Access control in decentralised collaborative systems present huge challenges especially where many autonomous entities including organizations, humans, software agents from different security domains seek to access and share resources in a secure and controlled way. Automated trust negotiation (ATN) is one approach that has been proposed for trust discovery and realisation, which enables entities viz. Strangers to access resources across autonomous boundaries through iterative exchange of credentials. Various negotiation strategies have been proposed to protect credential disclosure during trust negotiations. However in some domains such as e-health, not all entities are willing to negotiate credentials or disclose access policies directly to strangers regardless of negotiation strategies and instead prefer to negotiate and disclose sensitive information only to strangers within what we refer to as a circle of trust. In this paper, we introduce a formal model to describe how locally trusted intermediary parties can provide multiple negotiation and delegations hops to protect credentials and access policies. We propose a dynamic trust negotiations (DTN) model that not only protects sensitive information from disclosure but also reduces semantic issues that exist with credentials in decentralized systems. This work is currently being explored and implemented within the e-health domain: specifically in the MRC-funded Virtual Organisation for Trials of Epidemiological Studies (VOTES) project.
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