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@@ -298,6 +298,37 @@ and the IV :math:`AES\_IV_{i,j}` to give the cipher-text, :math:`X_{i,j}`.
Then the entire message (including the Version Byte and all Payload Bytes) are
passed through HMAC-SHA-256. The first 8 bytes of the MAC are appended to the message.
+Message authentication concerns
+-------------------------------
+
+To avoid unknown key-share attacks, the application must include identifying
+data for the sending and receiving user in the plain-text of (at least) the
+pre-key messages. Such data could be a user ID, a telephone number;
+alternatively it could be the public part of a keypair which the relevant user
+has proven ownership of.
+
+.. admonition:: Example attacks
+
+ 1. Alice publishes her public Curve25519 identity key, :math:`I_A`. Eve
+ publishes the same identity key, claiming it as her own. Bob downloads
+ Eve's keys, and associates :math:`I_A` with Eve. Alice sends a message to
+ Bob; Eve intercepts it before forwarding it to Bob. Bob believes the
+ message came from Eve rather than Alice.
+
+ This is prevented if Alice includes her user ID in the plain-text of the
+ pre-key message, so that Bob can see that the message was sent by Alice
+ originally.
+
+ 2. Bob publishes his public Curve25519 identity key, :math:`I_B`. Eve
+ publishes the same identity key, claiming it as her own. Alice downloads
+ Eve's keys, and associates :math:`I_B` with Eve. Alice sends a message to
+ Eve; Eve cannot decrypt it, but forwards it to Bob. Bob believes the
+ Alice sent the message to him, wheras Alice intended it to go to Eve.
+
+ This is prevented by Alice including the user ID of the intended recpient
+ (Eve) in the plain-text of the pre-key message. Bob can now tell that the
+ message was meant for Eve rather than him.
+
IPR
---