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Date: Tue, 17 Mar 2015 10:22:51 -0700
From: "Dennis E. Hamilton" <>
To: <>
Subject: RE: [PHC] pre-hashed passwords?

 -- in reply below to --
From: Jean-Philippe Aumasson [] 
Sent: Tuesday, March 17, 2015 06:09
Subject: [PHC] pre-hashed passwords?

(After reading this nice post about passwords including null bytes:

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)

   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).

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