[RFC] Improve Mantle Transaction hash

This RFC follows an older schema and will be updated when time allows.

Authors: Thomas Lavaur Approvals (research): Marcin Pawlowski Approvals (engineering): Daniel Sanchez Quiros, Youngjoon Lee

Motivation

As Mantle Transactions grow (notably with large CHANNEL_INSCRIBE Operations), computing ZkHash over the whole transaction becomes a noticeable cost during transaction construction and verification. This RFC proposes a small change that preserves the use of ZkHash while reducing the amount of data hashed inside the circuit.

Proposal

In version Mantle, the Mantle Transaction hash is defined as the ZkHash of the entire Mantle Transaction, excluding the Ledger Transaction proof and the Operation proofs. Currently we perform a zero-knowledge hashing on a Mantle Transaction (without proofs): tx_hash = ZkHash(mantle_tx_without_proofs). However, execution of this hashing algorithm is time consuming and grows with the size of the input. Therefore, we propose instead hashing the Mantle Transaction using the classic hash defined in Common Cryptographic Components, then hashing the result with ZkHash: tx_hash = ZkHash(classic_hash(mantle_tx_without_proofs)), where classic_hash() is the classic hash defined in Common Cryptographic Components.

Justification

The proposed change speeds up computation of the Mantle Transaction hash by moving most of the work out of ZkHash and into a classic hash, while still committing to the result inside ZkHash. This becomes an issue as Mantle Transactions grow, since hashing can take a substantial amount of time (for example, if the transaction contains a large CHANNEL_INSCRIBE Operation). Here we show the difference of computation speed for Mantle Transactions encoding and hashing containing a single Ledger Transaction and a single Inscription varying the size of the inscription to make the transaction longer.

+--------------+-----------------------+------------------+-------------------------------+---------------------------+
| payload_size | blake2b_poseidon2_avg | poseidon2_avg    | poseidon2 / blake2b_poseidon2 | blake2b+poseidon2 faster  |
+--------------+-----------------------+------------------+-------------------------------+---------------------------+
|           64 | 10.84 us              | 65.89 us         | 6.08x                         | 83.55%                    |
|          256 | 10.40 us              | 62.14 us         | 5.98x                         | 83.26%                    |
|        1,024 | 10.89 us              | 142.30 us        | 13.07x                        | 92.35%                    |
|        4,096 | 13.02 us              | 561.00 us        | 43.09x                        | 97.68%                    |
|       65,536 | 71.34 us              | 8.851 ms         | 124.07x                       | 99.19%                    |
|      524,288 | 657.20 us             | 70.69 ms         | 107.56x                       | 99.07%                    |
|    1,048,576 | 1.315 ms              | 143.30 ms        | 108.97x                       | 99.08%                    |
|    2,097,152 | 2.652 ms              | 283.90 ms        | 107.05x                       | 99.07%                    |
|    4,194,304 | 5.981 ms              | 563.70 ms        | 94.25x                        | 98.94%                    |
+--------------+-----------------------+------------------+-------------------------------+---------------------------+

We can clearly see that combining the classic and zk hashes yields a significant improvement. The material used for the benchmarks is the following:

  • CPU : 13th Gen Intel(R) Core(TM) i9-13980HX (24 cores / 32 threads)
  • RAM : 32GB - Speed: 5600 MT/s
  • Motherboard: Micro-Star International Co., Ltd. MS-17S1
  • OS : Ubuntu 22.04.5 LTS
  • Kernel : 6.8.0-59-generic

The code of the benchmarks can be found here.

Specifications Update

  • [v1.2.1] Mantle Specification