[laumars]

> General cryptography -- for example, message validation -- does not need to be slow. In fact, slow message validation would cripple hash functions for cryptography, increasing CPU load and reducing throughput.

There is definitely a trade off between the two (performance on servers vs rate of passwords an attacker can crack). But generally the advice is to go for the slowest you can afford. Hence why KDF's have an iteration parameter so passwords can be hardened as harder gets faster.

[ori_b]

No: This is why you have a split between KDFs and hashes. Where you use one, you would not want to use another.

Trying to make one that does both leads to something that sucks at both.

[laumars]

> No: This is why you have a split between KDFs and hashes. Where you use one, you would not want to use another.

That's not true either. Hashes are recommended to be used as input to KDFs.

"Modern password-based key derivation functions, such as PBKDF2 (specified in RFC 2898), use a cryptographic hash, such as SHA-2"

Source: https://en.wikipedia.org/wiki/Key_derivation_function#Key_st...

> Trying to make one that does both leads to something that sucks at both.

eh? Nobody is advocating that what-so-ever. Not me, not anybody.

I think you're now arguing with me for the sake of arguing with me. :-/

[ori_b]

Ok, that was sloppy phrasing on my part, but my point still stands: The entire reason that KDFs exist is to deal with the fact that hash functions are designed to be as fast as possible. If it made sense to make hash functions slow, then KDFs would not be needed.

While KDFs do use hash functions internally, the hash function is an implementation detail.

[laumars]

Ok, thank you. That does make more sense now :)

[alextgordon]

Ironically this whole thread played out in the opposite direction at the time of SHA3's announcement:

https://www.schneier.com/blog/archives/2012/10/keccak_is_sha...

https://www.imperialviolet.org/2012/10/21/nist.html

tl;dr: Keccak is not a good partner for PBKDF2. It has good hardware performance but comparatively poor software performance. This benefits attackers with FPGAs or better.

[brohee]

The hilarious part here is Wikipedia calling PBKDF2 modern. It's a 2000 minimal update (to generate more bits, to be kind of UTF-8 aware) of a 1993 standard.

At the time of the RFC publication it was already obvious its security was way behind bcrypt that was used in OpenBSD since 2.1 (June 1997), which did its best to be ASIC hostile, which isn't the case for PBKDF2.

In retrospect, NIST choosing PBKDF2 over bcrypt in NIST SP800-132 could be seen as part of the effort to weaken standards for NSA profit.