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 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.