/usr/man/cat.3/SSL_set_tmp_dh_callback.3

SSL_CTX_set_tmp_dh_callback(3OpenSSSSL_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,
                 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.

1.0.2t               Last change: 2019-09-10                    1

SSL_CTX_set_tmp_dh_callback(3OpenSSSSL_CTX_set_tmp_dh_callback(3)

     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. 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() 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();
      ...

1.0.2t               Last change: 2019-09-10                    2

SSL_CTX_set_tmp_dh_callback(3OpenSSSSL_CTX_set_tmp_dh_callback(3)

      /* 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);
        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(3), SSL_CTX_set_cipher_list(3),
     SSL_CTX_set_tmp_rsa_callback(3), SSL_CTX_set_options(3),
     ciphers(1), dhparam(1)

1.0.2t               Last change: 2019-09-10                    3

See also SSL_CTX_set_tmp_dh(3)
See also SSL_CTX_set_tmp_dh_callback(3)
See also SSL_set_tmp_dh(3)