=pod

=head1 NAME

EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal - EVP signature verification functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
			const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
 int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
 int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const unsigned char *sig, size_t siglen);

=head1 DESCRIPTION

The EVP signature routines are a high level interface to digital signatures.

EVP_DigestVerifyInit() sets up verification context B<ctx> to use digest
B<type> from ENGINE B<impl> and public key B<pkey>. B<ctx> must be initialized
with EVP_MD_CTX_init() before calling this function. If B<pctx> is not NULL the
EVP_PKEY_CTX of the verification operation will be written to B<*pctx>: this
can be used to set alternative verification options.

EVP_DigestVerifyUpdate() hashes B<cnt> bytes of data at B<d> into the
verification context B<ctx>. This function can be called several times on the
same B<ctx> to include additional data. This function is currently implemented
using a macro.

EVP_DigestVerifyFinal() verifies the data in B<ctx> against the signature in
B<sig> of length B<siglen>.

=head1 RETURN VALUES

EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0
or a negative value for failure. In particular a return value of -2 indicates
the operation is not supported by the public key algorithm.

Unlike other functions the return value 0 from EVP_DigestVerifyFinal() only
indicates that the signature did not not verify successfully (that is tbs did
not match the original data or the signature was of invalid form) it is not an
indication of a more serious error.

The error codes can be obtained from L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 NOTES

The B<EVP> interface to digital signatures should almost always be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the algorithm used and much more flexible.

In previous versions of OpenSSL there was a link between message digest types
and public key algorithms. This meant that "clone" digests such as EVP_dss1()
needed to be used to sign using SHA1 and DSA. This is no longer necessary and
the use of clone digest is now discouraged.

For some key types and parameters the random number generator must be seeded
or the operation will fail. 

The call to EVP_DigestVerifyFinal() internally finalizes a copy of the digest
context. This means that calls to EVP_VerifyUpdate() and EVP_VerifyFinal() can
be called later to digest and verify additional data.

Since only a copy of the digest context is ever finalized the context must
be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak
will occur.

=head1 SEE ALSO

L<EVP_DigestSignInit(3)|EVP_DigestSignInit(3)>,
L<EVP_DigestInit(3)|EVP_DigestInit(3)>, L<err(3)|err(3)>,
L<evp(3)|evp(3)>, L<hmac(3)|hmac(3)>, L<md2(3)|md2(3)>,
L<md5(3)|md5(3)>, L<mdc2(3)|mdc2(3)>, L<ripemd(3)|ripemd(3)>,
L<sha(3)|sha(3)>, L<dgst(1)|dgst(1)>

=head1 HISTORY

EVP_DigestVerifyInit(), EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal() 
were first added to OpenSSL 1.0.0.

=cut
