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Date: Tue, 19 Feb 2013 03:27:00 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Any "large verifiers" on the panel?

On Mon, Feb 18, 2013 at 09:54:01AM +0100, Patrick Mylund Nielsen wrote:
> Except this is not what happens today.

Oh, the wonders of top-posting.  I can't even comment on that paragraph
because it is unclear what exactly you were referring to. ;-)

> What does happen, and what absolutely hurts the most, is when somebody gets
> a hold of your entire database of (weak) digests and cracks them on their
> own or dumps them in a pastebin, e.g. without the usernames. The PHC
> schemes, IMHO, really need to be resistant to this.

Yes, PHC is precisely about coming up with password hashing schemes that
mitigate offline attacks.

> We _already have_
> effective ways to resist online attacks--exponential backoff, bans, even
> captchas...

Actually, dealing with online attacks is not always that easy - in
particular, if we consider that some may target password reuse from
other sites.  Some large sites are having a hard time dealing with
online attacks:

http://lists.openwall.net/full-disclosure/2012/05/17/11
https://ripe64.ripe.net/presentations/48-AbuseAtScale.pdf
https://speakerdeck.com/u/duosec/p/mike-hearn-at-ripe-64-abuse-at-scale

It's just beyond scope of PHC.

> To give an example: Blizzard Entertainment got a huge level of flak from
> the tech, and even the crypto, communities for using SRP when their
> verifiers were compromised, as people realized that SRP protects against
> MITM attacks, not offline attacks:
> http://arstechnica.com/security/2012/08/hacked-blizzard-passwords-not-hard-to-crack/

Sure.  SRP does not solve the problem that we're trying to focus on here.

> IMHO, it is a more useful exercise to think about leveraging clients, e.g.
> for key derivation,

It is OK to discuss this in PHC context, but how does it affect the
actual PHC submissions?  Well, scalability (via the cost settings) to
mobile devices is relevant, and it may contribute to us choosing one
password hashing scheme over another as a winner.  However, in order to
actually do the hashing on the client, a suitable protocol would need to
be used.  (And the security offered by that will vary across protocols,
as well as across actual implementations, both client- and server-side.)
I feel that as it relates to PHC winner selection, only the password
hashing scheme scalability aspect is relevant.

> than more secrets on the server side. The servers will be compromised.

This is a simplistic/idealistic view.  We're dealing with a problem
where we don't have the luxury of possibly arriving at a perfect
solution anyway.  Why should we discard other imperfect security
measures, then?

A password hashing scheme that is more costly for an attacker to compute
will result in that attacker cracking fewer passwords.  Similarly, a
password hashing implementation with a secret local parameter will
result in some attackers ending up with hashes, but not the local
parameter value.  In either case, the overall number of passwords
getting cracked - across multiple sites and attackers - is reduced.
Thus, support for a secret local parameter is in line with the goals of
PHC.

Now, one can argue that a local parameter stored in the exact same place
with the hashes is useless, and I agree - in cases where it's just an
implementation obscurity, the false sense of security aspect may
outweigh the arguable advantage of some attackers naively missing the
local parameter.  However, there are also cases where it won't be stored
in the exact same place, and a subset of reasonable attacks will likely
get to the hashes, but not to the local parameter value.  In PHC, we're
merely providing password hashing schemes that are usable with a local
parameter.  We do not control whether such uses will be reasonable or
not.  Yet discouraging such uses in general just because some are
unreasonable is not necessarily good.

Thus, the wording of in the call for submissions: a secret local
parameter is allowed as an optional input, but its support is not
required from PHC submissions.  It's then up to each individual
submissions' authors to include this functionality or not, according to
their beliefs, ease of inclusion of an extra parameter within their
design, etc.  I think this is how it should be.

Alexander

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