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>
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 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:

    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_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.

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

HISTORY
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.