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+=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, size_t 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 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.
+
+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
+for failure.
+
+EVP_DigestVerifyFinal() returns 1 for success; any other value indicates
+failure. A return value of zero indicates that the signature did not verify
+successfully (that is, 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.
+
+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 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)>,
+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)>
+
+=head1 HISTORY
+
+EVP_DigestVerifyInit(), EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal()
+were first added to OpenSSL 1.0.0.
+
+=head1 COPYRIGHT
+
+Copyright 2006-2017 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