Cryptology ePrint Archive: Report 2022/240

SNACKs: Leveraging Proofs of Sequential Work for Blockchain Light Clients

Hamza Abusalah and Georg Fuchsbauer and Peter Gaži and Karen Klein

Abstract: The success of blockchains has led to ever-growing ledgers that are stored by all participating full nodes. In contrast, light clients only store small amounts of blockchain-related data and rely on the mediation of full nodes when interacting with the ledger. A broader adoption of blockchains calls for protocols that make this interaction trustless.

We revisit the design of light-client blockchain protocols from the perspective of classical proof-system theory, and explain the role that proofs of sequential work (PoSWs) can play in it. To this end, we define a new primitive called succinct non-interactive argument of chain knowledge (SNACK), a non-interactive proof system that provides clear security guarantees to a verifier (a light client) even when interacting only with a single dishonest prover (a full node). We show how augmenting any blockchain with any graph-labeling PoSW (GL-PoSW) enables SNACK proofs for this blockchain. We also provide a unified and extended definition of GL-PoSWs covering all existing constructions, and describe two new variants. We then show how SNACKs can be used to construct light-client protocols, and highlight some deficiencies of existing designs, along with mitigations. Finally, we introduce incremental SNACKs which could provide a new approach to light mining.

Category / Keywords: cryptographic protocols / blockchain, light clients, proof of sequential work

Date: received 24 Feb 2022

Contact author: hamzaabusalah at gmail com, peter gazi at iohk io, georg fuchsbauer at tuwien ac at, karen klein at inf ethz ch

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Version: 20220225:081218 (All versions of this report)

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