Not sure which paper you are reading, but a dig signature is really a mechanism like:
- Hash message
- Encrypt hash with your private key
To validate a doc is signed then a reader
- Hash the doc
- Decrypt the signature with the senders public key and compare that with the above hash.
They come in 2 forms usually,
- The sig is appended to the doc (so you strip that before hash check)
- The sig is separate from the doc (wwe use this part a lot).
So signing is opposite of encrypt really in asymmetric world, where you encrypt data with the receiver’s public key and send to them. So signing is really encryption, this is the only encryption in bitcoin really, this signature part.