Implement Proof Of Possession capability for all public key crypto types

[drskalman]

Description

• Implement ProofOfPossession traits with default implementation.

• Implement their derive macro in a new proc-macro crate in pubkeycrypto create which eventually should contains all pubkey crypto scheme as recommended by Move crypto implementations out of sp-core #1975

• Derive ProofOfPossession for all pubkey crypto type beside BLS.

• TODO: Implement Nugget BLS proof of possession which should certifies that the unique secret key known to the prover is used to generate both public keys in G1 and G2.

• TODO: Implement sign Host function for BLS12-381 necessary to be able to generate proof of possion through runtime.

• TODO: Implement generation and verification of proof of possession functions in all RuntimeApp and RuntimeAppPublic s.

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[burdges]

We added backcerts of operator keys alraedy, no? Or was that an RFC but not yet implemented?

We could use backcerts for proof-of-possession. Ideally, we should place system randomness into those backcerts and place them on-chain, but this requires tweaking ed25519 I guess.

[drskalman]

@burdges Could you point to the backcerts PR or its RFC so I can take a look? Thank you.

[burdges]

polkadot-fellows/RFCs#48

It's seemingly just ownership proofs, but really they should sign the operators key, making them backcerts.

I proposed having some master session key too, but that's a bigger change. We could simply back certify both the operators' key and some piece of system randomness sampled at node startup.