Optimize single signature in Mithril Aggregator/Signer/Core

[jpraynaud]

We need to make some optimizations in the registration process of the single signatures in the Aggregator:

1. Single Signature Model

• Assumption: The single signature is the same for each won lottery index
• This means we could update the model of the SingleSignature, which will save bandwidth and storage
• The current model is to use a list/vec of SingleSignature:

pub struct SingleSignature {
    pub party_id: PartyId,
    pub index: LotteryIndex,
    pub signature: String,
}

• The updated model would be to use instead a SingleSignatures:

pub struct SingleSignatures {
    pub party_id: PartyId,
    pub won_indexes: Vec<LotteryIndex>,
    pub signature: String,
}

• Tasks:

• Validate if the model of the signature should be updated
• Adapt OpenApi specification
• Adapt Mithil Common entities
• Adapt Mithril Aggregator
• Adapt Mithril Signer

2. Split multi signer single signature registration process

• Issue: The handling of the RwLock must be optimized (the multi_signer is write locked at times where it could be read)
• Solution: The register_single_signatures method of the multi_signer should be split in:
  • check_registered: checks that none of the incoming single signatures are already stored in the Aggregator (read lock during this step)
  • verify: verify the validity of the single signatures with the mithril-core library (this is the operation that supposedly takes time) (read lock during this step)
  • store: store all the single signatures at once (write lock during this step)
• Tasks:

• Adapt the MultiSigner trait and its implementation with the new methods
• Adapt the http_server register_signatures route so that it interacts with the new methods of the multi signer and handle the locks on it
• Adapt the SingleSignatureStore to use save_single_signatures (input is multiple single signatures) instead of save_single_signature (input is one single signature)

3. Implement batch single signature verification in Mithril Core

• Issue: The last stage of the optimization is to verify the single signature once in the mithril-core library
• Solution: This means that the mithril-core library should provide a verification method that takes a single signature and a list of won lottery and batch the verification of the singe signatures
• Tasks:

• Add the batch method in Mithril Core
• Update the multi signer underlying call to Mithril Core with this new method
• Update if needed the protocol demo (which might not compile anymore depending on how the batch method is implemented)

[ghubertpalo]

The ticket is marked as done but the part 2 has not been completed.
We are going to see if parts 1 & 3 (done) are good enough first.