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?

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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 [512] (M || 01, 256); which was different than the original padding proposed by the Keccak team. It seems that now people are calling the "original" SHA3 Keccak hashing "Keccak" and the new NIST SHA3 standard as "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 = 5f16f4c7f149ac4f9510d9cf8cf384038ad348b3bcdc01915f95de12df9d1b02

Keccak-256 = 5f16f4c7f149ac4f9510d9cf8cf384038ad348b3bcdc01915f95de12df9d1b02

SHA3-256 (NIST Standard) = 7f5979fb78f082e8b1c676635db8795c4ac6faba03525fb708cb5fd68fd40c5e

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 sha3.

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