Implement server for Phase 2 of FROST for Zcash demos

[mpguerra]

Motivation

During the first set of demos we implemented socket based communications between the coordinator and participants with the coordinator demo acting as a server. In real world usage, the coordinator will not always be the server. Even in a restricted scenario like a single user using multiple devices, it’s unlikely that one device will be able to act as a server and the others will be able to connect to it. Thus, it makes sense to decouple the server from the coordinator.

Specifications

The server will need to implement the following operations:

• CreateNewSession
  • Input: signers identifiers
  • Output: session ID
• SendCommitment
  • Input: session ID, identifier, SigningCommitments
  • Output: acknowledgement
• GetSessionState
  • Input: session ID
  • Output: session state (WaitingForCommitments, CommitmentsReady, WaitingForSignatureShares, SignatureSharesReady)
• GetCommitments
  • Input: session ID
  • Output: list of commitments
• SendSigningPackage
  • Input: session ID, SigningPackage
  • Output: acknowledgement
• GetSigningPackage
  • Input: session ID
  • Output: SigningPackage
• SendSignatureShare
  • Input: session ID, identifier, SignatureShare
  • Output: acknowledgement
• GetSignatureShares
  • Input: session ID
  • Output: list of signature shares

The server will behave as follows:

• On CreateNewSession, generate random session ID, insert a new state into a map keyed by it.
  • State = WaitingForCommitments
• On SendCommitment, add commitment into list.
  • If list is complete, set State = CommitmentsReady
• On GetCommitments, send commitment list
• On SendSigningPackage, cache the package into the state
  • set State = WaitingForSignatureShares
• On GetSigningPackage, send the cached package
• On SendSignatureShare, add signature share into list.
  • If list is complete, set State = SignatureSharesReady
• On GetSignatureShares, send signature share list. Destroy session.

Implementation

The server can be an HTTP server which should make it easy for clients to integrate with.
Hyper could be used but it’s more low-level, it recommends axum or warp. Axum seems a bit easier and it integrates with tower which we use on Zebra.