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Date: Mon, 9 Dec 2013 12:55:04 -0500
From: Matt Weir <>
Subject: Intentionally increasing password hash collisions

Hi, I'm working on a submission to PHC. I wanted to give people an overview
of it and hopefully get some feedback.

In a nutshell I'm arguing there's many cases where it may be worthwhile to
intentionally increase collisions in password hashing algorithms to make
password reuse attacks more expensive.

Taking a step back, attackers typically crack password hashes for two

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

Now ideally the password reuse problem would be solved by everyone using a
password manager and choosing unique passwords for every single site. That
requires users to change their habits though. Another good solution would
be to use a computationally and/or memory expensive hashing algorithm,
(hence the PHC). I'm not arguing strong hashing isn't important. It is.
What I'm saying is adding collisions has the potential to enhance the cost
to an attacker for a password reuse attack regardless of the underlying
hashing algorithm chosen.

Because of that, this isn't a stand alone solution as it is meant to be
largely hash independent, (with a few requirements such as needed a long
random salt). The reason I'm submitting it to PHC is I'd like to have this
idea beat up and analyzed by as many people as possible. Here's a quick FAQ:

Q) How will collisions be created?
A) Collisions will be created by cutting bits off the end of a password
hash. The number of bits will be related to the number of collisions that
are desired to be generated vs the amount of risk the defender is willing
to accept for an attacker to be able to log in during an online guessing
attack via a collision vs the real password.

Q) Besides cutting bits, what else is required under this scheme?
A) A long random salt is required so the same set of collisions are not
shared between multiple sites that use the same password. There's a couple
of other additions I plan on discussion in the submission on how a defender
can minimize some of the risks associated with this scheme, (such as
preventing collisions with common passwords like '123456'), but they aren't
required. In fact I'm not altogether convinced that the security added by
some of the additions offsets their added complexity

Q) How does this make password reuse attacks more expensive?
A) Usually for an attacker, making offline guesses against a hash is
cheaper than making online guesses by trying to log in as the user. With
even the strongest hash an attacker can still make offline guesses at the
same rate as the defender can verify passwords. Online attacks on the other
hand require dealing with all the security mechanisms of the service
itself, (logging, rate limiting, IP blacklisting, etc). That's why
attackers take the time to crack hashes in the first place. Collisions
complicate hash cracking though. The attacker no longer knows if they
cracked the user's real password or created a collisions unless they verify
it some other way, (such as using that guess to successfully log into a
different site). This makes the decision to stop cracking a hash much more
problematic. Collisions also reduce the amount of mangling an attacker can
perform when performing an online attack since they will be “wasting”
guesses on collisions that have no relationship to the user’s real
password. In addition, hash collisions have the potential to substantially
disrupt the 3rd party password cracking marketplace since the person paying
will have a difficult time verifying if the 3rd party cracker is making a
legitimate effort or just generating collisions without attempting to find
the actual password.

Q) What is the recommended default bit length for hashes?
A) Following NIST's SP 800-63-1 Electronic Authentication Guideline,14 bits
for a site requiring Level 1 assurance, and 20 bits for a site requiring
Level 2 assurance.

Q) Will there be an attempt to measure actual cost to an attacker or risk
to the defender in more detail
A) Yup, I expect that will take up a significant portion of the submission.
Any suggestions on this will be apprecaited

Q) Is more information available about this?
A) Yes, I talked about this quite a bit on the crypt-dev mailing list about
a year ago when I was initially exploring the idea:

Q) Is there a name for this method?
A) Not yet, but I'm open to suggestions.

Matt Weir

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