I have reviewed this document as part of the security directorate's
ongoing effort to review all IETF documents being processed by the
IESG. These comments were written primarily for the benefit of the
security area directors. Document editors and WG chairs should treat
these comments just like any other last call comments.

The summary of the review is "Ready with Issue"

Well written document that clearly explains the new mechanism.

I think the last paragraph of the security consideration section needs a bit more details to 
describe the implication of using Unencoded-Digest with encrypted content to make it 
clear how this could lead to leak of information and what could be done to mitigate this risk.