Towards a Verified Reference Implementation of a Trusted Platform Module

We develop a reference implementation for a fragment of the API for a Trusted Platform Module. Our code is written in a functional language, suitable for verification with various tools, but is automatically translated to a subset of C, suitable for interoperability testing with production code, and for inclusion in a specification or standard for the API. One version of our code corresponds to the widely deployed TPM 1.2 specification, and is vulnerable to a recently discovered dictionary attack; verification of secrecy properties of this version fails producing an attack trace and highlights an ambiguity in the specification that has security implications. Another version of our code corresponds to a suggested amendment to the TPM 1.2 specification; verification of this version succeeds. From this case study we conclude that recent advances in tools for verifying implementation code for cryptographic APIs are reaching the point where it is viable to develop verified reference implementations. Moreover, the published code can be in a widely understood language like C, rather than one of the specialist formalisms aimed at modelling cryptographic protocols.