Identity protection attack description

#241

Good to describe that the known attack on Sigma is that the attacker create a new message_1, no attack is known where an active attacker modifies the ongoing connection (even if the lack of such attacks is likely not proven).

draft-ietf-lake-edhoc.md

@@ -1135,7 +1135,7 @@ For many constrained IoT devices it is problematic to support several crypto pri
 
 EDHOC inherits its security properties from the theoretical SIGMA-I protocol {{SIGMA}}. Using the terminology from {{SIGMA}}, EDHOC provides forward secrecy, mutual authentication with aliveness, consistency, and peer awareness. As described in {{SIGMA}}, peer awareness is provided to the Responder, but not to the Initiator.
 
-As described in {{SIGMA}}, different levels of identity protection are provided to the Initiator and the Responder. EDHOC protects the credential identifier of the Initiator against active attacks and the credential identifier of the Responder against passive attacks. The roles should be assigned to protect the most sensitive identity/identifier, typically that which is not possible to infer from routing information in the lower layers.
+As described in {{SIGMA}}, different levels of identity protection are provided to the Initiator and the Responder. EDHOC protects the credential identifier of the Initiator against active attacks and the credential identifier of the Responder against passive attacks. Unless the lower layers are encrypted in some way, an active attacker can eavesdrop on the destination address used for transporting message_1 and send its own message_1 to the same address. The roles should be assigned to protect the most sensitive identity/identifier, typically that which is not possible to infer from routing information in the lower layers.
 
 Compared to {{SIGMA}}, EDHOC adds an explicit method type and expands the message authentication coverage to additional elements such as algorithms, external authorization data, and previous messages. This protects against an attacker replaying messages or injecting messages from another session.