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

Taylor R Campbell <campbell+cfrg@mumble.net> Thu, 02 February 2017 21:57 UTC

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From: Taylor R Campbell <campbell+cfrg@mumble.net>
To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
In-reply-to: <666efaf7-b660-e20b-8a8a-8949a64e9bed@cs.tcd.ie> (stephen.farrell@cs.tcd.ie)
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Subject: Re: [Cfrg] erratum for hmac what do we think...
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> Date: Thu, 2 Feb 2017 02:24:44 +0000
> From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
> 
> There's an erratum posted for hmac [1] where I'd be
> interested in what folks here think.

Fix hash function H with block size b.  The alleged problem is that
for a key k, if |k| > b and h = H(k), then HMAC-H_h = HMAC-H_k.

What protocol would ever use HMAC-H with related keys of different
lengths like this for the same purpose?  What protocol would ever use
HMAC-H with keys of different lengths at all for the same purpose?

Needless flexibility like that strikes me as more likely to indicate a
deeper problem -- e.g., one reason that might happen is that the
protocol foolishly uses a password verbatim as an HMAC key.

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