[Lake] Ways forward on MTI cipher suite text

[Mališa Vučinić <[e-mail]>]

Dear all,

During the last LAKE interim meeting, we discussed the issue
of an MTI cipher suite and we agreed for the chairs to open a
thread on the subject. As a reminder, the previous discussion
points on this topic are summarized in github [1] and in
John’s mail dated 13 May 2021 [2].

We’d like to see if there is rough consensus in the WG on
this topic, at this moment in time. Knowing that the formal
analysis of the EDHOC-12 specification is under way, we
should keep in mind that additional input may arrive down the
road from teams working in the computational model.

As a reminder, the most recently discussed text for this
is in a PR [3] and states:

“For many constrained IoT devices it is problematic to support several crypto primitives. Existing devices can be expected to support either ECDSA or EdDSA. Cipher suites 0 (AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA, AES-CCM-16-64-128, SHA-256) and 1 (AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA, AES-CCM-16-64-128, SHA-256) only differ in size of the MAC length, so supporting one or both of these is no essential difference. Similarly for cipher suites 2 (AES-CCM-16-64-128, SHA-256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-256) and 3 (AES-CCM-16-128-128, SHA-256, 16, P-256, ES256, AES-CCM-16-64-128, SHA-256). To enable as much interoperability as possible, less constrained devices SHOULD implement all four cipher suites 0-3. Constrained endpoints SHOULD implement cipher suites 0 and 1, or cipher suites 2 and 3. Implementations only need to implement the algorithms needed for their supported methods.”

The options we see at this moment in time are:

Option 1: Keep current text as-is unless/until more feedback
is provided that motivates re-opening this issue
Option 2: Proceed with selecting a single MTI cipher suite

We'd like to know if the WG can live with Option 1. Note that
doesn't mean you think option 1 is perfect, just that it's
something with which you can live. If you prefer option 2 or
some other option please suggest specific text.

Mališa and Stephen

[1] https://github.com/lake-wg/edhoc/issues/22
[2] https://mailarchive.ietf.org/arch/msg/lake/75nRaD6czYG6RqLT06Qe8C_lsaM/
[3] https://github.com/lake-wg/edhoc/pull/225/files