.Dd $Mdocdate: November 2 2016 $
.Dt DSA_SET_METHOD 3
.Os
.Sh NAME
.Nm DSA_set_default_method ,
.Nm DSA_get_default_method ,
.Nm DSA_set_method ,
.Nm DSA_new_method ,
.Nm DSA_OpenSSL ,
.Nm DSA_set_default_openssl_method ,
.Nm DSA_get_default_openssl_method
.Nd select DSA method
.Sh SYNOPSIS
.In openssl/dsa.h
.In openssl/engine.h
.Ft void
.Fo DSA_set_default_method
.Fa "const DSA_METHOD *meth"
.Fc
.Ft const DSA_METHOD *
.Fn DSA_get_default_method void
.Ft int
.Fo DSA_set_method
.Fa "DSA *dsa"
.Fa "const DSA_METHOD *meth"
.Fc
.Ft DSA *
.Fo DSA_new_method
.Fa "ENGINE *engine"
.Fc
.Ft DSA_METHOD *
.Fn DSA_OpenSSL void
.Sh DESCRIPTION
A
.Vt DSA_METHOD
specifies the functions that OpenSSL uses for DSA operations.
By modifying the method, alternative implementations such as hardware
accelerators may be used.
See the
.Sx CAVEATS
section for how these DSA API functions are affected by the use of
.Xr engine 3
API calls.
.Pp
Initially, the default
.Vt DSA_METHOD
is the OpenSSL internal implementation, as returned by
.Fn DSA_OpenSSL .
.Pp
.Fn DSA_set_default_method
makes
.Fa meth
the default method for all
.Vt DSA
structures created later.
.Sy NB :
This is true only whilst no
.Vt ENGINE
has been set as a default for DSA, so this function is no longer
recommended.
.Pp
.Fn DSA_get_default_method
returns a pointer to the current default
.Vt DSA_METHOD .
However, the meaningfulness of this result is dependent on whether the
.Xr engine 3
API is being used, so this function is no longer recommended.
.Pp
.Fn DSA_set_method
selects
.Fa meth
to perform all operations using the key
.Fa dsa .
This will replace the
.Vt DSA_METHOD
used by the DSA key and if the previous method was supplied by an
.Vt ENGINE ,
the handle to that
.Vt ENGINE
will be released during the change.
It is possible to have DSA keys that only work with certain
.Vt DSA_METHOD
implementations (eg. from an
.Vt ENGINE
module that supports embedded hardware-protected keys),
and in such cases attempting to change the
.Vt DSA_METHOD
for the key can have unexpected results.
.Pp
.Fn DSA_new_method
allocates and initializes a
.Vt DSA
structure so that
.Fa engine
will be used for the DSA operations.
If
.Fa engine
is
.Dv NULL ,
the default engine for DSA operations is used, and if no
default
.Vt ENGINE
is set, the
.Vt DSA_METHOD
controlled by
.Fn DSA_set_default_method
is used.
.Sh THE DSA_METHOD STRUCTURE
.Bd -literal
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;
.Ed
.Sh RETURN VALUES
.Fn DSA_OpenSSL
and
.Fn DSA_get_default_method
return pointers to the respective
.Vt DSA_METHOD Ns s.
.Pp
.Fn DSA_set_method
returns non-zero if the provided
.Fa meth
was successfully set as the method for
.Fa dsa
(including unloading the
.Vt ENGINE
handle if the previous method was supplied by an
.Vt ENGINE ) .
.Pp
.Fn DSA_new_method
returns
.Dv NULL
and sets an error code that can be obtained by
.Xr ERR_get_error 3
if the allocation fails.
Otherwise it returns a pointer to the newly allocated structure.
.Sh SEE ALSO
.Xr dsa 3 ,
.Xr DSA_new 3
.Sh HISTORY
.Fn DSA_set_default_method ,
.Fn DSA_get_default_method ,
.Fn DSA_set_method ,
.Fn DSA_new_method ,
and
.Fn DSA_OpenSSL
were added in OpenSSL 0.9.4.
.Pp
.Fn DSA_set_default_openssl_method
and
.Fn DSA_get_default_openssl_method
replaced
.Fn DSA_set_default_method
and
.Fn DSA_get_default_method
respectively, and
.Fn DSA_set_method
and
.Fn DSA_new_method
were altered to use
.Vt ENGINE Ns s
rather than
.Vt DSA_METHOD Ns s
during development of the engine version of OpenSSL 0.9.6.
For 0.9.7, the handling of defaults in the
.Xr engine 3
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
.Xr engine 3
API now transparently overrides the behaviour of defaults in the
DSA API without requiring changing these function prototypes.
.Sh CAVEATS
As of version 0.9.7, DSA_METHOD implementations are grouped together
with other algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in
.Vt ENGINE
modules.
If a default
.Vt ENGINE
is specified for DSA functionality using an
.Xr engine 3
API function, that will override any DSA defaults set using the DSA API
.Pq ie. DSA_set_default_method .
For this reason, the
.Xr engine 3
API is the recommended way to control default implementations for
use in DSA and other cryptographic algorithms.