCipherSuite cipher-spec
Default: SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP
Context: server config, virtual host, directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl

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 renegotation 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 or Diffie-Hellman variants.
Authentication Algorithm:
RSA, Diffie-Hellman, DSS or none.
Cipher/Encryption Algorithm:
DES, Triple-DES, RC4, RC2, IDEA or none.
MAC Digest Algorithm:
MD5, SHA or SHA1.

An SSL cipher can also be an export cipher and is either a SSLv2 or SSLv3/TLSv1 cipher (here TLSv1 is equivalent to SSLv3). 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).
Tag Description
Key Exchange Algorithm:
kRSA RSA key exchange
kDHr Diffie-Hellman key exchange with RSA key
kDHd Diffie-Hellman key exchange with DSA key
kEDH Ephemeral (temp.key) Diffie-Hellman key exchange (no cert)
Authentication Algorithm:
aNULL No authentication
aRSA RSA authentication
aDSS DSS authentication
aDH Diffie-Hellman authentication
Cipher Encoding Algorithm:
eNULL No encoding
DES DES encoding
3DES Triple-DES encoding
RC4 RC4 encoding
RC2 RC2 encoding
IDEA IDEA encoding
MAC Digest Algorithm:
MD5 MD5 hash function
SHA1 SHA1 hash function
SHA SHA hash function
Aliases:
SSLv2 all SSL version 2.0 ciphers
SSLv3 all SSL version 3.0 ciphers
TLSv1 all TLS version 1.0 ciphers
EXP all export ciphers
EXPORT40 all 40-bit export ciphers only
EXPORT56 all 56-bit export ciphers only
LOW all low strength ciphers (no export, single DES)
MEDIUM all ciphers with 128 bit encryption
HIGH all ciphers using Triple-DES
RSA all ciphers using RSA key exchange
DH all ciphers using Diffie-Hellman key exchange
EDH all ciphers using Ephemeral Diffie-Hellman key exchange
ADH all ciphers using Anonymous Diffie-Hellman key exchange
DSS all ciphers using DSS authentication
NULL all ciphers using no encryption

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 (SSLv2, 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)

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 is ``ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP'' which means the following: first, remove from consideration any ciphers that do not authenticate, i.e. for SSL only the Anonymous Diffie-Hellman ciphers. Next, use ciphers using RC4 and RSA. Next include the high, medium and then the low security ciphers. Finally pull all SSLv2 and export ciphers to the end of the list.

$ openssl ciphers -v 'ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP'
NULL-SHA SSLv3 Kx=RSA Au=RSA Enc=None Mac=SHA1
NULL-MD5 SSLv3 Kx=RSA Au=RSA Enc=None Mac=MD5
EDH-RSA-DES-CBC3-SHA SSLv3 Kx=DH Au=RSA Enc=3DES(168) Mac=SHA1
... ... ... ... ...
EXP-RC4-MD5 SSLv3 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export
EXP-RC2-CBC-MD5 SSLv2 Kx=RSA(512) Au=RSA Enc=RC2(40) Mac=MD5 export
EXP-RC4-MD5 SSLv2 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export

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

SSLCipherSuite RSA:!EXP:!NULL:+HIGH:+MEDIUM:-LOW
Cipher-Tag Protocol Key Ex. Auth. Enc. MAC Type
RSA Ciphers:
DES-CBC3-SHA SSLv3 RSA RSA 3DES(168) SHA1
DES-CBC3-MD5 SSLv2 RSA RSA 3DES(168) MD5
IDEA-CBC-SHA SSLv3 RSA RSA IDEA(128) SHA1
RC4-SHA SSLv3 RSA RSA RC4(128) SHA1
RC4-MD5 SSLv3 RSA RSA RC4(128) MD5
IDEA-CBC-MD5 SSLv2 RSA RSA IDEA(128) MD5
RC2-CBC-MD5 SSLv2 RSA RSA RC2(128) MD5
RC4-MD5 SSLv2 RSA RSA RC4(128) MD5
DES-CBC-SHA SSLv3 RSA RSA DES(56) SHA1
RC4-64-MD5 SSLv2 RSA RSA RC4(64) MD5
DES-CBC-MD5 SSLv2 RSA RSA DES(56) MD5
EXP-DES-CBC-SHA SSLv3 RSA(512) RSA DES(40) SHA1 export
EXP-RC2-CBC-MD5 SSLv3 RSA(512) RSA RC2(40) MD5 export
EXP-RC4-MD5 SSLv3 RSA(512) RSA RC4(40) MD5 export
EXP-RC2-CBC-MD5 SSLv2 RSA(512) RSA RC2(40) MD5 export
EXP-RC4-MD5 SSLv2 RSA(512) RSA RC4(40) MD5 export
NULL-SHA SSLv3 RSA RSA None SHA1
NULL-MD5 SSLv3 RSA RSA None MD5
Diffie-Hellman Ciphers:
ADH-DES-CBC3-SHA SSLv3 DH None 3DES(168) SHA1
ADH-DES-CBC-SHA SSLv3 DH None DES(56) SHA1
ADH-RC4-MD5 SSLv3 DH None RC4(128) MD5
EDH-RSA-DES-CBC3-SHA SSLv3 DH RSA 3DES(168) SHA1
EDH-DSS-DES-CBC3-SHA SSLv3 DH DSS 3DES(168) SHA1
EDH-RSA-DES-CBC-SHA SSLv3 DH RSA DES(56) SHA1
EDH-DSS-DES-CBC-SHA SSLv3 DH DSS DES(56) SHA1
EXP-EDH-RSA-DES-CBC-SHA SSLv3 DH(512) RSA DES(40) SHA1 export
EXP-EDH-DSS-DES-CBC-SHA SSLv3 DH(512) DSS DES(40) SHA1 export
EXP-ADH-DES-CBC-SHA SSLv3 DH(512) None DES(40) SHA1 export
EXP-ADH-RC4-MD5 SSLv3 DH(512) None RC4(40) MD5 export