#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);
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.
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).
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.