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Date: Sun, 15 Dec 2013 20:00:48 -0500
From: Joseph Bonneau <>
Subject: Re: [PHC] Intentionally increasing password hash collisions

Sorry to be a week late here. Some very good discussion though, thanks a
lot for kicking it off Matt.

I've argued unsuccessfully for this change many times with websites
deploying it. I think Taylor's point #1 about game theory is on the right
track, though more simply I see two related things:
*Many sites can't fathom that they might leak their password database and
hence don't think it's worst investing time/thought into a feature that is
only useful after a leak.
*Savvier sites figure that if they were using a collision-rich hash
function (my preferred term) and they leaked their password database, this
would be too complicated to come out in news coverage as a mitigating
factor and hence they would take an equivalent publicity hit anyways, so
what would this really buy them?

I don't see Taylor's #2 as a major concern. Website users already can't
manage their own security against all sorts of risks (like how effectively
websites rate-limit to begin with) so making a smart choice on their behalf
here is sensible, though it's probably worth having a disclaimer for power

The other big objection is that the gain here may not be very big in
practice. Nobody I've talked to would accept less than 30 bit hashes,
assuming they can hope to keep attackers to 2^20 guesses or thereabouts
using rate-limiting and they want a security margin, and sometimes I've
heard 40 bits as the minimum. Given how weak most passwords are, with 40
bits leaked you're really only protecting pretty serious power users, and
even they're probably vulnerable with 40 bits leaked + 20 bits
rate-limiting at a second site where the password has been re-used. Even
with 30 bits leaked you've effectively given up the majority of your users

So I have supported this approach for a while but I think in the big
picture it's a pretty marginal improvement. Though it is very easy to
implement-the hurdle to implementing isn't writing one line of code to drop
bits, it's not knowing if this is a good idea at all and not wanting to do
the analysis. If there were a nice write-up and NIST or some other source
viewed as authoritative officially endorsed this approach perhaps that
would make a difference.



On Mon, Dec 9, 2013 at 5:25 PM, Matt Weir <> wrote:

> 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
> data.
> 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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