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Date: Mon, 20 Jan 2014 15:05:38 +0000
From: Peter Maxwell <peter@...icient.co.uk>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Native server relief support for password hashing in browsers

On 20 January 2014 11:19, Solar Designer <solar@...nwall.com> wrote:

> On Mon, Jan 20, 2014 at 02:13:20AM +0000, Peter Maxwell wrote:
> > [...] if we're going to go
> > to the bother of replacing the authentication mechanism on various
> > services, surely we can do better than client-side password hashing.
>
> I agree, which is a reason why I think a PHC candidate's builtin "server
> relief" support (if present) should be usable for some more advanced
> than "pass-the-hash" authentication mechanisms.  Ideally, it should fit
> some pre-existing ones.
>
>
​So if we're moving towards a PBKDF that is suitable for, say, SRP then the
"server relief" part of the PBKDF is pretty much redundant, is it not?
​
Have we had a discussion on the PHC candidates in reference to password
auth'd key exchange mechanisms as yet?



> > Instead of efforts being focused on complicated implementations of
> > client-side password hashing, the more sensible approach is probably
> > mandating a minimum password complexity for which the PHC algorithm can
> be
> > considered secure and accompanying that with advice on how to mitigate
> DoS
> > and brute-force attacks against the application.
>
> I think these areas of focus are orthogonal - well, except for us having
> limited resources and potentially focusing on some of these areas too
> much, neglecting the others.
>
>
It's orthogonal in the respect that no matter what password hash was used
in the Github example, those accounts would still have been compromised.
 It does however raise the point that if an attacker has access to a
password db that used suitably weak passwords then the chances are you
cannot protect against compromise, so in that sense the question is not
orthogonal.




> As to "a minimum password complexity for which the PHC algorithm can be
> considered secure", I think it depends on threat models, assumed
> per-{password, account} costs for certain kinds of attackers, and what
> is meant by "secure".  For practical purposes, the decision-making will
> usually be different: what's a reasonable tradeoff between security and
> usability.  In many cases, this will translate to using password policy
> settings that are below what we'd otherwise define as minimum "secure"
> ones, yet we'd want better password hashing in those cases as well (such
> as to better protect a subset of accounts against a subset of threats).
>
>
So far I've seen a *massive* amount of discussion on memory-hardness and
what processors can fill memory the quickest, without addressing the more
fundamental question of what we're considering to be our threat model and
what we consider secure in that model.  At the moment, we can probably
compare algorithm against algorithm but that assumption would not be valid
if someone presents a candidate algorithm that adjusts hardness based on
password complexity (which can be done but there are problems with the
approach).

We also cannot answer more fundamental questions like: for a standard
password db of say 1m passwords, how many more passwords will be protected
against an adversary with $x resources compared to using bcrypt.  Without
being able to answer those types of question, how do can you justify the
winning PHC algorithm?  For someone charged with securing passwords, would
it be more cost-effective to enforce a strong password policy instead of
the extra hardware needed for the PHC algorithm?  If my own experience in
security is anything to go by, risk is the governing factor and I'm not
seeing any sufficient justification on why a company or institution should
spend increased hardware resources for storing passwords when you cannot
measure projected risk reduction.

Anyway, to cut to the chase - we should be able to model performance of
candidates using a suitably small number of variables and making some
reasonable assumptions (I'd already started on this last year and if I can
get a good run of spare time might actually be able to finish it).




> I think this is mostly beyond scope of PHC (although these things may
> reasonably be worked on by the same people).  The only portion under
> scope is "per-{password, account} costs for certain kinds of attackers"
> for different defender's cost settings, similar to info included in the
> scrypt paper, but expanded to cover not only ASICs, but also relevant
> pre-existing hardware (that some attackers are likely to use).
>
>
>
​On the contrary, I think these questions are absolutely core to the PHC:
how can you ​justify using the winning PHC algorithm otherwise?

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