miTLS is a verified reference implementation of the TLS protocol. Our code fully supports its wire formats, ciphersuites, sessions and connections, re-handshakes and resumptions, alerts and errors, and data fragmentation, as prescribed in the RFCs; it interoperates with mainstream web browsers and servers. At the same time, our code is carefully structured to enable its modular, automated verification, from its main API down to computational assumptions on its cryptographic algorithms.
The stable version of miTLS including the new 0.9 release are written in F# and specified in F7. We present security specifications for its main components, such as authenticated stream encryption for the record layer and key establishment for the handshake. We describe their verification using the F7 refinement typechecker. To this end, we equip each cryptographic primitive and construction of TLS with a new typed interface that captures its security properties, and we gradually replace concrete implementations with ideal functionalities. We finally typecheck the protocol state machine, and thus obtain precise security theorems for TLS, as it is implemented and deployed. We also revisit classic attacks and report a few new ones.
TLS is possibly the most used secure communications protocol, with a long history of flaws and fixes, ranging from its protocol logic to its cryptographic design, and from the Internet standard to its diverse implementations.
This work is the result of the long-term collaboration between Microsoft Research and Inria, the two partners in the Microsoft-Inria Joint Centre.