Gregory G Rose ggr at qualcomm.com
Wed Apr 24 12:50:59 EDT 2002

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In article <7xu1q1mdwd.fsf at ruckus.brouhaha.com>,
Paul Rubin  <phr-n2002a at nightsong.com> wrote:
>I should re-read Krawczyk's HMAC paper to see what that fancy padding
>is really supposed to do.  Remember that the underlying encryption
>here (SHA-OFB) probably isn't the greatest in the world either.

The proof of security of NMAC (that HMAC is based
on) requires that the inner and outer hash
functions be independent. For HMAC, this is
approximated by using the padding so that the keys
are effectively different, making the hash
operations effectively independent (wave hands
here).

>And it actually might be faster to use two separate hash-based auth
>keys than deal with the fancy HMAC padding, if a simple double hash
>has some vulnerabilities.

Two different keys would certainly satisfy the
conditions.

Greg.
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