[no subject]

I do have two comments on the Security Considerations section.

 1. PGP is mentioned in the 3rd paragraph. S/MIME should also be 
 listed. S/MIME is included in every major mail agent (except the one 
 from Qualcomm). While S/MIME suffers from the lack of ubiquitous 
 certificate enrollment, PGP suffers from the lack of integration into 
 mail stream mail agents. In my opinion, either mention both or 
 neither.

 2. At least two more items should be added to the number list of 
 "mistakes." The first is replay detection. The second is the use of 
 confidentiality mechanisms without integrity mechanisms. 

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