phc-discussions - Re: [PHC] Upgrade HKDF to HKDF2?

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Date: Sun, 09 Mar 2014 07:28:02 -0700
From: Jeremi Gosney <epixoip@...dshell.nl>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Upgrade HKDF to HKDF2?

On 3/8/2014 10:48 AM, Bill Cox wrote:
> HMAC: Input collisions galore!  Really?!?  Why are we using this for
> hashing passwords???
> PBKDF2: Inherits HMAC collisions

HMAC itself isn't the problem. PBKDF2 doesn't inherit an issue from
HMAC, it creates an issue with HMAC.

Collisions only occur when using the password as the key and the salt as
the message. Using the salt as the key and the password as the message
resolves this issue. Generate salts that are the exactly $blocksize in
length, and you don't have to worry about null padding issues, either.

PBKDF2PEBKAC.