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diff --git a/docs/olm.rst b/docs/olm.rst index af42215..34bf9ba 100644 --- a/docs/olm.rst +++ b/docs/olm.rst @@ -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 --- |