Expand description
Recover threshold signatures over an externally synchronized sequencer of items.
This module allows a dynamic set of participants to collectively produce threshold signatures for any ordered sequence of items.
The primary use case for this primitive is to allow blockchain validators to agree on a series of state roots emitted from an opaque consensus process. Because some chains may finalize transaction data but not the output of said transactions during consensus, agreement must be achieved asynchronously over the output of consensus to support state sync and client balance proofs.
_For applications that want to collect threshold signatures over concurrent, sequencer-driven broadcast, check out crate::ordered_broadcast.
§Architecture
The core of the module is the Engine. It manages the agreement process by:
- Requesting externally synchronized commonware_cryptography::Digests
- Signing said digests with BLS commonware_cryptography::bls12381::primitives::poly::PartialSignature
- Multicasting partial signatures to other validators
- Recovering commonware_cryptography::bls12381::primitives::poly::Signatures from a quorum of partial signatures
- Monitoring recovery progress and notifying the application layer of recoveries
The engine interacts with four main components:
- crate::Automaton: Provides external digests
- crate::Reporter: Receives agreement confirmations
- crate::Monitor: Tracks epoch transitions
- crate::ThresholdSupervisor: Manages validator sets and network identities
§Design Decisions
§Missing Signature Resolution
The engine does not try to “fill gaps” when threshold signatures are missing. When validators fall behind or miss signatures for certain indices, the tip may skip ahead and those signatures may never be emitted by the local engine. Before skipping ahead, we ensure that at-least-one honest validator has the threshold signature for any skipped index.
Like other consensus primitives, aggregation’s design prioritizes doing useful work at tip and minimal complexity over providing a comprehensive recovery mechanism. As a result, applications that need to build a complete history of all formed types::Certificates must implement their own mechanism to synchronize historical results.
§Recovering Threshold Signatures
In aggregation, participants never gossip recovered threshold signatures. Rather, they gossip types::TipAcks
with partial signatures over some index and their latest tip. This approach reduces the overhead of running aggregation
concurrently with a consensus mechanism and consistently results in local recovery on stable networks. To increase
the likelihood of local recovery, participants should tune the Config::activity_timeout to a value larger than the expected
drift of online participants (even if all participants are synchronous the tip advancement logic will advance to the f+1th highest
reported tip and drop all work below that tip minus the Config::activity_timeout).
Modules§
- types
- Types used in aggregation.
Structs§
- Config
- Configuration for the super::Engine.
- Engine
- Instance of the engine.