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Community decisions about access control in virtual communities are non-monotonic in nature. This means that they cannot be expressed in current, monotonic trust management languages such as the family of Role Based Trust Management languages (RT). To solve this problem we propose RT ⊖ , which adds a restricted form of negation to the standard RT language, thus admitting a controlled form of non-monotonicity. The semantics of RT ⊖ is discussed and presented in terms of the well-founded semantics for Logic Programs. Finally we discuss how chain discovery can be accomplished for RT ⊖ .

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

In this paper we introduce Standard TuLiP -a new logic based Trust Management system. In Standard TuLiP, security decisions are based on security credentials, which can be issued by different entities and stored at different locations. Standard TuLiP directly supports the distributed credential storage by providing a sound and complete Lookup and Inference AlgoRithm (LIAR). In this paper we focus on (a) the language of Standard TuLiP and (b) on the practical considerations which arise when deploying the system. These include credential encoding, system architecture, system components and their functionality, and also the usability issues.

This thesis would have never become a reality without an enormous help of my two promotors: Sandro Etalle and Pieter Hartel. I am grateful to Sandro Etalle for countless number of things. First of all, I thank Sandro for his commitment to my work. I cannot recall a single moment, when I had the feeling that the work I was doing was not important to him. Sandro's honesty was an important driving force that helped me to retain self-confidence and belief in that I am able to finish this Ph.D. thesis. Everything I learnt about what a good technical paper should look like, and how to approach the difficult task of writing a good scientific paper professionally comes from Sandro. I wish every Ph.D. student to have this level of commitment and this level of knowledge from the promotor as I had from Sandro Etalle. I also would like to thank Sandro and his wife Nicole for warm reception in Trento. I would like to thank Pieter Hartel for giving me the opportunity of doing Ph.D. in his research group. Excellent working conditions, friendly atmosphere, and unbelievable level of flexibility in the working hours are the testimonies of Pieter's deep understanding for even highly eccentric Ph.D. student behaviour. I appreciate both Pieter's and his wife Marijke's concern to make us foreign students feel a bit more at home. I am especially thankful to Pieter for his involvement in improving both my written and spoken English. Regardless of the extent to which I met Pieter's expectations, I believe that my English is better than it was four years ago when I was starting my Ph.D. journey. Finally, I am impressed with Pieter's extra-natural ability to "process" hundreds of pages in only a few days and still being able to find not only spelling mistakes, but also technical inconsistencies that both me and Sandro overlooked. If there are any mistakes in this thesis, it is most certainly because I forgot to include some of the Pieter's comments. I thank Jeroen Doumen, who was my daily advisor for majority of time. I thank him for all discussions we had: it was very important to me. In a special way I am grateful to Jan Kuper for his enthusiasm in helping me to understand basics of mathematical logic during the individual course he gave me. I am often returning to his visualisations of the darkest corners of deductive systems. I also thank Jerry den Hartog. It was always a pleasure (and often the necessity) to "use" his sharp brain. In particular, I thank Jerry that he spontaneously agreed to provide the Dutch translation of the summary of this thesis. I would like to thank Mehmet Akşit, Krzysztof Apt, Willem Jonker, Marten van Sinderen, Sabrina De Capitani Di Vimercati, and Will Winsborough for accepting the invitation to be part of the graduation committee of this thesis. It is a great honour for me. I thank the Freeband consortium for the funding for my research. In particular, I would like to acknowledge all the members of the I-Share project, especially Inald Lagendijk, Dick Epema, Johan Pouwelse, Arno Bakker, and Jan-David Mol. I ...