[tveita]

I do mean the hash function "SHA-512/256", as defined in FIPS 180-4 [1]. It is basically a version of SHA-512 that truncates the final result to 256 bits (Like SHA-384). It is not vulnerable to length extension, because unlike SHA-256, the final hash does not contain enough state to continue hashing.

I wouldn't consider SHA-224 immune to length extension since it only truncates 32 bits, which is low enough to brute force.

[1] http://csrc.nist.gov/publications/drafts/fips180-4/Draft-FIP...

[daeken]

Ahh, quite right. Sorry about that. And yeah, I agree on SHA-224; it's okay in certain circumstances (you're rarely going to be able to pull off 2^31 (average) requests), but it's almost definitely not the right choice.