phc-discussions - RE: [PHC] pre-hashed passwords?

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Date: Tue, 17 Mar 2015 10:22:51 -0700
From: "Dennis E. Hamilton" <dennis.hamilton@....org>
To: <discussions@...sword-hashing.net>
Subject: RE: [PHC] pre-hashed passwords?

 -- in reply below to --
From: Jean-Philippe Aumasson [mailto:jeanphilippe.aumasson@...il.com] 
Sent: Tuesday, March 17, 2015 06:09
To: discussions@...sword-hashing.net
Subject: [PHC] pre-hashed passwords?

(After reading this nice post about passwords including null bytes:
http://blog.ircmaxell.com/2015/03/security-issue-combining-bcrypt-with.html)

Has anyone already seen password hashes "pre-hashing" a password, to
handle length limitations? Things like

password_hash(hash('sha256', $password, true), PASSWORD_DEFAULT)

password_hash(hash_hmac('sha256', $password, $key, true), PASSWORD_DEFAULT)

<orcmid>  
   PBKDF2 does that when the password length exceeds a block-size 
   limitation. I have also seen deployed usage of PBKDF2 where all 
   passwords are pre-hashed (and therefore within the limitation).  
   (Unfortunately, there is a resulting pass-the-hash attack in 
   the case at hand.)

   If you are going to do this, it seems to me that an HMAC 
   form should be used, with a likely-unique key (some transform 
   of a crypto-quality IV, for example).
</orcmid>