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Date: Fri, 24 Jan 2014 05:37:52 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] cache timing attacks (Re: [PHC] Initial multiply-compute-hardened Catena-3 benchmark)

On Fri, Jan 24, 2014 at 12:57:22AM +0000, Marsh Ray wrote:
> 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.

I agree.

Salt-only-dependent lookups are to be avoided.  If/when the function
does start making lookups based on heavily-processed-password, it is OK
and desirable for all of those to be dependent on the salt as well.
Then without having at least made a likely guess at the password, salt
bits are not leaked.

... which leads us to a new concern: what if the same-machine attacker
without knowledge of the salts does make a likely guess at the password
(e.g., guesses "123456", which might be correct with 1% probability when
there's no password policy)?  This attacker can then try to infer salt
bits, still not knowing whether the password guess was right or not.
Then, having obtained (via cache timings) the possible salt _assuming_
that the password was guessed correctly, the attacker can use further
cache timing attacks to test whether the password guess was in fact
correct or not.  If not, repeat for second most common password.  Nasty.

Now what, do we resort to (postponed) indexing by only the password or
only the salt?  I think not, as this seems to fully(?) kill the benefits
of indexing by any secret at all, as compared to Catena's approach.
With indexing by only one of {password, salt}, the cracker's inner loop
may iterate over the other, and thus benefit from advance computation of
indices for prefetching, meaning no memory latency dependency anymore.

Looks like this is another aspect of the cache timing resistance vs.
offline attack resistance tradeoff.

Mitigating factor: the attacker will need to watch multiple legitimate
logins to test each candidate password in this way.  This may only be
feasible against really weak passwords, which are much quicker tested
for by attempted logins (but this may be logged and rate-limited).

> 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.

I interpreted your 10k as a specific example, with the general idea also
relevant to more complicated passwords.

Then, slowing down offline attacks even on hashes of top 10k common
passwords makes some sense.  Sure, such attacks will be practical for
almost anyone, but the percentage cracked e.g. by a single-computer
amateur attacker on the first day may vary depending on how good or bad
the hashing scheme is, and this affects what percentage of accounts are
exposed to abuse (e.g. retrieval of additional private info from them)
until the service provider is able to respond.  For example, with 100M
accounts and a speed of 5000 c/s, it'd take 5.5 hours to test all of
them for the password "123456".  So it's 1% cracked in 5.5 hours if
there was no password policy.  That's awful (it's 1 million accounts!),
yet it's by far not as bad as what it could be with fast hashes (e.g.,
same attacker cracking over 50% in first hour).  Probing for "123456"
could also be done online, so at least some minimal password policy is
needed even without hash leaks, and this would make better password
hashing even more relevant.

Alexander

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