This is a perhaps unusual feature request: the autograph service provides an API that clients can use to request signatures. Most of the time, clients call /sign/data with a raw message to sign, and that works fine with openpgp. But in some cases, we want to just accept a pre-computed hash to reduce the bandwidth usage between a client and the autograph service.

Trying to implement this with the openpgp package seems rather challenging. From what I can tell, the packet#Signature.Sign() function takes an initialized hash, appends a suffix and finishes the computation of the digest, then signs. There's no obvious way to provide the function with a hash that's already computed. I'm also failing to think of a way to hack around this without touching the openpgp/packet package directly.

Would it be possible to implement a SignHash function in the packet package that takes a pre-computed hash? It might also be useful to export buildHashSuffix for client to assemble the hash themselves.