DSA_set_method(3)

     NAME
          DSA_set_default_method, DSA_get_default_method,
          DSA_set_method, DSA_new_method, DSA_OpenSSL - select DSA
          method

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

           void DSA_set_default_method(const DSA_METHOD *meth);

           const DSA_METHOD *DSA_get_default_method(void);

           int DSA_set_method(DSA *dsa, const DSA_METHOD *meth);

           DSA *DSA_new_method(ENGINE *engine);

           DSA_METHOD *DSA_OpenSSL(void);


     DESCRIPTION
          A DSA_METHOD specifies the functions that OpenSSL uses for
          DSA 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 DSA API functions are affected by the use of
          ENGINE API calls.

          Initially, the default DSA_METHOD is the OpenSSL internal
          implementation, as returned by DSA_OpenSSL().

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

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

          DSA_set_method() selects meth to perform all operations
          using the key rsa. This will replace the DSA_METHOD used by
          the DSA 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 DSA keys that only work
          with certain DSA_METHOD implementations (eg. from an ENGINE
          module that supports embedded hardware-protected keys), and
          in such cases attempting to change the DSA_METHOD for the
          key can have unexpected results.

          DSA_new_method() allocates and initializes a DSA structure
          so that engine will be used for the DSA operations. If
          engine is NULL, the default engine for DSA operations is
          used, and if no default ENGINE is set, the DSA_METHOD
          controlled by DSA_set_default_method() is used.

     THE DSA_METHOD STRUCTURE
          struct
           {
               /* name of the implementation */
                  const char *name;

               /* sign */
                  DSA_SIG *(*dsa_do_sign)(const unsigned char *dgst, int dlen,
                                           DSA *dsa);

               /* pre-compute k^-1 and r */
                  int (*dsa_sign_setup)(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
                                           BIGNUM **rp);

               /* verify */
                  int (*dsa_do_verify)(const unsigned char *dgst, int dgst_len,
                                           DSA_SIG *sig, DSA *dsa);

               /* compute rr = a1^p1 * a2^p2 mod m (May be NULL for some
                                                    implementations) */
                  int (*dsa_mod_exp)(DSA *dsa, BIGNUM *rr, BIGNUM *a1, BIGNUM *p1,
                                           BIGNUM *a2, BIGNUM *p2, BIGNUM *m,
                                           BN_CTX *ctx, BN_MONT_CTX *in_mont);

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

               /* called at DSA_new */
                  int (*init)(DSA *DSA);

               /* called at DSA_free */
                  int (*finish)(DSA *DSA);

                  int flags;

                  char *app_data; /* ?? */

           } DSA_METHOD;


     RETURN VALUES
          DSA_OpenSSL() and DSA_get_default_method() return pointers
          to the respective DSA_METHODs.

          DSA_set_default_method() returns no value.
          DSA_set_method() returns non-zero if the provided meth was
          successfully set as the method for dsa (including unloading
          the ENGINE handle if the previous method was supplied by an
          ENGINE).

          DSA_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, DSA_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 DSA functionality using an ENGINE API
          function, that will override any DSA defaults set using the
          DSA API (ie.  DSA_set_default_method()). For this reason,
          the ENGINE API is the recommended way to control default
          implementations for use in DSA and other cryptographic
          algorithms.

     SEE ALSO
          dsa(3), DSA_new(3)

     HISTORY
          DSA_set_default_method(), DSA_get_default_method(),
          DSA_set_method(), DSA_new_method() and DSA_OpenSSL() were
          added in OpenSSL 0.9.4.

          DSA_set_default_openssl_method() and
          DSA_get_default_openssl_method() replaced
          DSA_set_default_method() and DSA_get_default_method()
          respectively, and DSA_set_method() and DSA_new_method() were
          altered to use ENGINEs rather than DSA_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 DSA
          API without requiring changing these function prototypes.