Attack, Solution and Verification for Shared Authorisation Data in TCG TPM

The Trusted Platform Module (TPM) is a hardware chip designed to enable computers to achieve greater security. Proof of possession of authorisation values known as authdata is required by user processes in order to use TPM keys. If a group of users are to be authorised to use a key, then the authdata for the key may be shared among them. We show that sharing authdata between users allows a TPM impersonation attack, which enables an attacker to completely usurp the secure storage of the TPM. The TPM has a notion of encrypted transport session, but it does not fully solve the problem we identify. We propose a new authorisation protocol for the TPM, which we call Session Key Authorisation Protocol (SKAP). It generalises and replaces the existing authorisation protocols (OIAP and OSAP). It allows authdata to be shared without the possibility of the impersonation attack, and it solves some other problems associated with OIAP and OSAP. We analyse the old and the new protocols using ProVerif. Authentication and secrecy properties (which fail for the old protocols) are proved to hold of SKAP.