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DSA_SET_METHOD(3) Library Functions Manual DSA_SET_METHOD(3)

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

#include <openssl/dsa.h>

DSA_set_default_method(const DSA_METHOD *meth);

const DSA_METHOD *

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

DSA_new_method(ENGINE *engine);


A DSA_METHOD object contains pointers to the functions used for DSA operations. By default, the internal implementation returned by DSA_OpenSSL() is used. By selecting another method, alternative implementations such as hardware accelerators may be used.

DSA_set_default_method() selects meth as the default method for all DSA structures created later. If any ENGINE was registered with ENGINE_register_DSA(3) that can be successfully initialized, it overrides the default.

DSA_get_default_method() returns a pointer to the current default method, even if it is actually overridded by an ENGINE.

DSA_set_method() selects meth to perform all operations using the key dsa. This replaces the DSA_METHOD used by the DSA key and if the previous method was supplied by an ENGINE, ENGINE_finish(3) is called on it. It is possible to have DSA keys that only work with certain DSA_METHOD implementations (e.g. 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 is used for the DSA operations. If engine is NULL, ENGINE_get_default_DSA(3) is used. If that returns NULL, the default method controlled by DSA_set_default_method() is used.

The DSA_METHOD structure is defined as follows:

     /* 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_OpenSSL() and DSA_get_default_method() return pointers to the respective DSA_METHOD.

DSA_set_method() returns 1 on success or 0 on failure. Currently, it cannot fail.

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.

DSA_meth_new(3), DSA_new(3), ENGINE_get_default_DSA(3), ENGINE_register_DSA(3), ENGINE_set_default_DSA(3)

DSA_set_default_method(), DSA_get_default_method(), DSA_set_method(), DSA_new_method(), and DSA_OpenSSL() first appeared in OpenSSL 0.9.5 and have been available since OpenBSD 2.7.
April 18, 2018 OpenBSD-current