NAME

       EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal, EVP_DigestVerify -
       EVP signature verification functions

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);

DESCRIPTION

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

       EVP_DigestVerifyInit() sets up verification context ctx to use digest type from ENGINE e
       and public key pkey. ctx must be created with EVP_MD_CTX_new() before calling this
       function. If pctx is not NULL, the EVP_PKEY_CTX of the verification operation will be
       written to *pctx: this can be used to set alternative verification options. Note that any
       existing value in *pctx is overwritten. The EVP_PKEY_CTX value returned must not be freed
       directly by the application if ctx is not assigned an EVP_PKEY_CTX value before being
       passed to EVP_DigestSignInit() (which means the EVP_PKEY_CTX is created inside
       EVP_DigestSignInit() and it will be freed automatically when the EVP_MD_CTX is freed).

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

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

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

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

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, 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 ERR_get_error(3).

NOTES

       The 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 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_free() or a memory leak will occur.

SEE ALSO

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

HISTORY

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

COPYRIGHT

       Copyright 2006-2018 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 <https://www.openssl.org/source/license.html>.