International Association
for Cryptologic Research

Chameleon hash (CH) is a trapdoor hash function. Generally it is hard to find collisions, but with the help of a trapdoor, finding collisions becomes easy. CH plays an important role in converting a conventional blockchain to a redactable one. However, most of existing CH schemes are too weak to support redactable blockchains. The currently known CH schemes serving for redactable blockchains have the best security of so-called ``full collision resistance (f-CR)'', but they are built either in the random oracle model or rely on heavy tools like the simulation-sound extractable non-interactive zero-knowledge (SSE-NIZK) proof system. Moreover, up to now there is no CH scheme with post-quantum f-CR security in the standard model. Therefore, no CH can support redactable blockchains in a post-quantum way without relying on random oracles. In this paper, we introduce a variant of CH, namely tagged chameleon hash (tCH). Tagged chameleon hash takes a tag into hash evaluations and collision finding algorithms. We define two security notions for tCH, restricted collision resistance (r-CR) and full collision resistance (f-CR), and prove the equivalence between r-CR and f-CR when tCH works in the one-time tag mode. We propose a tCH scheme from lattices without using any NIZK proof, and prove that its restricted collision resistance is reduced to the Short Integer Solution (SIS) assumption in the standard model. We also show how to apply tCH to a blockchain in one-time tag mode so that the blockchain can be compiled to a redactable one. Our tCH scheme provides the first post-quantum solution for redactable blockchains, without resorting to random oracles or NIZK proofs. Besides, we also construct a more efficient tCH scheme with r-CR tightly reduced to SIS in the random oracle model, which may be of independent interest.