# Are there use cases where a signature itself needs to be signed?

I'm writing some code to digitally sign certain files (JSON, if you must know), and I'm trying to understand the use cases, not being a crypto guy myself.

I understand there are use cases where several parties all need to sign the same content, e.g. the two parties who enter a contract sign it to indicate their approval.

But are there use cases where B signs not just the content (of the contract, or whatever), but the concatenation of content plus the previously-created signature by A of the content?

Update: given this discussion, I created a proposal that I believe covers the case in case you are interested: http://upon2020.com/blog/2014/03/digital-signatures-on-json-payloads-lets-call-it-jsonsig/

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You should take a look at JSON Web Signature (JWS). I didn't read it myself, so I can't vouch for its quality. –  CodesInChaos 13 hours ago
Duh! I was looking for something like that and failed to find it. Thanks for pointing it out. –  Johannes Ernst 13 hours ago
From what I can tell, this is about representing the signature/metadata in JSON, assuming blobs as payload, so it doesn't really address my use case. –  Johannes Ernst 13 hours ago

Yes. An independent witness to the signing may vouch for the initial signing, and do so by signing the whole document. E.g. this could indicate that the signing was done by an officer of the company and not a rogue employee.

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Thank you for actually answering my question :-) –  Johannes Ernst 12 hours ago
@JohannesErnst I'd rather encapsulate it, like {data={...}, sigs=[{sig1}, {sig2}]}. If one signature needs to be signed, the outer signature can encapsulate the previous one, else add them to the array. –  CodesInChaos 13 hours ago