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The initial TODO was simply to use one ExternalBlock per all batches in the
block. This would require publishing ExternalBlock after the last batch,
requiring knowing the last batch. While we could add such a pipeline, it'd
require:

1) Initial preprocesses using a distinct message from BatchPreprocess
2) An additional message sent after all BatchPreprocess are sent

Unfortunately, both would require tweaks to the SubstrateSigner which aren't
worth the complexity compared to the solution here, at least, not at this time.

While this will cause, if a Tributary is signing a block whose total batch data
exceeds 25 kB, to use multiple transactions which could be optimized out by
'better' local data pipelining, that's an extreme edge case. Given the temporal
nature of each Tributary, it's also an acceptable edge.

This does no longer achieve synchrony over external blocks accordingly. While
signed batches have synchrony, as they embed their block hash, batches being
signed don't have cryptographic synchrony on their contents. This means
validators who are eclipsed may produce invalid shares, as they sign a
different batch. This will be introduced in a follow-up commit.

2023-08-31 23:00:25 -04:00

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Tributary

A tributary is a side-chain, created for a specific multisig instance, used as a verifiable broadcast layer.

Transactions

Key Gen Commitments

DkgCommitments is created when a processor sends the coordinator key_gen::ProcessorMessage::Commitments. When all validators participating in a multisig publish DkgCommitments, the coordinator sends the processor key_gen::CoordinatorMessage::Commitments, excluding the processor's own commitments.

Key Gen Shares

DkgShares is created when a processor sends the coordinator key_gen::ProcessorMessage::Shares. The coordinator additionally includes its own pair of MuSig nonces, used in a signing protocol to inform Substrate of the key's successful creation.

When all validators participating in a multisig publish DkgShares, the coordinator sends the processor key_gen::CoordinatorMessage::Shares, excluding the processor's own shares and the MuSig nonces.

Key Gen Confirmation

DkgConfirmed is created when a processor sends the coordinator key_gen::ProcessorMessage::GeneratedKeyPair. The coordinator takes the MuSig nonces they prior associated with this DKG attempt and publishes their signature share.

When all validators participating in the multisig publish DkgConfirmed, an extrinsic calling validator_sets::pallet::set_keys is made to confirm the keys.

Setting the keys on the Serai blockchain as such lets it receive Batchs, provides a BFT consensus guarantee, and enables accessibility by users. While the tributary itself could offer both the BFT consensus guarantee, and verifiable accessibility to users, they'd both require users access the tributary. Since Substrate must already know the resulting key, there's no value to usage of the tributary as-such, as all desired properties are already offered by Substrate.

Note that the keys are confirmed when Substrate emits a KeyGen event, regardless of if the Tributary has the expected DkgConfirmed transactions.

Batch

When TODO, a Batch transaction is provided. This is used to have the group acknowledge and synchronize around a batch, without the overhead of voting in its acknowledgment.

When a Batch transaction is included, participants are allowed to publish transactions to produce a threshold signature for the batch synchronized over.

Substrate Block

SubstrateBlock is provided when the processor sends the coordinator substrate::ProcessorMessage::SubstrateBlockAck.

When a SubstrateBlock transaction is included, participants are allowed to publish transactions for the signing protocols it causes.

Batch Preprocess

BatchPreprocess is created when a processor sends the coordinator coordinator::ProcessorMessage::BatchPreprocess and an Batch transaction allowing the batch to be signed has already been included on chain.

When t validators have published BatchPreprocess transactions, if the coordinator represents one of the first t validators to do so, a coordinator::ProcessorMessage::BatchPreprocesses is sent to the processor, excluding the processor's own preprocess.

BatchShare is created when a processor sends the coordinator coordinator::ProcessorMessage::BatchShare. The relevant Batch transaction having already been included on chain follows from coordinator::ProcessorMessage::BatchShare being a response to a message which also has that precondition.

When the t validators who first published BatchPreprocess transactions have published BatchShare transactions, if the coordinator represents one of the first t validators to do so, a coordinator::ProcessorMessage::BatchShares with the relevant shares (excluding the processor's own) is sent to the processor.

Sign Preprocess

SignPreprocess is created when a processor sends the coordinator sign::ProcessorMessage::Preprocess and a SubstrateBlock transaction allowing the transaction to be signed has already been included on chain.

When t validators have published SignPreprocess transactions, if the coordinator represents one of the first t validators to do so, a sign::ProcessorMessage::Preprocesses is sent to the processor, excluding the processor's own preprocess.

SignShare is created when a processor sends the coordinator sign::ProcessorMessage::Share. The relevant SubstrateBlock transaction having already been included on chain follows from sign::ProcessorMessage::Share being a response to a message which also has that precondition.

When the t validators who first published SignPreprocess transactions have published SignShare transactions, if the coordinator represents one of the first t validators to do so, a sign::ProcessorMessage::Shares with the relevant shares (excluding the processor's own) is sent to the processor.

Sign Completed

SignCompleted is created when a processor sends the coordinator sign::ProcessorMessage::Completed. As soon as 34% of validators send Completed, the signing protocol is no longer further attempted.

Re-attempts

Key generation protocols may fail if a validator is malicious. Signing protocols, whether batch or transaction, may fail if a validator goes offline or takes too long to respond. Accordingly, the tributary will schedule re-attempts. These are communicated with key_gen::CoordinatorMessage::GenerateKey, coordinator::CoordinatorMessage::BatchReattempt, and sign::CoordinatorMessage::Reattempt.

TODO: Document the re-attempt scheduling logic.

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