Date: Tue, 19 Feb 2013 01:42:42 +0100
From: Patrick Mylund Nielsen <patrick@...rickmylund.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Any "large verifiers" on the panel?

>> 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?

> I'm not sure if I was unclear here: PHC needs to resist offline attacks
since we can assume that the database of verifiers/digests will be
compromised at some point. I don't think this is simplistic/ideal, just
realistic.

Sorry, I do agree that it doesn't hurt to have e.g. a secret that is e.g.
on the disk instead of the database--I'm just worried that such secrets
would become central to the strength of the algorithm, i.e. developers rely
too much on the secret, and offline attacks become trivial if you have both
the secrets and the salts+digests. I didn't mean that it should be ignored,
but that I hope it is considered a "bonus" and not a condition for
security. Of course, if the latter happens, we will have gone full circle
if developers using bcrypt decide to switch to this faster construction,
but don't handle the secrets differently. (Consider that very few
production applications today actually have the kind of meaningful
separation this would require, sans maybe DB vs. disk--it's quite hard to
give some application access to both digests and a secret, and not grant an
attacker the same access if he compromises the machine it's on.)

At the end of the day, I think the point I'm trying to get across is:
Developers want libraries like py-bcrypt that they can import, run phc(pwd)
and save the result. If it uses another secret, they will put it in a
config file or hard-code it in the application without any special
ACL/privilege separation. Also, there is a good chance that vulnerabilities
on the machine or the network provides a way to access the files on the
machine, or at least the database containing the digests. If we make other
assumptions then we risk developers not using the construction over bcrypt
because it is too much work (a major contributor to why bcrypt is fairly
popular today vs. PBKDF2/scrypt for password authentication, IMHO), or
worse, hurting the end users more than they would have been if their
provider had used bcrypt. (Yeah, you could argue that "the developers
should have known better, and spent more time on it", or "The users should
have chosen stronger and unique passwords." Of course, that's not the world
we live in...)


On Tue, Feb 19, 2013 at 12:38 AM, Patrick Mylund Nielsen <
patrick@...rickmylund.com> wrote:

> > Oh, the wonders of top-posting.  I can't even comment on that paragraph because
> it is unclear what exactly you were referring to. ;-)
>
> That attackers are compromising servers and sniffing passwords over the
> wire, waiting for re-authentications, rather than just grabbing the
> database.
>
> > Actually, dealing with online attacks is not always that easy - in particular,
> if we consider that some may target password reuse from other sites.
>
> Sure, but if you actually get the username and password right, the PHC
> can't protect you anyway. My point was that it should not be the focus of
> the PHC to be resistant to online (slow) attacks since that is not where
> we're hurting the most, currently.
>
> > 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.
> ...
> > 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.
>
> This is the point I was trying to get across: It wouldn't hurt to be able
> to strengthen the construction somehow by using a secret that is stored
> elsewhere, but the security of the algorithm should not be contingent upon
> it.
>
> > 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?
>
> I'm not sure if I was unclear here: PHC needs to resist offline attacks
> since we can assume that the database of verifiers/digests will be
> compromised at some point. I don't think this is simplistic/ideal, just
> realistic.
>
>
>
> On Tue, Feb 19, 2013 at 12:27 AM, Solar Designer <solar@...nwall.com>wrote:
>
>> 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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