Hello, 

the ID is essentially a IANA registration of 2 elements, with a good and detailed analysis of applicability scenarios.

The only nit suggestion is to reformulate the paragraph where "msg" and "msgi" are introduced into the specification; the current version is:

   In order to differentiate a PASSporT for an individual message from a
   PASSporT used to secure a telephone call or message stream, this
   document defines a new "msg" PASSporT Type. "msg" PASSporTs may carry
   a new optional JWT [RFC7519] claim "msgi" which provides a digest
   over a MIME body that contains a text or multimedia message.

which does not put in evidence this is a "definition". I would suggest to change it as:

   A PASSporT for an individual message must be different from a
   PASSporT used to secure a telephone call or message stream. As such this
   document defines a new PASSporT Type:

     "msg"

   A "msg" PASSporTs may carry a new optional JWT [RFC7519] claim named

     "msgi" 

   which provides a digest over a MIME body that contains a text or multimedia message.

No further suggestions from my side.

all the best
Claudio