SSL_CTX_set_cipher_list, SSL_set_cipher_list — choose list of available SSL_CIPHERs
int SSL_CTX_set_cipher_list(SSL_CTX *ctx, const char *str); int SSL_set_cipher_list(SSL *ssl, const char *str);
SSL_CTX_set_cipher_list() sets the list of available ciphers for ctx using the control string str. The format of the string is described in ciphers(1). The list of ciphers is inherited by all ssl objects created from ctx.
SSL_set_cipher_list() sets the list of ciphers only for ssl.
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 at all.
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 don't 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 SSL_CTX_set_tmp_rsa_callback(3)). RSA ciphers using EDH need a certificate and key and additional DH-parameters (see SSL_CTX_set_tmp_dh_callback(3)).
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 (see SSL_CTX_set_tmp_dh_callback(3)).
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.
SSL_CTX_set_cipher_list() and SSL_set_cipher_list() return 1 if any cipher could be selected and 0 on complete failure.