Many sources refer to the use of the "SHA3" cryptographic hash function within Ethereum, but the Ethereum project began before the SHA3 standard was finalised, and there was some confusion/controversy around the final parameters selected for the standard. Which function specifically did Ethereum actually end up with? Is it the same as the actual SHA3 standard, or one of the variants within the standard? Or is it a parameterisation of Keccak that doesn't match any of the variants?
Ethereum uses KECCAK-256. It should be noted that it does not follow the FIPS-202 based standard (a.k.a SHA-3), which was finalized in August 2015.
According to this, NIST changed the padding to
SHA3-256(M) = KECCAK  (M || 01, 256).
This was different from the padding proposed by the Keccak team in The Keccak SHA-3 submission version 3 (final, winning version). The difference is the additional '01' bits appended to the message. People are now calling the "submitted version 3" SHA-3 Keccak hashing "Keccak" and the finalized NIST SHA-3 standard "SHA-3".
Using this online generator and the Solidity Online Compiler, I tested the difference between Keccak-256 and SHA3-256. I hashed the word testing using Ethereum and the 2 SHA3 hashing algorithms:
Ethereum SHA3 function in Solidity =
SHA3-256 (NIST Standard) =
7For clarity's sake I think we should call it "Keccak-256" rather than anything that explicitly says "SHA3" in it. Jan 24, 2016 at 9:22
2You are not alone. See this EIP. Jan 24, 2016 at 9:25
2Oh, and excellent answer by the way. Everyone reading this, please upvote this answer. It is an ideal example of the kind of answers we'd like to have on this site!! Jan 24, 2016 at 9:53
4Solidity 0.4.3 has introduced
keccak256, an alias which can safely be used as a replacement for
sha3.– eth ♦Nov 11, 2016 at 11:12
1@MattSayar [kɛtʃak], like “ketchak”, as seen here: quora.com/Cryptography-How-should-you-pronounce-Keccak– FRAGAOct 5, 2017 at 5:51
To make it clearer that Ethereum uses KECCAK-256 instead of the NIST standardized SHA-3 hash function, Solidity 0.4.3 has introduced
keccak256. (It is an alias to
sha3, meaning that
keccak256 produces identical results to
sha3, but with the intent to avoid confusion, especially for developers new to Ethereum.) It's recommended that new code use
keccak256 instead of