Gas Cost of Keccak-256 and SHA-256 in an Ethereum Smart Contract

Abstract

Smart contracts often store a hash of a record instead of the record, and Ethereum offers Keccak-256 as an opcode and SHA-256 as a precompiled contract. A Hardhat experiment on a course-evaluation contract compared their gas cost in 18 scenarios (2 algorithms, 3 functions, 3 input lengths) of 10 transactions each, and reported Keccak-256 as cheaper by 0.44–2.66%. We re-analyze the reported mean, minimum, maximum, and standard deviation. All 18 scenarios are reproduced exactly by nine repeated transactions at the minimum and one first transaction at the maximum, so the means mix two regimes. For the 850-character submit and verify functions the repeated transactions sit at the EIP-7623 calldata floor (56,030 gas) under both algorithms, which turns a first-transaction gap of 2,518–2,521 gas into a reported mean gap of 252. The first-transaction gaps follow 455 + 76.4w gas for w input words, within 8 gas, about 12 times the cost-schedule difference of 6 gas per word. Keccak-256 stays cheaper, by 0.4–4.1% of first-transaction gas, but the size of the gap is not a property of the hash functions alone. Per-transaction logs and the contract source are needed to confirm the reconstruction.