[tor-dev] Comments on proposal 224 updates

[Nick Mathewson]

I'm behind on this!  But let me comment on the stuff that got merged recently.

    "prop224: Remove the MAINT_INTRO feature."
    "prop224: Clarify backwards compatibility sections slightly."
    "prop224: Clarify use of shared random values."
    "prop224: Fix undefined variables in the ntor section."

Looks ok.

    "prop224: Add missing key expansion section for rendezvous crypto."

KDF-Tor is a legacy algorithm, and we want to get away from SHA1. We
don't use KDF-Tor with NTOR. Let's kill it for new use. I'd suggest
HKDF-SHA256 or SHAKE, both of which we have implemented already.

   "prop224: In cells, replace TYPE/LEN/KEY with just TYPE/KEY."

I'm not sure about this change.  If we ever add a third key type, all
of the cells will be unparseable.

   "prop224: Various improvements."

There's no point in using HKDF with SHAKE; we should just use SHAKE as the KDF.