phc-discussions - Re: [PHC] HKDF has collisions with different salts

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Date: Sat, 15 Mar 2014 01:38:19 +0000
From: Peter Gutmann <pgut001@...auckland.ac.nz>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] HKDF has collisions with different salts

Bill Cox <waywardgeek@...il.com> writes:

>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.

This dates back to Abadi and Needham's "Prudent Engineering Practice for
Cryptographic Protocols", circa 1996 or so.  One of their design principles
was "every message should say what it means", i.e. there should be no
ambiguity about interpreting its content.  Take for example SSHv1
fingerprints:

  hash( n || e )

where n and e are the raw values.  This creates an equivalence class of keys
where you can interpret the composite string 'n || e' as a variety of
different keys based on which bits you choose to be n and which you choose to
be e.  So defining an input format that makes it very clear what data belongs
to which element is critically important.  This can be done either explicitly
(PBKDF2 does this, with distinct input parameters for the salt, iteration
count, and secret value), or explicitly (by defining a marshalled format for
the data for a generic KDF).

Peter.