EVP_VerifyInit, EVP_VerifyUpdate, EVP_VerifyFinal — EVP signature verification functions
int EVP_VerifyInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl); int EVP_VerifyUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt); int EVP_VerifyFinal(EVP_MD_CTX *ctx,unsigned char *sigbuf, unsigned int siglen,EVP_PKEY *pkey);
int EVP_VerifyInit(EVP_MD_CTX *ctx, const EVP_MD *type);
The EVP signature verification routines are a high level interface to digital signatures.
EVP_VerifyInit_ex() sets up verification context ctx to use digest type from ENGINE impl. ctx must be initialized by calling EVP_MD_CTX_init() before calling this function.
EVP_VerifyUpdate() 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.
EVP_VerifyFinal() verifies the data in ctx using the public key pkey and against the siglen bytes at sigbuf.
EVP_VerifyInit() initializes verification context ctx to use the default implementation of digest type.
EVP_VerifyInit_ex() and EVP_VerifyUpdate() return 1 for success and 0 for failure.
EVP_VerifyFinal() returns 1 for a correct signature, 0 for failure and -1 if some other error occurred.
The error codes can be obtained by ERR_get_error(3).
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.
Due to the link between message digests and public key algorithms the correct digest algorithm must be used with the correct public key type. A list of algorithms and associated public key algorithms appears in EVP_DigestInit(3).
The call to EVP_VerifyFinal() 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.
Older versions of this documentation wrongly stated that calls to EVP_VerifyUpdate() could not be made after calling EVP_VerifyFinal().
Since the public key is passed in the call to EVP_SignFinal() any error relating to the private key (for example an unsuitable key and digest combination) will not be indicated until after potentially large amounts of data have been passed through EVP_SignUpdate().
It is not possible to change the signing parameters using these function.
The previous two bugs are fixed in the newer EVP_VerifyDigest*() function.
EVP_VerifyInit(), EVP_VerifyUpdate() and EVP_VerifyFinal() are available in all versions of SSLeay and OpenSSL.
EVP_VerifyInit_ex() was added in OpenSSL 0.9.7