Verifies a detached PKCS#7 signature specified by Signature
against a hash value contained in Hash and a public key contained in Cert.
Returns True if the signature is verified, or False otherwise.
Usage:
Set Cert = CM.ImportCertFromFile("c:\mycert.cer")
Set Hash = Context.CreateHash ' SHA by default
Hash.AddText "secret"
SignatureValid = Msg.VerifySignature(Signature, Hash, Cert)
Related Section: 9.4 Generating and Verifying Detached PKCS#7 Signatures.