SSL_CTX_set_tmp_rsa_callbackOpenSSL_CTX_set_tmp_rsa_callback(3)





NAME
       SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa,
       SSL_CTX_need_tmp_rsa, SSL_set_tmp_rsa_callback,
       SSL_set_tmp_rsa, SSL_need_tmp_rsa - handle RSA keys for
       ephemeral key exchange

SYNOPSIS
        #include <openssl/ssl.h>

        void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
                   RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
        long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);

        void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
                   RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
        long SSL_need_tmp_rsa(SSL *ssl)

        RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));

DESCRIPTION
       SSL_CTX_set_tmp_rsa_callback() sets the callback func-
       tion for ctx to be used when a temporary/ephemeral RSA
       key is required to tmp_rsa_callback.  The callback is
       inherited by all SSL objects newly created from ctx with
       <SSL_new(3)|SSL_new(3)>. Already created SSL objects are
       not affected.

       SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA
       key to be used to be rsa. The key is inherited by all
       SSL objects newly created from ctx with
       <SSL_new(3)|SSL_new(3)>. Already created SSL objects are
       not affected.

       SSL_CTX_need_tmp_rsa() returns 1, if a tempo-
       rary/ephemeral RSA key is needed for RSA-based strength-
       limited 'exportable' ciphersuites because a RSA key with
       a keysize larger than 512 bits is installed.

       SSL_set_tmp_rsa_callback() sets the callback only for
       ssl.

       SSL_set_tmp_rsa() sets the key only for ssl.

       SSL_need_tmp_rsa() returns 1, if a temporary/ephemeral
       RSA key is needed, for RSA-based strength-limited
       'exportable' ciphersuites because a RSA key with a key-
       size larger than 512 bits is installed.

       These functions apply to SSL/TLS servers only.

NOTES
       When using a cipher with RSA authentication, an
       ephemeral RSA key exchange can take place. In this case
       the session data are negotiated using the ephemeral/tem-
       porary RSA key and the RSA key supplied and certified by
       the certificate chain is only used for signing.

       Under previous export restrictions, ciphers with RSA
       keys shorter (512 bits) than the usual key length of
       1024 bits were created. To use these ciphers with RSA
       keys of usual length, an ephemeral key exchange must be
       performed, as the normal (certified) key cannot be
       directly used.

       Using ephemeral RSA key exchange yields forward secrecy,
       as the connection can only be decrypted, when the RSA
       key is known. By generating a temporary RSA key inside
       the server application that is lost when the application
       is left, it becomes impossible for an attacker to
       decrypt past sessions, even if he gets hold of the nor-
       mal (certified) RSA key, as this key was used for sign-
       ing only. The downside is that creating a RSA key is
       computationally expensive.

       Additionally, the use of ephemeral RSA key exchange is
       only allowed in the TLS standard, when the RSA key can
       be used for signing only, that is for export ciphers.
       Using ephemeral RSA key exchange for other purposes vio-
       lates the standard and can break interoperability with
       clients.  It is therefore strongly recommended to not
       use ephemeral RSA key exchange and use EDH (Ephemeral
       Diffie-Hellman) key exchange instead in order to achieve
       forward secrecy (see SSL_CTX_set_tmp_dh_callback(3)).

       On OpenSSL servers ephemeral RSA key exchange is there-
       fore disabled by default and must be explicitly enabled
       using the SSL_OP_EPHEMERAL_RSA option of
       SSL_CTX_set_options(3), violating the TLS/SSL standard.
       When ephemeral RSA key exchange is required for export
       ciphers, it will automatically be used without this
       option!

       An application may either directly specify the key or
       can supply the key via a callback function. The callback
       approach has the advantage, that the callback may gener-
       ate the key only in case it is actually needed. As the
       generation of a RSA key is however costly, it will lead
       to a significant delay in the handshake procedure.
       Another advantage of the callback function is that it
       can supply keys of different size (e.g. for
       SSL_OP_EPHEMERAL_RSA usage) while the explicit setting
       of the key is only useful for key size of 512 bits to
       satisfy the export restricted ciphers and does give away
       key length if a longer key would be allowed.

       The tmp_rsa_callback is called with the keylength needed
       and the is_export information. The is_export flag is
       set, when the ephemeral RSA key exchange is performed
       with an export cipher.

EXAMPLES
       Generate temporary RSA keys to prepare ephemeral RSA key
       exchange. As the generation of a RSA key costs a lot of
       computer time, they saved for later reuse. For demon-
       stration purposes, two keys for 512 bits and 1024 bits
       respectively are generated.

        ...
        /* Set up ephemeral RSA stuff */
        RSA *rsa_512 = NULL;
        RSA *rsa_1024 = NULL;

        rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
        if (rsa_512 == NULL)
            evaluate_error_queue();

        rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
        if (rsa_1024 == NULL)
          evaluate_error_queue();

        ...

        RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
        {
           RSA *rsa_tmp=NULL;

           switch (keylength) {
           case 512:
             if (rsa_512)
               rsa_tmp = rsa_512;
             else { /* generate on the fly, should not happen in this example */
               rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
               rsa_512 = rsa_tmp; /* Remember for later reuse */
             }
             break;
           case 1024:
             if (rsa_1024)
               rsa_tmp=rsa_1024;
             else
               should_not_happen_in_this_example();
             break;
           default:
             /* Generating a key on the fly is very costly, so use what is there */
             if (rsa_1024)
               rsa_tmp=rsa_1024;
             else
               rsa_tmp=rsa_512; /* Use at least a shorter key */
           }
           return(rsa_tmp);
        }

RETURN VALUES
       SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_call-
       back() do not return diagnostic output.

       SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1
       on success and 0 on failure. Check the error queue to
       find out the reason of failure.

       SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1
       if a temporary RSA key is needed and 0 otherwise.

SEE ALSO
       ssl(3), SSL_CTX_set_cipher_list(3),
       SSL_CTX_set_options(3), SSL_CTX_set_tmp_dh_callback(3),
       SSL_new(3), ciphers(1)



0.9.7c                     2001-SSL_CTX_set_tmp_rsa_callback(3)
