MirBSD manpage: SSL_CTX_set_tmp_dh(3), SSL_CTX_set_tmp_dh_callback(3), SSL_set_tmp_dh(3), SSL_set_tmp_dh_callback(3)



     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 per-
     manent 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

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     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 (which it is). 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 dur-
     ing 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 com-
     puter time needed during negotiation, but it is not very
     large, so application authors/users should consider to
     always enable this option. We did this in MirBSD for you.

     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) applica-
     tion. 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

     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

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     the ephemeral DH key exchange is performed with an export


     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);
      /* "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);

      /* "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;

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         switch (keylength) {
         case 512:
           if (!dh_512)
             dh_512 = get_dh512();
           dh_tmp = dh_512;
         case 1024:
           if (!dh_1024)
             dh_1024 = get_dh1024();
           dh_tmp = dh_1024;
           /* Generating a key on the fly is very costly, so use what is there */


     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)

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