SSL_CTX_SET_CIPHER_LIST

Section: OpenSSL (3)
Updated: 2021-03-26
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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 preference. 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.

COPYRIGHT

Licensed under the OpenSSL license (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.