[TLS] Binder key labels for imported PSKs

["Christopher Wood" <[e-mail]>]

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