phc-discussions - RE: [PHC] Any "large verifiers" on the panel?

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Date: Mon, 25 Feb 2013 14:34:15 +0100 (CET)
From: Stefan.Lucks@...-weimar.de
To: Marsh Ray <maray@...rosoft.com>
cc: Jeffrey Goldberg <Jeffrey@...dmark.org>,
        "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] Any "large verifiers" on the panel?

On Sun, 17 Feb 2013, Marsh Ray wrote:

> While I'm not at liberty to disclose the exact number of password 
> authentications we process, I can say that it really comes down to 
> deciding how much CPU load you're willing to put on the system. Many 
> systems, you specify a password only once to login, and everything after 
> that is done with cookies. So even a very high work factor setting may 
> not represent a noticeable hit on overall server load.

Ouch! So I log in once at your site (*), and soon close the connection. 
But, since one or more cookies are left on my computer, any trojan that 
later takes over my computer will be able to log in at your site, again?

That is really poor security, and a good reason to delete cookies very 
frequently, I think!

> Another anecdote comes from Moxie Marlinspike when he was at Twitter. We 
> were discussing memory-hard password hashing functions, and his response 
> was to the effect of "yeah we would definitely not be able to handle 
> near as many simultaneous auths as we do now if the shared memory bus of 
> the multicore server were constantly saturated."

Indeed, that is an issue for memory-hard password hashing functions.(**)

Actually, the current way of applying a password-hashing function by the 
server is sub-optimal, at least.

Ideally, given a (slow, memory-hard, or whatver) function F and a 
cryptographic hash function H, the password hash should be X := 
H(F(password, salt, ...)). Now, the client could compute Y := F(password, 
salt, ...), and the server would only have to compute H(Y). So the server 
would neither need many CPU cycles, nor much memory -- and still, 
password, cracking would not get any simpler.

The only assumption is that F cannot be so slow or memory-demanding that 
it would not run reasonably fast on the client at hand.

So long

Stefan

P.S.: Sorry for my late response. I had been on a vacation last week.

-------------
(*)  I understand that it is not really "your" site, of course. ;-)
(**) I'd prefer to call them Password Scrambling Functions, but that is
      just me and my taste.

--
------  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-Universität Weimar, Germany--