.Dd $Mdocdate: November 3 2016 $ .Dt EVP_DIGESTVERIFYINIT 3 .Os .Sh NAME .Nm EVP_DigestVerifyInit , .Nm EVP_DigestVerifyUpdate , .Nm EVP_DigestVerifyFinal .Nd EVP signature verification functions .Sh SYNOPSIS .In openssl/evp.h .Ft int .Fo EVP_DigestVerifyInit .Fa "EVP_MD_CTX *ctx" .Fa "EVP_PKEY_CTX **pctx" .Fa "const EVP_MD *type" .Fa "ENGINE *e" .Fa "EVP_PKEY *pkey" .Fc .Ft int .Fo EVP_DigestVerifyUpdate .Fa "EVP_MD_CTX *ctx" .Fa "const void *d" .Fa "unsigned int cnt" .Fc .Ft int .Fo EVP_DigestVerifyFinal .Fa "EVP_MD_CTX *ctx" .Fa "unsigned char *sig" .Fa "size_t siglen" .Fc .Sh DESCRIPTION The EVP signature routines are a high level interface to digital signatures. .Pp .Fn EVP_DigestVerifyInit sets up verification context .Fa ctx to use digest .Fa type from .Vt ENGINE .Fa e and public key .Fa pkey . .Fa ctx must be initialized with .Xr EVP_MD_CTX_init 3 before calling this function. If .Fa pctx is not .Dv NULL , the .Vt EVP_PKEY_CTX of the verification operation will be written to .Pf * Fa pctx : this can be used to set alternative verification options. .Pp .Fn EVP_DigestVerifyUpdate hashes .Fa cnt bytes of data at .Fa d into the verification context .Fa ctx . This function can be called several times on the same .Fa ctx to include additional data. This function is currently implemented using a macro. .Pp .Fn EVP_DigestVerifyFinal verifies the data in .Fa ctx against the signature in .Fa sig of length .Fa siglen . .Pp 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. .Pp In previous versions of OpenSSL, there was a link between message digest types and public key algorithms. This meant that "clone" digests such as .Xr EVP_dss1 3 needed to be used to sign using SHA1 and DSA. This is no longer necessary and the use of clone digest is now discouraged. .Pp The call to .Fn EVP_DigestVerifyFinal internally finalizes a copy of the digest context. This means that calls to .Xr EVP_VerifyUpdate 3 and .Xr EVP_VerifyFinal 3 can be called later to digest and verify additional data. .Pp Since only a copy of the digest context is ever finalized, the context must be cleaned up after use by calling .Xr EVP_MD_CTX_cleanup 3 or a memory leak will occur. .Sh RETURN VALUES .Fn EVP_DigestVerifyInit and .Fn 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. .Pp Unlike other functions, the return value 0 from .Fn EVP_DigestVerifyFinal only indicates that the signature did not verify successfully. That is it did not match the original data or the signature was of invalid form. It is not an indication of a more serious error. .Pp The error codes can be obtained from .Xr ERR_get_error 3 . .Sh SEE ALSO .Xr ERR 3 , .Xr evp 3 , .Xr EVP_DigestInit 3 , .Xr EVP_DigestSignInit 3 .Sh HISTORY .Fn EVP_DigestVerifyInit , .Fn EVP_DigestVerifyUpdate , and .Fn EVP_DigestVerifyFinal were first added to OpenSSL 1.0.0.