Tobias "Tobi" Buchberger
Tox  is a free and open source peer-to-peer instant messaging protocol and implementation, that aims to provide secure messaging. It’s intended as an end-to-end encrypted (E2EE) and distributed Skype replacement. Tox’ cryptography is based on the NaCl library from Daniel J. Bernstein . The cryptographic primitives for the key exchange (X25519), authentication (Poly1305) and symmetric encryption (XSalsa20) are state of the art peer-reviewed algorithms.
Unfortunately Tox’ authenticated key exchange (AKE) during Tox’ cryptographic handshake is a "home-brewed" cryptographic protocol (remember: do not roll your own crypto!) and is known to be vulnerable to key compromise impersonation (KCI) attacks . In this talk we will show why this vulnerability is challenging to exploit in practice. However, we will also present a fix to this vulnerability by designing and implementing a new cryptographic Tox handshake with formally-verified security properties.