Re: [Cfrg] erratum for hmac what do we think...

Peter Gutmann <pgut001@cs.auckland.ac.nz> Fri, 03 February 2017 10:24 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Hugo Krawczyk <hugo@ee.technion.ac.il>
Thread-Topic: [Cfrg] erratum for hmac what do we think...
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Date: Fri, 3 Feb 2017 10:24:06 +0000
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Subject: Re: [Cfrg] erratum for hmac what do we think...
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Hugo Krawczyk <hugo@ee.technion.ac.il> writes:

>For example, people wanted to use unlimited passphrases, and between having
>people truncate long keys or hash them first, the latter seemed the more
>robust solution. (BTW, the right way to deal with these issues using HMAC is
>to use HKDF.)

Another solution to the fact that there are two external-format keys that map
to a single internal-format key (i.e. the > blocksize key and its hash) would
be to reserve the first bit of the internal-format key to denote hashed vs.
non-hashed.  OTOH then people would complain about losing one bit of strength,
or someone would propose an attack based on knowing that the first bit will
almost always be zero (for non-hashed), or something similar.  You can't win
:-).

Peter.