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Date: Sun, 17 Feb 2013 00:20:55 -0800
From: Jeremi Gosney <>
To: Marsh Ray <>
CC: Jens Christian Hillerup <>, 
 Jens Steube <>,
 "" <>
Subject: Re: [PHC] Different cost settings and optional input

On 2/16/2013 11:18 PM, Marsh Ray wrote:
> Salting for Linkedin would have increased the difficulty for the attackers *hardly at all*. [...] So...salts aren't always be the big win they look like on paper because cracking tools have gotten far more faster than users have gotten better at generating and remembering entropic strings.

False. Assuming a unique salt per user, salting would have slowed down
our attacks by a factor of 6.4 million. At the time that we were
cracking the LinkedIn hashes, I was pulling about 15.5 billion hashes
per second. With 6.4 million unique salts, I would have been reduced to
an effective rate of about 2.4 *thousand* per second.  At that rate it
would have taken over an hour and a half to just run through rockyou.txt
without any rules -- and 101 days just to run through rockyou.txt with
d3ad0ne.rule. Even with the GPU power that I have now (I can pull about
63 G/s with SHA-1) it still would take a full month to run through just
rockyou.txt with d3ad0ne.rule. And mind you, we have a *lot* more
horsepower than your typical password cracker.

That is what we call "crippling." A little salt goes a long way.

> GPU cracking has gotten so fast that precomputed rainbow tables often not even worth the trouble.

Agreed, but they are becoming more relevant now that we are able to
generate tables that are too large to distribute, and provide remote
chain lookups over the internet as a service. This is useful for lengths
that are too large to brute force in a reasonable amount of time, even
with lots of GPUs.

But rainbow tables is not what I had in mind when I said that using a
fixed salt defeats the purpose of salting. I meant you don't get the
benefit of dramatically slowing all other bulk attacks.

> When you can compute Gigahashes per second on a single video card, a dictionary proves to be more trouble than it's worth for the initial pass.

Well no, that's not true at all. We rely very heavily on our
dictionaries. Dictionary-based attacks are *always* the first pass --
and the second, and the third, and the fourth, and the fifth. All the
GPU means is that we can run more rule-based and hybrid attacks in a
shorter amount of time. Even with ungodly amounts of power, we rarely do
any brute forcing. And when we do, it's usually just for specific masks
and small keyspaces.

> So if a defender said "forget the salt...we'll thwart dictionary attacks by preventing duplicate passwords", could he possibly be better off with this strategy?

No. Just because you do not allow duplicate passwords on your site
doesn't mean these people aren't 1) re-using passwords on other sites,
and 2) aren't thinking like other people who use the same passwords on
other sites. People have been trained to pick passwords specific ways,
that's why the markov model works so well. If you prohibit people from
using Password1, they'll use Password!.  So if a straight dictionary
attack doesn't get the job done, a rule-based attack or hybrid attack
probably will.

> But when new user B independently chooses existing user A's password, we know two things: that the password is not unguessable and probably worth adding to a dictionary. We can't allow B to use it. We also have informed B that some user account has that password

Hmm, sounds like a great way for an attacker to start harvesting
passwords from a site without ever having to compromise it. ;)


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