/usr/man/cat.3/DSA_OpenSSL.3

DSA_set_method(3)            OpenSSL            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 dependent 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

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DSA_set_method(3)            OpenSSL            DSA_set_method(3)

     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.

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DSA_set_method(3)            OpenSSL            DSA_set_method(3)

     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.

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See also DSA_get_default_method(3)
See also DSA_new_method(3)
See also DSA_set_default_method(3)
See also DSA_set_method(3)