phc-discussions - HKDF has collisions with different salts

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Message-ID: <CAOLP8p5zkq6wgngcgnEEh0caz9gQbE-0QDeA+uViRhWGhF1PUw@mail.gmail.com>
Date: Fri, 14 Mar 2014 12:23:29 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: HKDF has collisions with different salts

FYI, if you want to use HKDF and want the output to change with any
change in input, you have to add the salt length to the input key
material.

HKDF inherits the same input collisions from HMAC that PBKDF2
inherited, but they apply to salt instead of the password, which is of
course much better.  Still, I want the output has to change when *any*
input changes, preferably with no known collisions.

Just adding the saltSize parameter to the key material I pass to HKDF.
 That fixes the issue.

Another thing to be careful about: you if you use the
reset/input/result API, you have to zero the context yourself.
hkdfReset does not do it for you.

Bill