A Logic for Analysing Subterfuge in Delegation Chains

Trust Management is an approach to construct and interpret the trust relationships among public-keys that are used to mediate security-critical actions. Cryptographic credentials are used to specify delegation of authorisation among public keys. Existing trust management schemes are operational in nature, defining security in terms of specific controls such as delegation chains, threshold schemes, and so forth. However, they tend not to consider whether a particular authorisation policy is well designed in the sense that a principle cannot somehow bypass the intent of a complex series of authorisation delegations via some unexpected circuitous route. In this paper we consider the problem of authorisation subterfuge, whereby, in a poorly designed system, delegation chains that are used by principals to prove authorisation may not actually reflect the original intention of all of the participants in the chain. A logic is proposed that provides a systematic way of determining whether a particular delegation scheme using particular authorisation is sufficiently robust to be able to withstand attempts at subterfuge. This logic provides a new characterisation of certificate reduction that, we argue, is more appropriate to open systems.