Generates a digital signature by encrypting the underlying hash value
with a private key pertaining to the context used to create this hash object.
If KeyExchange is True, the key-exchange key will be used. Otherwise,
the singature key will be used.
Return Value: a CryptoBlob object containing signature blob.
Set Blob = Hash.Sign( True )
Related Section: 6.3 Creating and Verifying Digital Signatures.