Messages signed by multiple parties

Developers
1

I’d like to be able to used a chained message signature from canister API that uses ICP’s caller-based authentiation.

Here’s the use case:

  1. User A signs message M
  2. User B approves and signs message M (now Mprime?)
  3. User B then sends message Mp to Canister C via API
  4. Canister C uses ICP’s caller based auth to see that the caller is B.
  5. Canister C then wants to take Mp and deconstruct it to M, proving that it was signed by A.

Essentially, I’m looking for some way of allowing multiple parties to sign a message such that I can know the initial principal that wrote the message, and can verify that another principal has signed off on the initial message. The ordering (knowing that A wrote M) is important.

Ideally this operation would be atomic and not require calls to the management canister to fetch a principal’s public key. It would be lovely if some of these operations could be abstracted away, similar to ICP’s caller based authentication.

2

There is no way to do this atomically at this time. What you could do today with the existing threshold signature implementation is

  1. A signs M generating sig_A
  2. A sends M and sig_A to B
  3. B combines M and sig_A into a single input (for example by concatenating them together, or putting them into a JSON struct)
  4. B signs the combination of M and sig_A generating sig_B
  5. C takes M, sig_A and sig_B and verifies both signatures.

Implementing a multi-canister threshold signature would be an interesting extension to what we currently have, but this is not (afaik) currently something on the roadmap.

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