[TLS] Binder key labels for imported PSKs

"Christopher Wood" <caw@heapingbits.net> Mon, 02 September 2019 23:29 UTC

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Date: Mon, 02 Sep 2019 16:29:25 -0700
From: "Christopher Wood" <caw@heapingbits.net>
To: "TLS@ietf.org" <TLS@ietf.org>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/D7VEcn0itiBlN5-G8UGyfQ_rToo>
Subject: [TLS] Binder key labels for imported PSKs
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Hi folks,


Per Jonathan Hoyland's recommendation, we're considering adding a new binder_key label ("imp binder") for imported PSKs. Specifically, this changes the key schedule from this:

~~~
              0
              |
              v
    PSK ->  HKDF-Extract = Early Secret
              |
              +-----> Derive-Secret(., "ext binder" | "res binder", "")
              |                     = binder_key
~~~

to this:

~~~
              0
              |
              v
    PSK ->  HKDF-Extract = Early Secret
              |
              +-----> Derive-Secret(., "ext binder"
              |                      | "res binder"
              |                      | "imp binder", "")
              |                     = binder_key
 ~~~

Details can be found in the PR [1]. 

This does not seem to affect the interoperability story (imported keys are further differentiated from non-imported keys). However, it's non trivial, so we'd like feedback from the group before merging the change.

Thanks!
Chris (no hat)

[1] https://github.com/tlswg/draft-ietf-tls-external-psk-importer/pull/10