A vector model of trust to reason about trustworthiness of entities for developing secure systems

Security services rely to a great extent on some notion of trust. In all security mechanisms there is an implicit notion of trustworthiness of the involved entities. Security technologies like cryptographic algorithms, digital signature, access control mechanisms provide confidentiality, integrity, authentication, and authorization thereby allow some level of 'trust' on other entities. However, these techniques provide only a restrictive (binary) notion of trust and do not suffice to express more general concept of 'trustworthiness'. For example, a digitally signed certificate does not tell whether there is any collusion between the issuer and the bearer. In fact, without a proper model and mechanism to evaluate and manage trust, it is hard to enforce trust-based security decisions. Therefore there is a need for more generic model of trust. However, even today, there is no accepted formalism for specifying and reasoning with trust. Secure systems are built under the premise that concepts like "trustworthiness" or "trusted" are well understood, without agreeing to what "trust" means, what constitutes trust, how to measure it, how to compare or compose two trusts, and how a computed trust can help to make a security decision.