=pod

=head1 NAME

EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal,
EVP_DigestVerify - 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, size_t cnt);
 int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const unsigned char *sig,
                           size_t siglen);
 int EVP_DigestVerify(EVP_MD_CTX *ctx, const unsigned char *sigret,
                      size_t siglen, const unsigned char *tbs, size_t tbslen);

=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<e> and public key B<pkey>. B<ctx> must be created
with EVP_MD_CTX_new() 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. Note that any existing
value in B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be freed
directly by the application if B<ctx> is not assigned an EVP_PKEY_CTX value before
being passed to EVP_DigestVerifyInit() (which means the EVP_PKEY_CTX is created
inside EVP_DigestVerifyInit() and it will be freed automatically when the
EVP_MD_CTX is freed).

No B<EVP_PKEY_CTX> will be created by EVP_DigestSignInit() if the passed B<ctx>
has already been assigned one via L<EVP_MD_CTX_set_ctx(3)>. See also L<SM2(7)>.

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>.

EVP_DigestVerify() verifies B<tbslen> bytes at B<tbs> against the signature
in B<sig> of length B<siglen>.

=head1 RETURN VALUES

EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0
for failure.

EVP_DigestVerifyFinal() and EVP_DigestVerify() return 1 for success; any other
value indicates failure.  A return value of zero indicates that the signature
did not verify successfully (that is, B<tbs> did not match the original data or
the signature had an invalid form), while other values indicate a more serious
error (and sometimes also indicate an invalid signature form).

The error codes can be obtained from L<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.

EVP_DigestVerify() is a one shot operation which verifies a single block of
data in one function. For algorithms that support streaming it is equivalent
to calling EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal(). For
algorithms which do not support streaming (e.g. PureEdDSA) it is the only way
to verify data.

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.
If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to
external circumstances (see L<RAND(7)>), the operation will fail.

The call to EVP_DigestVerifyFinal() internally finalizes a copy of the digest
context. This means that 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_free() or a memory leak
will occur.

=head1 SEE ALSO

L<EVP_DigestSignInit(3)>,
L<EVP_DigestInit(3)>,
L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,
L<MD5(3)>, L<MDC2(3)>, L<RIPEMD160(3)>,
L<SHA1(3)>, L<dgst(1)>,
L<RAND(7)>

=head1 HISTORY

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

=head1 COPYRIGHT

Copyright 2006-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
