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Date: Mon, 9 Dec 2013 17:25:19 -0500
From: Matt Weir <>
Subject: Re: [PHC] Intentionally increasing password hash collisions

Thanks Taylor,
    So there's another question that I've been thinking about that hits
some of the points you brought up. What do you do when systems that most
need hash collisions are the ones least likely to implement it? Aka we'd
see the largest overall security benefit and least downsides of using this
strategy on sites that don't practice strong security and contain low value

If this proves to be a useful technique, ideally I'd like to see it
incorporated into common forum software, (PhpBB, VBulletin, etc), as the
default option. Also by default it would only apply the shorter hashes to
accounts that do not have administrative privileges. Site administrators
would be able to disable this if they choose. That's just like in PhpBB
right now where you can choose a more time intensive hash than the default
if you want.

By including this scheme as the default option of common software it helps
address the game theory question you brought up. While it might not be in
the site's best interests to go with such a scheme, (side note: In this
case a site's interests and a site's users' interests are two different
things), it may be in the forum software maker's interests to enhance the
security of the community as a whole. Since it would be the default option,
sites that spend the time hardening their systems might deem it worthwhile
to opt out if they want, but the vast majority of low security sites would
end up using this scheme.

That's not to say that this should only be applied to low security sites.
In higher security sites such a scheme might make lateral movement more
costly and easier to detect. Likewise if a site does get popped, using a
scheme like this could help provide some PR benefit. All I'm saying is that
there's ways this scheme could be deployed to a large enough audience that
it could start to make an impact even though the benefits to each site to
individually opt into it are low.

As to your second point about experienced users not being able to pick
their own security, I think it'd be good to define what "experienced user"
means. I would argue that in this context "experienced user" would be
someone who uses an unique password for the site in question that isn't one
of the top 20k most likely passwords. I'm using this definition because
those are the advanced users that will see the least benefit from this
scheme. I'm focusing on the importance of "unique" since if you reuse your
password, even if it's "uncrackable", it might end up in a wordlist someday
because the one of the other sites stored the password in plaintext.
Likewise, if a user's password is unique, even if it's crackable it would
provide very little benefit to an attacker. Aka if the attacker has access
to the user's hashes, they probably also have access to their data on the
site. I also like this definition because "unique" is much easier to define
that "strong" ;p Side note, I'm still struggling with what to call these
users since "advanced" or "experienced" carries too much luggage in my
opinion. I'm currently leaning towards "password-vault users" since that's
pretty much required to choose an unique password per site. The downside is
that ignores a user who for whatever reason may deem it important to have
an unique password for a particular site which this scheme is applied, but
not to others. In that case they don't need to use a password-vault.

So the question to then ask is how many users are password vault users, and
how prevalent is password reuse? There's some studies on this out there. My
favorite is

Really though the question is do the negative aspects of this scheme to
password vault users outweigh the positive aspect it provides to non
password vault users? That takes a little more arguing than I'd like to go
into this e-mail, and quite honestly I'm still working on that. My current
feeling is that in a lot of cases the answer is "yes". While I can go into
the numbers from previous studies like the one I mentioned above, one quick
reasoning is that the rate at which we're seeing password reuse attacks
occurring in the wild says that this is certainly a problem that needs to
be addressed, and the added risk imposed on password vault uses by this
scheme is low in a vast majority of cases. Aka most people's accounts
aren't worth the investment of a 20k node botnet to crack.

To further minimize the risk, and to pull the responsibility off of users
to be secure, (we need to take into account how people behave when
designing systems), I think it's useful to instead look at the worth of
user accounts when determining when to apply this scheme. That's why in the
above suggestion the default option would be to not apply reduced hashes to
administrator accounts. Those are the accounts that are much more likely to
be targeted directly by an online attack where the attacker is willing to
invest the resources to obtain a login via collision.

Now we can start looking at the overlap in Venn Diagrams of:
1) Systems/Sites that are secure vs hacking attempts
2) Users on those sites that pick passwords that aren't guessable in an
online attack
3) Users that would be inconvenienced or harmed if their account on that
site was compromised
4) Accounts that would be worthwhile for an attacker to invest the
time/resources to find a login via password collision
5) Accounts have the reduced password hash scheme applied to them

And compare it to the Venn Diagram of:
1) Systems that will be successful hacked
2) Users who reused password on that site with another site the attacker
wants access too. Note in this case reuse doesn't require a direct reuse
but variations as well. Aka attackers will often mangle known passwords.
Aka if they crack "password1" they might also try submitting "password2"
and "password3"
3) Users who had "crackable" passwords given the hashing scheme of the site.
4) Users who would be inconvenienced or harmed if the attacker compromised
their account referenced in #2

As to your third point about "rate limiting on the internet is hard" I
agree with you. On the flip side, a lot of valuable sites, (google,
facebook, twitter, etc), do perform rate limiting which can increase the
costs of making large number of guesses against them. Aka while the sites
that make use of reduced hashes may not employ rate limiting, when
attackers attempt to use those collisions vs high value sites it can become
expensive thus protecting the users who had accounts on both those sites.
Of course your comment mostly was focused on the fact that if rate limiting
is not used, or if an attacker devotes enough resources to bypass rate
limiting, they will eventually be able to gain access to an account with
reduced hashes. I'm not arguing with that so much as saying there's a lot
of situations where the cost/benefit ratio of reduced hashes benefits the
defender more than the attacker. In situations where it doesn't exceptions
can be made for classes of accounts, individual accounts, or entire sites.

Matt Weir

On Mon, Dec 9, 2013 at 2:05 PM, Taylor Hornby <> wrote:

> On 12/09/2013 10:55 AM, Matt Weir wrote:
> > 1) If an attacker gains access to the hashes but does not have access to
> > the individual user accounts, (an example would be a SQL injection attack
> > with only SELECT privileges), then by cracking the hash they can log in
> as
> > the user
> >
> > 2) The attacker is attempting to gain knowledge about user's raw password
> > for use in attacking other sites/services.
> >
> > The core idea behind this submission is that it may be worth giving up
> the
> > security in use case 1, as well as making it possible for an attacker to
> > log into a site via a collision, with the end goal of making use case 2
> > more costly for an attacker. Or to put it another way, there's a lot of
> > sites on the internet that are not valuable to users, but are valuable to
> > attackers looking to steal credentials for use in attacking more valuable
> > sites.
> I'm inclined to like this idea, but three objections are always raised,
> and never seem to be resolved:
> 1. Game theory: A rational website would not decrease its own security
> to make the user's account on other websites (business competitors) more
> secure.
> 2. "Experienced" users cannot choose their own level of security. They
> are forced to have their security downgraded because other users are
> re-using passwords on other websites when they are not. They're using a
> 64-character random ASCII password but someone could still log in to
> their account after a million online requests. This almost punishes good
> user behavior.
> 3. Rate limiting and account lockout across the Internet is hard.
> You can't just sleep(5000) before each authentication request, because
> requests can be made in parallel. You can't rate limit based on the
> source address, since it's easy to get tons of IPs (botnets, IPv6). You
> can rate limit based on the account, but this makes it easier to DoS a
> specific user and doesn't stop attackers from sending parallel requests
> to many *different* accounts.
> If hashes are 20 bits, and the attacker has 2^20 IP addresses, I don't
> see how you could reasonably prevent them from getting into at least one
> account without knowing the real password.
> I am very interested to see if you've solved any of these problems.
> This idea was also discussed after I proposed it to the GRC newsgroups
> in 2011, there might be something useful there:
> See the thread "Are Short Password Hashes Safer?", which starts with
> (I am "FireXware")
> --
> Taylor Hornby

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