phc-discussions - Re: [PHC] on timing attacks

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Date: Thu, 1 May 2014 15:08:58 +0200
From: Thomas Pornin <pornin@...et.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] on timing attacks

On Thu, May 01, 2014 at 08:45:07AM +0400, Solar Designer wrote:
> Historically, salts haven't been considered secret, but there's this
> precomputation attack and there are timing attacks on comparison of
> hashes (of course, it can also be dealt with by using a constant-time
> comparison function, if one is available or can be implemented).  Now
> side-channel attacks on hash functions themselves are added to these two
> attack categories, for a total of three (or are there more?)
> 
> I think it's fair game to assume or stipulate that salts, while not
> secret for purposes of possible offline attacks, are stored along with
> the hashes and are not revealed separately, except if mandated by the
> protocol (such as for server relief), in which case some security
> tradeoffs are accepted (and need to be understood and possibly mitigated
> by other means).

I think it is my cue to point out that with Makwa, the delegation allows
to offload the bulk of the computation to an untrusted system (who can
be the client itself) without revealing anything, in particular without
giving away the salt. And, more generally, since the system to which
computation is delegated does not receive anything secret, it cannot
leak any secret either.

There can be side-channel leaks on the source system -- the server --
but since that system does not have to do the heavy computational work,
suboptimal but leak-safe implementation techniques can be used. In a
basic Makwa implementation, the password _length_ may leak, but not the
password contents, and that's for a one-shot initial operation which is
done within microseconds, so the attack window is not wide.

	--Thomas Pornin