=pod

=head1 NAME

DSA_sign, DSA_sign_setup, DSA_verify - DSA signatures

=head1 SYNOPSIS

 #include <openssl/dsa.h>

 int	DSA_sign(int type, const unsigned char *dgst, int len,
		unsigned char *sigret, unsigned int *siglen, DSA *dsa);

 int	DSA_sign_setup(DSA *dsa, BN_CTX *ctx, BIGNUM **kinvp,
                BIGNUM **rp);

 int	DSA_verify(int type, const unsigned char *dgst, int len,
		unsigned char *sigbuf, int siglen, DSA *dsa);

=head1 DESCRIPTION

DSA_sign() computes a digital signature on the B<len> byte message
digest B<dgst> using the private key B<dsa> and places its ASN.1 DER
encoding at B<sigret>. The length of the signature is places in
*B<siglen>. B<sigret> must point to DSA_size(B<dsa>) bytes of memory.

DSA_sign_setup() may be used to precompute part of the signing
operation in case signature generation is time-critical. It expects
B<dsa> to contain DSA parameters. It places the precomputed values
in newly allocated B<BIGNUM>s at *B<kinvp> and *B<rp>, after freeing
the old ones unless *B<kinvp> and *B<rp> are NULL. These values may
be passed to DSA_sign() in B<dsa-E<gt>kinv> and B<dsa-E<gt>r>.
B<ctx> is a pre-allocated B<BN_CTX> or NULL.
The precomputed values from DSA_sign_setup() B<MUST NOT be used> for
more than one signature: using the same B<dsa-E<gt>kinv> and
B<dsa-E<gt>r> pair twice under the same private key on different
plaintexts will result in permanently exposing the DSA private key.

DSA_verify() verifies that the signature B<sigbuf> of size B<siglen>
matches a given message digest B<dgst> of size B<len>.
B<dsa> is the signer's public key.

The B<type> parameter is ignored.

The PRNG must be seeded before DSA_sign() (or DSA_sign_setup())
is called.

=head1 RETURN VALUES

DSA_sign() and DSA_sign_setup() return 1 on success, 0 on error.
DSA_verify() returns 1 for a valid signature, 0 for an incorrect
signature and -1 on error. The error codes can be obtained by
L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 CONFORMING TO

US Federal Information Processing Standard FIPS 186 (Digital Signature
Standard, DSS), ANSI X9.30

=head1 SEE ALSO

L<dsa(3)|dsa(3)>, L<ERR_get_error(3)|ERR_get_error(3)>, L<rand(3)|rand(3)>,
L<DSA_do_sign(3)|DSA_do_sign(3)>

=head1 HISTORY

DSA_sign() and DSA_verify() are available in all versions of SSLeay.
DSA_sign_setup() was added in SSLeay 0.8.

=cut
