phc-discussions - Re: A weak password attack against PolyPassHash

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Date: Wed, 2 Apr 2014 20:19:15 -0400
From: Justin Cappos <jcappos@....edu>
To: Andy Lutomirski <luto@...capital.net>
Cc: discussions <discussions@...sword-hashing.net>, 
	santiago torres <sat417@...dents.poly.edu>
Subject: Re: A weak password attack against PolyPassHash

>
> First, a question: how do you verify that you've correctly recovered
> the global secret?  If you can't check that, then won't *any* set of
> passwords appear valid?  I'll assume that the server stores a hash of
> the constant term.
>

What you say will work perfectly fine.   In the C implementation, I believe
the code places a secure hash of the secret in the last few bytes of the
constant term, but your solution is fine too.

Second, an attack, based on the observation that the distribution of
> passwords is, in practice, far from uniform:

Suppose that k shares are needed to unlock the database.  Select, at

random, k users.  For each of them, calculate H(salt, "123456").  Then

try unlocking the database.  Repeat until you succeed.

<snip>

Mitigating this type of attack may be difficult, unless defenders are
> willing to choose a rather large value for k.
>

Your point here is well taken.   PPH shouldn't have *threshold keys* that
are weak, if at all possible.   However, for *thresholdless keys* (which
would likely be generated for random outside users upon demand), this does
not pose a problem.

We do assume that many people that have administrator passwords use
reasonable security.   The paper talks about how effective PPH is on a
leaked database from Sony that has very weak passwords for admins (like
Password1).   PPH still dramatically improves the security in this case,
but is not a panacea for extremely weak admin passwords.

Thanks for the hard questions!   Ask more!   :)

Justin

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