Functions to sign and verify data.
using openssl evp.c funtions
[specs taken from PHP doc's]
sign - computes a signature for the specified data by
using SHA1 for hashing followed by encryption using the
private key. Note that the data itself is not encrypted.
verify - verifies that the signature is correct for the
specified data using the public key. This must be the
public key corresponding to the private key used for