[cryptography] any reason PBKDF2 shouldn't be used for storing hashed passwords?

Solar Designer solar at openwall.com
Wed Aug 15 22:27:31 EDT 2012


On Thu, Aug 16, 2012 at 02:25:34AM +0200, Patrick Mylund Nielsen wrote:
> PBKDF2 is certainly decent, and often the easiest choice if you intend to
> comply with e.g. FIPS 140-2/ISO 27001, but the biggest argument against it
> is that it _isn't_ difficult to parallelize, since it is just e.g.
> HMAC-SHA256. Each guess might require sequential iteration, but you can
> still compute many guesses simultaneously.

Exactly.

> bcrypt is slightly more computationally expensive

On current GPUs, that's "a lot", not "slightly".  GPUs currently deliver
CPU-like speeds for bcrypt, but a lot higher speeds for PBKDF2-HMAC-SHA*.

(I expect that we'll get faster attacks on bcrypt using general-purpose
hardware with AVX2 and Intel MIC, though.)

> (although its 55-byte input limit is often ignored,)

The limit is actually at 72 chars.  55 was an error in the paper.

> > PBKDF2-HMAC-SHA1 significantly weaker than PBKDF2-HMAC-SHA512?
> 
> SHA-1 is still safe in an HMAC construction, however there's no convincing
> reason not to use at least PBKDF2-HMAC-SHA256 that I can think of. You
> don't need it to be fast. Going beyond that won't really give you much
> security compared to using a memory-hard function.

There's a lot of difference between SHA-1 and SHA-512 in terms of
friendliness to current GPUs.  SHA-512 is a lot less GPU-friendly.

SHA-256 is inbetween.  This is related to SHA-512 making more complete
use of 64-bit CPUs.  If you use more of the available resources and do
so wisely, then the attacker will similarly have to use more resources.
This alone accounts for a 2x difference between SHA-256 and SHA-512 when
we consider attackers' GPU implementations.  Further difference comes
from SHA-512's increased register pressure (on GPU's 32-bit registers).

Overall, SHA-256 is 2-3 times slower than SHA-1, and SHA-512 is a
further 4-14 times slower than SHA-256 on current GPUs with currently
available implementations.  SHA-512 is 10 to 30 times slower than SHA-1
on current GPUs with currently available implementations.

Still PBKDF2-HMAC-SHA-512 is a lot more susceptible to attacks with
current GPUs than bcrypt, let alone scrypt.

Alexander



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