Ubuntu Manpage: SSL_CTX_set_cipher_list, SSL_set_cipher_list, SSL_CTX_set

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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