Redacting Blockchain Without Exposing Chameleon Hash Collisions

Abstract

AbstractA new implementation of a redactable blockchain is proposed: a construction where trusted authorities can change past blocks without invalidating the blockchain and without needing a hard fork. Like previous works, it uses a chameleon hash function—a special kind of hash function where it is possible to compute collisions efficiently if a trapdoor is known. One problem with most chameleon hash functions is that collision-resistance and security are invalidated or severely weakened if a collision is publicized. It turns out that redacting a blockchain in existing constructions reveals collisions. Previous works dealt with this using public-key encryption schemes and non-interactive zero-knowledge proofs to achieve what is known as Enhanced or Full Collision-Resistance. Here instead, we propose a simpler and hopefully faster construction. We use a subset of chameleon hash schemes where the trapdoor also allows for first-preimage computation. We build a redactable blockchain using this stronger property where redactions do not expose collisions.KeywordsRedactable blockchainChameleon hashPost-quantum cryptography