phc-discussions - password obfuscation

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Date: 23 Feb 2015 22:28:10 -0000
From: "D. J. Bernstein" <djb@...yp.to>
To: discussions@...sword-hashing.net
Subject: password obfuscation

A year ago Wired reported that a "Cryptography Breakthrough Could Make
Software Unhackable":

   http://www.wired.com/2014/02/cryptography-breakthrough/

This article

   * advertised the concept of software obfuscation: your software
     "could be captured by the enemy, interrogated, and disassembled,
     but it couldn’t be forced to reveal your secrets”;

   * said that previous obfuscation techniques were at most a "speed
     bump" ("An attacker might need a few days to unlock the secrets
     hidden in your software, instead of a few minutes"); and

   * said that a new "obfuscation scheme is unbreakable" and is the
     "best possible obfuscator" if "a certain newfangled problem about
     lattices is as hard to solve as the team thinks it is".

This sounds like a great replacement for password hashing: obfuscate a
program such as "allow access if the input is XYZZY" to obtain a program
that "couldn't be forced to reveal" the password XYZZY.

Apon, Huang, Katz, and Malozemoff presented an implementation of this
new obfuscation scheme at the Crypto 2014 rump session (with a limit on
the "depth" of the obfuscated program, but implementing _any_ of this is
impressive). They issued a challenge---a 25-gigabyte obfuscation of a
14-bit password---and said that breaking it was "not feasible".

You might complain that you can't afford 25 gigabytes per obfuscated
password (and you might wonder what happens to passwords longer than 14
bits). But if you have only a few passwords and lots of disk space then
you might not think that this is a showstopper.

My main point in this message is that, contrary to what the Wired
article might make you think, this "cryptography breakthrough" is much,
much, much less _secure_ than any normal password-hashing function. So
please don't throw away PHC!

Concretely, the "Obviouscation team" broke the challenge in 19 minutes
on a cluster of 21 PCs. I'm a small part of this team. We announced the
solution on Twitter in October, and have now released the paper and our
latest software:

   http://obviouscation.cr.yp.to

The original implementation took 4 hours to check a password; our attack
is 10000 times faster. We gained about 50x from removing redundancies in
how each password is checked, and another 200x from a meet-in-the-middle
attack that exploits the matrix structure of this "obfuscation" and ends
up sharing most computations across passwords---a huge security failure
that of course wouldn't be tolerated in PHC. The paper also explains
further speedups, and generalizations to other "obfuscated" programs.

---Dan