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           key exchange


            #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)


           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.


           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 if either the DH parameters are
           supplied via callback or the SSL_OP_SINGLE_DH_USE option of
           SSL_CTX_set_options(3) is set (or both).  It will  immediately create a
           DH key if 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.
           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. Files dh1024.pem and dh512.pem
           contain 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 ciphersuites
           are advised to either use SSL_CTX_set_tmp_dh() in combination with
           SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore
           keylength and is_export and simply supply at least 2048-bit parameters
           in the callback.


           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();
            /* Set up ephemeral DH parameters. */
            DH *dh_2048 = NULL;
            FILE *paramfile;
            paramfile = fopen("dh_param_2048.pem", "r");
            if (paramfile) {
              dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
            } else {
              /* Error. */
            if (dh_2048 == NULL) {
             /* Error. */
            if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
           SSL_CTX_set_options(3), ciphers(1), dhparam(1)

    1.0.1e 2016-01-07 SSL_CTX_set_tmp_dh_callback(3)


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