[Ace] minor comments on draft-ietf-ace-oscore-profile-16

[Benjamin Kaduk <[e-mail]>]

Hi all,

I was going through the four drafts that have been "waiting for writeup"
for a while, to check that the latest changes are good and they are ready
to go once the last point from the secdir review of
draft-ietf-ace-dtls-authorize is wrapped up.  In short: they are, but I had
a couple comments on the OSCORE profile that might help improve it.

In section 2, we have some discussion:

   The use of nonces during the exchange prevents the reuse of an
   Authenticated Encryption with Associated Data (AEAD) nonces/key pair
   for two different messages.  Reuse might otherwise occur when client
   and RS derive a new Security Context from an existing (non- expired)
   access token, as might occur when either party has just rebooted, and
   might lead to loss of both confidentiality and integrity.  Instead,
   by using nonces as part of the Master Salt, the request to the authz-
   info endpoint posting the same token results in a different Security
   Context, by OSCORE construction, since even though the Master Secret,
   Sender ID and Recipient ID are the same, the Master Salt is different
   (see Section 3.2.1 of [RFC8613]).  If nonces were reused, a node
   reusing a non-expired old token would be susceptible to on-path
   attackers provoking the creation of OSCORE messages using old AEAD
   keys and nonces.

Where we talk about how the nonces (N1 and N2) exchanged during the
authz-info request/response are used to prevent the use of nonce+key
combinations for the AEAD used for the OSCORE messages.  But there's really
two classes of nonce: the ones for the AEAD, and the ones used in
constructing the master salt.  Whenever we just say "nonce" or "nonces"
there is potential for ambiguity, so we might want to add an adjective
every time we use the word, as tedious as it is to do so.


Also in Section 2, I just wanted to check on the location of the "mutual
authentication" indication -- currently it's show after the second OSCORE
Response, but I am not sure why it is not achieved after just the first
Request/Response exchange that performs proof of possession.

Thanks!

-Ben