SSL_CTX_SET_TMP_DH_CAL... BSD Library Functions Manual SSL_CTX_SET_TMP_DH_CAL...

     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

     #include <openssl/ssl.h>

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

     SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);

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

     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 by ctx.  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.

     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(1) dhparam 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 openssl(1) dhparam
     application.  Generation of custom DH parameters during installation should
     still be preferred to stop an attacker from specializing on a commonly used
     group.  The 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 ciphersuites 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.

     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.

     Set up DH parameters with a key length of 2048 bits.  Error handling is
     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) {
             /* Error. */

     openssl(1), ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),

     SSL_CTX_set_tmp_dh_callback() and SSL_CTX_set_tmp_dh() first appeared in
     SSLeay 0.8.0 and have been available since OpenBSD 2.4.

     SSL_set_tmp_dh_callback() and SSL_set_tmp_dh() first appeared in OpenSSL
     0.9.2b and have been available since OpenBSD 2.6.

BSD                              March 30, 2020                              BSD