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>

 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));

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, 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 create 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 parameters, 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_SINGLE_DH_USE has an impact on the computer time needed during negotiation, but it is not very large, so application 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 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. Authors may also generate their own set of parameters using dhparam(1), but a user may not be sure how the parameters were generated. The generation of DH parameters during installation is therefore recommended.

An application may either directly specify the DH parameters 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.

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);
 }

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.

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