ISeCure The ISC Int'l Journal of Information Security July 2013, Volume 5, Number 2 (pp. 155–170) http://www.isecure-journal.org GTrust: A Group Based Trust Model Mansooreh Ezhei 1,∗ and Behrouz Tork Ladani 1 1 Department of Computer Engineering, University of Isfahan, Isfahan, Iran ARTICLE I N F O. Article history: Received: 13 March 2013 Revised: 4 September 2013 Accepted: 2 November 2013 Published Online: 20 March 2014 Keywords: Trust, Group, Metagraph, GTrust. ABSTRACT Nowadays, the growth of virtual environments such as virtual organizations, social networks, and ubiquitous computing, have led to the adoption of trust concept. One of the methods of making trust in such environments is to use a long-term relationship with a trusted partner. The main problem of this kind of trust, which is based on personal experiences, is its limited domain. Moreover, both parties of such trust relationship will face big problems of collecting data and forming reasonable and reliable beliefs. Considering the concept of “group” in modeling trust is a way to overcome the above mentioned problems. Since, group-based trust is more suited with the nature of trust in new virtual environments. In this paper, a new trust model called “GTrust” is proposed in which trust is considered as a collective and shared feature of all group members. Therefore, group membership is used as the judgment criteria regarding a person’s expected behavior and how he can be a trustee. GTrust is based on Metagraphs which are graphical data structures for representing a collection of directed set-to-set mappings. We show that by using GTrust, large trust spaces between unknown individuals can be shaped effectively. The proposed model not only offers a better description of human sense of trust when considering communities, but also provides the setting for evaluating the trust of individuals whom we do not know, and therefore provides an extended evaluation domain. © 2013 ISC. All rights reserved. 1 Introduction T rust is an important yet complex relation among agents in social environments. With the growth of virtual environments such as virtual organizations, social networks, and ubiquitous computing, there is an increasing interest in bringing this concept into play. Therefore, modeling, evaluation and management of trust are of high importance. Trust plays important roles in the coalition formation, quality assessment * Corresponding author. Email addresses: m ezhei@eng.ui.ac.ir (M. Ezhei), ladani@eng.ui.ac.ir (B. Tork Ladani) ISSN: 2008-2045 © 2013 ISC. All rights reserved. and information credibility, and in determining how information flows between agents. Trust can be defined as the decision to rely on another party (i.e. person, group, or organization) under a condition of risk [1]. Experimental trust is a kind of trust that is acquired through continuous interaction with entities who have been known for a long time [2]. Most of the current trust management models are based on this viewpoint which normally uses directional graphs to describe the trust between persons. Even in models which try to infer indirect (transient) trust, only trust between persons is considered [3]. The main problem with experimental trust models ISeCure