A person can "prove" their trustworthiness by fulfilling an assigned responsibility - and as an extension of that, by not letting down expectations. The responsibility can be either material (such as delivering a mail package on time) or non-material (such as keeping an important secret).
One can confide worries and secrets to a trustworthy person in the reasonable expectation that such information will not get repeated without one's permission. In general, in order for trust to be earned, worth and integrity must be proven over time.
In systems, a trusted component has a set of properties which another component can rely on. If A trusts B, this means that a violation in those properties of B might compromise the correct operation of A. Observe that those properties of B trusted by A might not correspond quantitatively or qualitatively to B’s actual properties. This happens when the designer of the overall system does not take the relation into account. In consequence, trust should be placed to the extent of the component’s trustworthiness. The trustworthiness of a component is thus, not surprisingly, defined by how well it secures a set of functional and non-functional properties, deriving from its architecture, construction, and environment, and evaluated as appropriate.
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- Paulo Verissimo, Miguel Correia, Nuno F. Neves, Paulo Sousa. Intrusion-Resilient Middleware Design and Validation. In Annals of Emerging Research in Information Assurance, Security and Privacy Services, H. Raghav Rao and Shambhu Upadhyaya (eds.), Elsevier, to appear. 2008.