Commit d48dc819 authored by Richard van der Hoff's avatar Richard van der Hoff Committed by GitHub
Browse files

Document the unknown key-share attacks and mitigation (#29)

parent 351b26fa
......@@ -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
---
......
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