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Date: Mon, 27 Jan 2014 16:39:40 +0100
From: Alexandre Anzala-Yamajako <>
Subject: Re: [PHC] Opinions sought on whether a specific side-channel leakage
 is ok.

Even though the trick is neat I would personally be against such an
exploitable side channel.
Not everybody is going to use your scheme and people like to reuse
passwords across different website which means that your scheme allows me
to check the expected complexity of the password of a given user on another
Am i wrong here ?

2014-01-27 Peter Maxwell <>

> Without exposing too much of my intended design, I'd like to garner some
> opinion if that is possible.
> As a specific feature of my design, I'm intending on adjusting the
> computational & memory work requirement based on password complexity, in a
> somewhat probabilistic manner.
> So, lets say we can associate a given password with a scalar password
> complexity measure in the interval [0,1] with an as yet to be defined
> distribution.  To keep things nice and fair, lets assume that upon a
> password registration with password complexity p_c the algorithm calculates
> an interval [p_min,p_max], 0 < p_min < p_c < p_max < 1, st. the integral
> over the distribution from p_min to p_max is kept at a fixed global
> constant and the interval is, intuitively at least, centred around p_c.
>  Now, as a final step, the algorithm chooses a random value within
> [p_min,p_max], say p_r, and determines a computational & memory work
> requirement deterministically based on p_r, for example a function w: [0,1]
> -> R where w(p_r) maps the complexity measure to something tangible.
> Each password complexity measure is kept a secret and after registration
> of a password is not known even to the server.  This allows another little
> trick that very much benefits the defender but isn't relevant here, only
> that the complexity measure is considered secret.
> The consequence of this model is that the defender can force weak
> passwords to take considerably more work per password guess, thus
> ameliorating much of the problem of users picking poor passwords.  Instead
> of an attacker being able to get some proportion of passwords almost
> guaranteed, they'll have to work just as hard for the low entropy passwords
> as the high entropy ones while the defender doesn't - on average - face any
> computational disadvantage, assuming login frequency is not dependent on
> password complexity and that I get my maths correct (which is dubious given
> I can't concentrate due to the pile-driving going on in the building site
> next door... the whole building is thumping at around 1Hz).
> The problem - and it's a not an insignificant problem - is that there is a
> rather obvious side-channel: the time taken to compute a valid login
> betrays some information about the password complexity.  Given password
> complexity is at least partially related to length, it may fall foul of the
> call for submission guideline:
> *"Resilience to side-channel attacks (timing attacks, leakages, etc.). In
> particular, information should not leak on a password's length."*
> The algorithm doesn't leak password bits, it only leaks a password
> complexity range that may give some information as to password length or a
> fairly large set to which the password may belong.  The other item to note
> is that repeated measurements don't give more information, the best an
> attacker can do is invert w() to get p_r, and get a rough idea of the range
> [p_min,p_max] but they cannot accurately determine p_c.  The interval
> p_min,p_max would be controlled by the previous mentioned fixed constant
> with obvious implications.
> The question is, whether this is an acceptable trade-off, at least as an
> optional feature?

Alexandre Anzala-Yamajako

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