Hi Kevin,
I've done this via a third party library, but it isn't functionality that is included in NetTalk. Are you trying to create a digital signature by any chance?
Capesoft SafeUpdate creates (and reads) standard signatures using Canonical XML, SHA-1 hashing and Base64 encoding. If that is what you are looking for then CapeSoft SafeUpdate contains all the code that you need already.
cheers,
Sean