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by scatbot 198 days ago
Not really. BLAKE3 isn’t a reduced-round tweak of BLAKE2 like K12 is for Keccak. It's a different construction that still meets its full security target. K12 and TurboSHAKE on the other hand are literally the same permutation with fewer rounds, which actually reduces Keccak's security margin. The situations are not really comparable.
2 comments
oconnor663 198 days ago
BLAKE3 does reduce the round count relative to BLAKE2, and the underlying compression functions are similar enough that it is an apples-to-apples comparison. Our rationale for doing that was described in https://eprint.iacr.org/2019/1492.pdf, which also argued that Keccak could reduce their round count even farther than they did.
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robobully 198 days ago
> BLAKE3 isn’t a reduced-round tweak of BLAKE2 <...> It's a different construction

My initial argument was meant to highlight the difference between BLAKE and its successors. However, I have no idea what you back your statements with, BLAKE3 in fact _is_ BLAKE2s with reduced round + tree-based structure on top of it. The authors even directly mention it in the spec.

> K12 and TurboSHAKE on the other hand are literally the same permutation with fewer rounds

It's true for TurboSHAKE, but as for K12, it builds a tree-based structure on top of TurboSHAKE by the virtue of Sakura encoding (similar to what Bao encoding is used for in BLAKE3).

IANAC, so I won't make any claims about cryptographical strengths of the functions.

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oconnor663 196 days ago
> similar to what Bao encoding is used for in BLAKE3

In my mind Sakura and Bao are doing very different things. Sakura is a general framework for defining sound hash functions, while Bao is a BLAKE3-specific interleaving of hash function input and output that let's you do partial/streaming verification.

There was an interesting issue where it turned out that the Sakura security properties (which I studied while I was working on Bao, very helpful) weren't sufficient for what Bao wanted to do. I didn't realize that until a couple colleagues pointed it out years later. Luckily it didn't turn out to be a break in practice: https://github.com/oconnor663/bao/issues/41

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