Last Call Review of draft-campbell-sip-messaging-smime-03

Request Review of draft-campbell-sip-messaging-smime
Requested rev. no specific revision (document currently at 05)
Type Last Call Review
Team Security Area Directorate (secdir)
Deadline 2018-10-10
Requested 2018-09-12
Draft last updated 2018-10-08
Completed reviews Secdir Last Call review of -03 by Liang Xia (diff)
Genart Last Call review of -03 by Peter Yee (diff)
Assignment Reviewer Liang Xia
State Completed
Review review-campbell-sip-messaging-smime-03-secdir-lc-xia-2018-10-08
Reviewed rev. 03 (document currently at 05)
Review result Ready
Review completed: 2018-10-08


In general, this draft is clear and well written.

I have 2 comments as below:
1. For Section 6, is it required to protect the UA capabilities negotiation messages? Have you considered the possible downgrade attack, like: the adversary tampered the UA capabilities negotiation message to make the UA work without s/mime? 

2. For Section 12, is there any general way to defend against the attacks from malicious or compromised intermediaries mentioned in this section? Maybe some guidances are helpful here.