[TLS] Key Hierarchy

Eric Rescorla <ekr@rtfm.com> Sun, 20 September 2015 23:58 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 20 Sep 2015 16:58:12 -0700
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Subject: [TLS] Key Hierarchy
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https://github.com/tlswg/tls13-spec/pull/248

Folks,

Hugo Krawczyk, Hoeteck Wee, and Bjorn Tackmann suggested a revision
to the key hierarchy that separates out the computation of the MS from the
computation of the keys that are derived from ES and SS. Specifically,
xES and xES are to be used to derive their respective traffic keys and
intermediate values mES and mES which are then used with HKDF-Extract
to generate MS.

Aside from some analytic advantages, this also allows us to use
the HKDF-Extract and HKDF-Expand APIs from RFC 5869 which is
convenient (it's also compatible with all-in-one HKDF APIs).

The PR is at:
https://github.com/tlswg/tls13-spec/pull/248

I think this is a good change, but comments welcome.

-Ekr