To verify the Stringsignature,digest, an
instance of OpenSSL::Digest, must be
provided to re-compute the message digest of the original data,
also a String. The return value is true
if the signature is valid, false otherwise. A PKeyError is raised should errors
occur. Any previous state of the Digest instance
is irrelevant to the validation outcome, the digest instance is reset to
its initial state during the operation.