The control string str should be universally usable and not depend
on details of the library configuration (ciphers compiled in). Thus no
syntax checking takes place. Items that are not recognized, because the
corresponding ciphers are not compiled in or because they are mistyped,
are simply ignored. Failure is only flagged if no ciphers could be collected
It should be noted, that inclusion of a cipher to be used into the list is
a necessary condition. On the client side, the inclusion into the list is
also sufficient. On the server side, additional restrictions apply. All ciphers
have additional requirements. ADH ciphers dont need a certificate, but
DH-parameters must have been set. All other ciphers need a corresponding
certificate and key.
A RSA cipher can only be chosen, when a RSA certificate is available.
RSA export ciphers with a keylength of 512 bits for the RSA key require
a temporary 512 bit RSA key, as typically the supplied key has a length
of 1024 bit (see
RSA ciphers using EDH need a certificate and key and additional DH-parameters
A DSA cipher can only be chosen, when a DSA certificate is available.
DSA ciphers always use DH key exchange and therefore need DH-parameters
When these conditions are not met for any cipher in the list (e.g. a
client only supports export RSA ciphers with a asymmetric key length
of 512 bits and the server is not configured to use temporary RSA
keys), the no shared cipher (SSL_R_NO_SHARED_CIPHER) error is generated
and the handshake will fail.
If the cipher list does not contain any SSLv2 cipher suites (this is the
default) then SSLv2 is effectively disabled and neither clients nor servers
will attempt to use SSLv2.