So in order for you to copyright your piece of work - any work -- it has to meet criteria of minimum creative value. According to "An artist’s guide to copyrights" by Laura Levin-Dando:
a broad range of creative works, such as photographs, visual art, audio recordings, sheet music, novels, and screenplays. If you expended creative effort to make something, you created it independently without copying someone else’s work, and it is embodied in a physical format
Therefore, things that deemed to lack creative value or are facts (phone numbers) cannot be copyrighted. Where does this leave sequence of characters like digital signatures or hashes? They arguably don't have artistic value on its own - while they are unique, it just machine-generated sequence of bytes. It would be different if, for example, you used bunch of existing or valid hashes to compose them in ASCII art - but it would protect that piece of ASCII art only, wouldnt it?
Plus, from my point of view - they are factual things.
1.0.2.003 has md5hash
aea603dba12c6e23b7b0ab9db4f81120 - this is a fact. A file, containing list of hashes for list of files is not copyright protected on its own, even if it is a part of something which partially protected by copyright (software, video game).
Am I missing something?
P.S. If we delve into patent territory, then you can patent the mechanism by which has generated but not the product -- hash/digital signature -- itself.
P.P.S I'm talking in this specific case about signatures used to sign files but not the authoritative ones like SLL Certificates, signed and what not or personal digital signatures. Just a signature for the encrypted container that is regenerated anew every time it is created.