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. Summary: Needs some additional explanation The basic mechanisms look fine. What I am less happy with are the security considerations. The security challenge in SIP is trying to introduce security into a legacy infrastructure that has none. As such, there is inevitably an element of attempting to nail jello to a wall: the nails are strong enough but the jello is not. I think there needs to be more discussion of the potential shortcomings of the input data. The other issue I had was with the requirements for administration of keys. There is a MUST here: "The operator MUST change the key on a frequent basis." What is the security concern driving this requirement? Changing keys has security costs as well as benefits. It is not something that should be done for the sake of it.