openssl/doc/crypto/DSA_set_method.pod

=pod

=head1 NAME

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

=head1 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);

=head1 DESCRIPTION

A B<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 B<ENGINE> API calls.

Initially, the default DSA_METHOD is the OpenSSL internal implementation,
as returned by DSA_OpenSSL().

DSA_set_default_method() makes B<meth> the default method for all DSA
structures created later. B<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 B<meth> to perform all operations using the key
B<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 B<engine>
will be used for the DSA operations. If B<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.

=head1 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;

=head1 RETURN VALUES

DSA_OpenSSL() and DSA_get_default_method() return pointers to the respective
B<DSA_METHOD>s.

DSA_set_default_method() returns no value.

DSA_set_method() returns non-zero if the provided B<meth> was successfully set as
the method for B<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 L<ERR_get_error(3)|ERR_get_error(3)> if the allocation
fails. Otherwise it returns a pointer to the newly allocated structure.

=head1 NOTES

As of version 0.9.7, DSA_METHOD implementations are grouped together with other
algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in B<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.

=head1 SEE ALSO

L<dsa(3)|dsa(3)>, L<DSA_new(3)|DSA_new(3)>

=head1 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 B<ENGINE>s rather than
B<DSA_METHOD>s 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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