NAME

       EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal - 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, unsigned int cnt);
        int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t siglen);

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
       impl and public key pkey. ctx must be initialized with EVP_MD_CTX_init() 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.

       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.

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 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.

       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.

SEE ALSO

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

HISTORY

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