The Trust-Free Aggregation Layer of the Unicity Infrastructure
Unicity is a novel blockchain infrastructure for enabling users to execute off-chain peer-to-peer token transactions while preventing parallel states of tokens (double-spending) with minimal blockchain complexity and storage. A key component of the infrastructure is the Aggregation Layer responsible for storing information about the spent states of tokens and providing compact cryptographic proofs of no double-spending for the users without making any compromises in trust. Aggregation Layer is a layer 2 style service that holds an append-only key-value repository that periodically certifies its state using a traditional blockchain that we call the Consensus Layer. Every time while certifying a changed state the Aggregation Layer provides a cryptographic proof to the Consensus Layer about the append-only consistency of the key-value store. We use the Radix Sparse Merkle Trees (RSMTs) to create the cryptographic digest r_i of the store in every round i and authentication paths of special type in order to prove that the next digest r_{i+1} was obtained while only adding some key-value pairs (k,v) to the repository. The proof verification code is implemented as an Algebraic Intermediate Representation (AIR) circuit on top of the Plonky3 STARK toolkit. Our implementation uses no trusted setup, achieves throughput of 10,000 insertions per second and and millisecond range verification time on a single consumer-class CPU.