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

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 certified by the certificate chain is only used for
       signing.  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 (certified) 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.

       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. This application guarantees that "strong"
       primes are used.

       Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current version of the OpenSSL
       distribution contain the 'SKIP' DH parameters, which use safe primes and were generated verifiably
       pseudo-randomly.  These files can be converted into C code using the -C option of the dhparam(1)
       application. Generation of custom DH parameters during installation should still be preferred to stop an
       attacker from specializing on a commonly used group. File dh1024.pem contains old parameters that must
       not be used by applications.

       An application may either directly specify the DH parameters or can supply the DH parameters via a
       callback function.

       Previous versions of the callback used is_export and keylength parameters to control parameter generation
       for export and non-export cipher suites. Modern servers that do not support export cipher suites are
       advised to either use SSL_CTX_set_tmp_dh() or alternatively, use the callback but ignore keylength and
       is_export and simply supply at least 2048-bit parameters in the callback.

EXAMPLES

       Setup DH parameters with a key length of 2048 bits. (Error handling partly left out.)

       Command-line parameter generation:

        $ openssl dhparam -out dh_param_2048.pem 2048

       Code for setting up parameters during server initialization:

        SSL_CTX ctx = SSL_CTX_new();

        DH *dh_2048 = NULL;
        FILE *paramfile = fopen("dh_param_2048.pem", "r");

        if (paramfile) {
            dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
            fclose(paramfile);
        } else {
            /* Error. */
        }
        if (dh_2048 == NULL)
            /* Error. */
        if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
            /* Error. */
        ...

RETURN VALUES

       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() 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(7), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3), ciphers(1), dhparam(1)

COPYRIGHT

       Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.

       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>.