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Date: Fri, 12 Dec 2014 10:41:09 +0100 (CET)
From: Stefan.Lucks@...-weimar.de
To: discussions@...sword-hashing.net
Subject: Re: [PHC] How important is salting really?

On Thu, 11 Dec 2014, Taylor Hornby wrote:

> Another reason is that without a salt, the attacker can pre-compute
> lookup tables before they get access to the hashes. In terms of
> organized crime that means there'll be large-scale cracking services
> that can invest in creating MASSIVE rainbow tables and make a profit
> selling access to them. With salt, that entire effort has to be re-done
> for each breach (and hash within a breach).

Actually, you don't need *rainbow* tables for that!

Computing a list with a few million pairs

 	(Hash(PW_i), PW_i),

(with a few million likely passwords PW_i) isn't a big deal. Probably, 
many users actually log in with one of the PW_i on the list. Cracking a 
likely password on any machine is just a single look-up from the list.

Rainbow tables are only interesting, if you want a much longer list, such 
that you cannot afford to store all the (Hash(PW_i), PW_i) any more.

I have two further points:

   1. Observing two (or more) users on some server did choose the same
      password isn't much of an attack. Except that one can draw
      conclusions about server's user base and their typical password
      strength: For a fixed number of users:

 	more password collisions ~ weaker passwords.

   2. Traditionally, the salt is often chosen randomly. But there is
      no need for a *random* salt at all. The salt just needs to be unique.

      E.g., S = H(date, user_id, server_id) would make an excellent salt:

       * H is a cryptographic hash function,

       * date is date (and time) when the password is set,

       * user_id is a unique user id, and

       * server_id uniquely describes the server
         (URL, or IP-address, or whatever)

      If you know date, user_id and server_id, you can compute S on the
      fly, rather than storing the salt.

So long

Stefan


------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
     <http://www.uni-weimar.de/cms/medien/mediensicherheit/home.html>
--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universit├Ąt Weimar, Germany--

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