phc-discussions - Re: [PHC] How important is salting really?

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Message-ID: <20141223015907.17d6e174@lambda>
Date: Tue, 23 Dec 2014 01:59:07 +0000
From: Brandon Enright <bmenrigh@...ndonenright.net>
To: "Jeremy Spilman" <jeremy@...link.co>
Cc: discussions@...sword-hashing.net, epixoip <epixoip@...dshell.nl>,
 bmenrigh@...ndonenright.net
Subject: Re: [PHC] How important is salting really?

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On Mon, 22 Dec 2014 16:49:16 -0800
"Jeremy Spilman" <jeremy@...link.co> wrote:

> When might the target size be one? Well, for example, in a targeted
> or on-demand attack, versus the general case of an attacker trying to
> re-sell in bulk the weakest passwords in the database. In this case
> you might look at what is the average cost to crack just one account.
> You can't rely on hiding among millions of salts to make the attack
> slower.

Right.  Salting only makes sure work done on one hash can't be re-used
for other hashes.  Salting doesn't make things less secure but in some
scenarios it doesn't add any security either.

This is why we need a layered approach to password hashing / storage.

* We need salting to help slow down parallel cracking of big dumps.

* We need salting to prevent pre-computation (rainbow tables).

* We need personalization keys to add an additional secret that in some
  scenarios (where the key remains secret), can prevent any cracking
  whatsoever.

* We need slow hashes to make repeated guessing costly.

* We need high-memory usage to make parallel guessing costly.

* We need to minimize the reusability of work from one guess on the same
  hash to the next (anti-stream processing).

* Users needs to choose good passwords.

A failure in any of these makes the job of the cracker easier.  Salting
is just *one* protection out of many.  It helps in some situations, it
does nothing in others.

Brandon

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