DH_set_method(3)

     NAME
          DH_set_default_method, DH_get_default_method, DH_set_method,
          DH_new_method, DH_OpenSSL - select DH method

     SYNOPSIS
           #include <openssl/dh.h>
           #include <openssl/engine.h>

           void DH_set_default_method(const DH_METHOD *meth);

           const DH_METHOD *DH_get_default_method(void);

           int DH_set_method(DH *dh, const DH_METHOD *meth);

           DH *DH_new_method(ENGINE *engine);

           const DH_METHOD *DH_OpenSSL(void);


     DESCRIPTION
          A DH_METHOD specifies the functions that OpenSSL uses for
          Diffie-Hellman operations. By modifying the method,
          alternative implementations such as hardware accelerators
          may be used. IMPORTANT: See the NOTES section for important
          information about how these DH API functions are affected by
          the use of ENGINE API calls.

          Initially, the default DH_METHOD is the OpenSSL internal
          implementation, as returned by DH_OpenSSL().

          DH_set_default_method() makes meth the default method for
          all DH structures created later. NB: This is true only
          whilst no ENGINE has been set as a default for DH, so this
          function is no longer recommended.

          DH_get_default_method() returns a pointer to the current
          default DH_METHOD.  However, the meaningfulness of this
          result is dependant on whether the ENGINE API is being used,
          so this function is no longer recommended.

          DH_set_method() selects meth to perform all operations using
          the key dh.  This will replace the DH_METHOD used by the DH
          key and if the previous method was supplied by an ENGINE,
          the handle to that ENGINE will be released during the
          change. It is possible to have DH keys that only work with
          certain DH_METHOD implementations (eg. from an ENGINE module
          that supports embedded hardware-protected keys), and in such
          cases attempting to change the DH_METHOD for the key can
          have unexpected results.

          DH_new_method() allocates and initializes a DH structure so
          that engine will be used for the DH operations. If engine is
          NULL, the default ENGINE for DH operations is used, and if
          no default ENGINE is set, the DH_METHOD controlled by
          DH_set_default_method() is used.

     THE DH_METHOD STRUCTURE
           typedef struct dh_meth_st
           {
               /* name of the implementation */
                  const char *name;

               /* generate private and public DH values for key agreement */
                  int (*generate_key)(DH *dh);

               /* compute shared secret */
                  int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);

               /* compute r = a ^ p mod m (May be NULL for some implementations) */
                  int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
                                          const BIGNUM *m, BN_CTX *ctx,
                                          BN_MONT_CTX *m_ctx);

               /* called at DH_new */
                  int (*init)(DH *dh);

               /* called at DH_free */
                  int (*finish)(DH *dh);

                  int flags;

                  char *app_data; /* ?? */

           } DH_METHOD;


     RETURN VALUES
          DH_OpenSSL() and DH_get_default_method() return pointers to
          the respective DH_METHODs.

          DH_set_default_method() returns no value.

          DH_set_method() returns non-zero if the provided meth was
          successfully set as the method for dh (including unloading
          the ENGINE handle if the previous method was supplied by an
          ENGINE).

          DH_new_method() returns NULL and sets an error code that can
          be obtained by ERR_get_error(3) if the allocation fails.
          Otherwise it returns a pointer to the newly allocated
          structure.

     NOTES
          As of version 0.9.7, DH_METHOD implementations are grouped
          together with other algorithmic APIs (eg. RSA_METHOD,
          EVP_CIPHER, etc) in ENGINE modules. If a default ENGINE is
          specified for DH functionality using an ENGINE API function,
          that will override any DH defaults set using the DH API (ie.
          DH_set_default_method()). For this reason, the ENGINE API is
          the recommended way to control default implementations for
          use in DH and other cryptographic algorithms.

     SEE ALSO
          dh(3), DH_new(3)

     HISTORY
          DH_set_default_method(), DH_get_default_method(),
          DH_set_method(), DH_new_method() and DH_OpenSSL() were added
          in OpenSSL 0.9.4.

          DH_set_default_openssl_method() and
          DH_get_default_openssl_method() replaced
          DH_set_default_method() and DH_get_default_method()
          respectively, and DH_set_method() and DH_new_method() were
          altered to use ENGINEs rather than DH_METHODs during
          development of the engine version of OpenSSL 0.9.6. For
          0.9.7, the handling of defaults in the ENGINE API was
          restructured so that this change was reversed, and behaviour
          of the other functions resembled more closely the previous
          behaviour. The behaviour of defaults in the ENGINE API now
          transparently overrides the behaviour of defaults in the DH
          API without requiring changing these function prototypes.