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Date: Fri, 24 Jan 2014 00:57:22 +0000
From: Marsh Ray <maray@...rosoft.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] cache timing attacks (Re: [PHC] Initial
 multiply-compute-hardened Catena-3 benchmark)

From: Solar Designer [mailto:solar@...nwall.com] 
>
> On Thu, Jan 23, 2014 at 10:33:59PM +0000, Marsh Ray wrote:
> > Now say the PHC algorithm imposes memory-latency hardness by using
> > the supplied password to generate a seed with which to initialize a PRNG
> > sequence to pick memory addresses within a large memory block. Going
> > by the general rule that 50% of users choose one of the 10,000 most
> > common passwords, the attacker only needs to learn just one or two
> > cache line addresses in order to have a better than 50% chance of
> > guessing the user's password on the first try.
>
> This sounds about right, but:
>
> This assumes that the salt is known to the attacker,

Rather, even worse, no salt at all is used in this part of the computation. I was just trying to give an example of an obviously broken case.

> which is usually the case when the attacker has a copy of the salts+hashes database and can run offline attacks already.

Yep, the main value of a PHC function and the work factor is obtained only after the attacker learns one or more of the stored hash values. Otherwise all we'd ever need is a time-invariant memcmp().

But we also can't regress (relative to time-invariant memcmp) in the other (hopefully usual!) case where the attacker does *not* know the stored hash value. So leaking meaningful information about salt or derived values via side channels (with an attacker-chosen password input) should be considered a weakness too.

> In order to check the 10k most common passwords against cache line addresses, the attacker needs
> to partially compute hashes - up to the point of first password-dependent lookup. If this corresponds
> to a substantial portion of the full hash computation, then that attacker hasn't gained all that much
>  - only a speedup of their offline attack by a certain factor, which we may try to make reasonably small. 

Yeah. But let's be clear here that nothing can save users who choose such common passwords anyway, because any defender with 5,000 logins-per-reasonable-time will have chosen a work factor small enough to make that calculation practical.

> So I currently think that a combination of the two approaches will work best

Interesting.

- Marsh

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