SSL_CTX_set_tmp_dh_callback(OpenSSSL_CTX_set_tmp_dh_callback(3)





NAME
       SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh,
       SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys
       for ephemeral key exchange

SYNOPSIS
        #include <openssl/ssl.h>

        void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);

        void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_set_tmp_dh(SSL *ssl, DH *dh)

        DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));

DESCRIPTION
       SSL_CTX_set_tmp_dh_callback() sets the callback function
       for ctx to be used when a DH parameters are required to
       tmp_dh_callback.  The callback is inherited by all ssl
       objects created from ctx.

       SSL_CTX_set_tmp_dh() sets DH parameters to be used to be
       dh.  The key is inherited by all ssl objects created
       from ctx.

       SSL_set_tmp_dh_callback() sets the callback only for
       ssl.

       SSL_set_tmp_dh() sets the parameters only for ssl.

       These functions apply to SSL/TLS servers only.

NOTES
       When using a cipher with RSA authentication, an
       ephemeral DH key exchange can take place. Ciphers with
       DSA keys always use ephemeral DH keys as well.  In these
       cases, the session data are negotiated using the
       ephemeral/temporary DH key and the key supplied and cer-
       tified by the certificate chain is only used for sign-
       ing.  Anonymous ciphers (without a permanent server key)
       also use ephemeral DH keys.

       Using ephemeral DH key exchange yields forward secrecy,
       as the connection can only be decrypted, when the DH key
       is known. By generating a temporary DH 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 normal (cer-
       tified) key, as this key was only used for signing.

       In order to perform a DH key exchange the server must
       use a DH group (DH parameters) and generate a DH key.
       The server will always generate a new DH key during the
       negotiation, when the DH parameters are supplied via
       callback and/or when the SSL_OP_SINGLE_DH_USE option of
       SSL_CTX_set_options(3) is set. It will immediately cre-
       ate a DH key, when DH parameters are supplied via
       SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not
       set. In this case, it may happen that a key is generated
       on initialization without later being needed, while on
       the other hand the computer time during the negotiation
       is being saved.

       If "strong" primes were used to generate the DH parame-
       ters, it is not strictly necessary to generate a new key
       for each handshake but it does improve forward secrecy.
       If it is not assured, that "strong" primes were used
       (see especially the section about DSA parameters below),
       SSL_OP_SINGLE_DH_USE must be used in order to prevent
       small subgroup attacks. Always using SSL_OP_SIN-
       GLE_DH_USE has an impact on the computer time needed
       during negotiation, but it is not very large, so appli-
       cation authors/users should consider to always enable
       this option.

       As generating DH parameters is extremely time consuming,
       an application should not generate the parameters on the
       fly but supply the parameters.  DH parameters can be
       reused, as the actual key is newly generated during the
       negotiation. The risk in reusing DH parameters is that
       an attacker may specialize on a very often used DH
       group. Applications should therefore generate their own
       DH parameters during the installation process using the
       openssl dhparam(1) application. In order to reduce the
       computer time needed for this generation, it is possible
       to use DSA parameters instead (see dhparam(1)), but in
       this case SSL_OP_SINGLE_DH_USE is mandatory.

       Application authors may compile in DH parameters. Files
       dh512.pem, dh1024.pem, dh2048.pem, and dh4096 in the
       'apps' directory of current version of the OpenSSL dis-
       tribution contain the 'SKIP' DH parameters, which use
       safe primes and were generated verifiably pseudo-ran-
       domly.  These files can be converted into C code using
       the -C option of the dhparam(1) application.  Authors
       may also generate their own set of parameters using
       dhparam(1), but a user may not be sure how the parame-
       ters were generated. The generation of DH parameters
       during installation is therefore recommended.

       An application may either directly specify the DH param-
       eters or can supply the DH parameters via a callback
       function. The callback approach has the advantage, that
       the callback may supply DH parameters for different key
       lengths.

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

EXAMPLES
       Handle DH parameters for key lengths of 512 and 1024
       bits. (Error handling partly left out.)

        ...
        /* Set up ephemeral DH stuff */
        DH *dh_512 = NULL;
        DH *dh_1024 = NULL;
        FILE *paramfile;











        ...
        /* "openssl dhparam -out dh_param_512.pem -2 512" */
        paramfile = fopen("dh_param_512.pem", "r");
        if (paramfile) {
          dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
          fclose(paramfile);
        }
        /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
        paramfile = fopen("dh_param_1024.pem", "r");
        if (paramfile) {
          dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
          fclose(paramfile);
        }
        ...

        /* "openssl dhparam -C -2 512" etc... */
        DH *get_dh512() { ... }
        DH *get_dh1024() { ... }

        DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
        {
           DH *dh_tmp=NULL;

           switch (keylength) {
           case 512:
             if (!dh_512)
               dh_512 = get_dh512();
             dh_tmp = dh_512;
             break;
           case 1024:
             if (!dh_1024)
               dh_1024 = get_dh1024();
             dh_tmp = dh_1024;
             break;
           default:
             /* Generating a key on the fly is very costly, so use what is there */
             setup_dh_parameters_like_above();
           }
           return(dh_tmp);
        }

RETURN VALUES
       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_call-
       back() do not return diagnostic output.

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

SEE ALSO
       ssl(3), SSL_CTX_set_cipher_list(3),
       SSL_CTX_set_tmp_rsa_callback(3), SSL_CTX_set_options(3),
       ciphers(1), dhparam(1)



0.9.7c                     2001-0SSL_CTX_set_tmp_dh_callback(3)
