Ubuntu Manpage: SSL_CTX_set_cipher_list, SSL_set_cipher_list, SSL_CTX_set_ciphersuites, SSL_set_ciphersuites

Provided by: libssl-doc_1.1.1-1ubuntu2.1~18.04.23_all

NAME

       SSL_CTX_set_cipher_list, SSL_set_cipher_list, SSL_CTX_set_ciphersuites, SSL_set_ciphersuites - choose
       list of available SSL_CIPHERs

SYNOPSIS

        #include <openssl/ssl.h>

        int SSL_CTX_set_cipher_list(SSL_CTX *ctx, const char *str);
        int SSL_set_cipher_list(SSL *ssl, const char *str);

        int SSL_CTX_set_ciphersuites(SSL_CTX *ctx, const char *str);
        int SSL_set_ciphersuites(SSL *s, const char *str);

DESCRIPTION

       SSL_CTX_set_cipher_list() sets the list of available ciphers (TLSv1.2 and below) 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. This function does not impact TLSv1.3 ciphersuites. Use
       SSL_CTX_set_ciphersuites() to configure those.

       SSL_set_cipher_list() sets the list of ciphers (TLSv1.2 and below) only for ssl.

       SSL_CTX_set_ciphersuites() is used to configure the available TLSv1.3 ciphersuites for ctx. This is a
       simple colon (":") separated list of TLSv1.3 ciphersuite names in order of perference. Valid TLSv1.3
       ciphersuite names are:

       TLS_AES_128_GCM_SHA256
       TLS_AES_256_GCM_SHA384
       TLS_CHACHA20_POLY1305_SHA256
       TLS_AES_128_CCM_SHA256
       TLS_AES_128_CCM_8_SHA256

       An empty list is permissible. The default value for the this setting is:

       "TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256"

       SSL_set_ciphersuites()  is  the  same as SSL_CTX_set_ciphersuites() except it configures the ciphersuites
       for ssl.

NOTES

The control string str for SSL_CTX_set_cipher_list() and SSL_set_cipher_list() 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 unless the security level excludes it. 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 ciphers using DHE 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 an 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.

RETURN VALUES

       SSL_CTX_set_cipher_list()  and  SSL_set_cipher_list()  return  1 if any cipher could be selected and 0 on
       complete failure.

       SSL_CTX_set_ciphersuites() and SSL_set_ciphersuites() return 1 if  the  requested  ciphersuite  list  was
       configured, and 0 otherwise.

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