Adopting the Noise Key Exchange in Tox

Tox is a P2P instant messaging protocol that aims to provide secure messaging. It's implemented in a FOSS library called "c-toxcore" (GPLv3). The project started in the wake of Edward Snowden's disclosure of global surveillance. It's intended as an end-to-end encrypted and distributed Skype replacement. The cryptographic primitives for the key exchange (X25519), authentication (Poly1305) and symmetric encryption (XSalsa20) are state of the art peer-reviewed algorithms. Tox' authenticated key exchange (AKE) during Tox' handshake works, but it is a self-made cryptographic protocol and is known to be vulnerable to key compromise impersonation (KCI) attacks. This vulnerability enables an attacker, who compromised the static long-term private X25519 key of a Tox party Alice, to impersonate any other Tox party (with certain limitations) to Alice (reverse impersonation) and to perform Man-in-the-Middle attacks. The objective of this project is to implement a new KCI-resistant handshake based on NoiseIK in c-toxcore, which is backwards compatible to the current KCI-vulnerable handshake to enable interoperability. Further Noise's rekey feature will be evaluated for adoption.