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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 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.

1.0.2h                            2016-05-03                 DSA_set_method(3)

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