alt Hacker NewsI really never managed to understand why we keep suggesting SHA-256 even though we know it's vulnerable to length extension attacks?
Everywhere else in cryptography we favor recommendations that are harder to misuse, or that shrink the surface area of "things you have to think about and get right", but for some reason SHA-256 keeps finding its way into new designs (such as Git 3.0), even though things would be much nicer if I never had to remember that length extensions are a thing :/
On another note, as someone who defaults to BLAKE3 unless I have a good reason not to, it would be nice to have more context on Filippo's comment about it being very fast "but complex and uncommon enough that you want to have a good reason to use it and a good reason to trust your implementation".
I've peeked into the guts of Zig's BLAKE3 implementation before, it's merkle-tree-ish and not very scary looking, the primitives are a lot more recognizable than Keccak for example...
Because length extension attacks only matter if you're doing something you should never be doing: hand-rolling your own keyed hash constructions. Everybody uses HMAC (you even see it in SHA3 systems), and so length extension doesn't matter. Meanwhile, SHA2 has hardware support.
I like BLAKE3 just fine. Nobody's really going to ding you for using it (or SHA3 for that matter). But it would not be sound practice to tear SHA2 out of a system that was already using it.
Two other notes:
(1) The article explicitly recommends HMAC-SHA2, not SHA2 by itself.
(2) Where it does recommend SHA2 directly (for hashing, not authentication), it recommends 512/256, which is truncated and can't be length-extended.