Closed user group based on a Peer-to-Peer system require a membership mechanism that is fully decentralized. Several suggestions have been made in literature. However, the proposed mechanisms have some significant drawbacks since any peer can easily bypass the proposed membership mechanism. In this paper we present a new fully decentralized and scure membership mechanism for Peer-to-Peer systems. A quorum of peers decides about membership requests from new peer. In order to prevent the requesting peer from manipulating the membership decision two mechanisms are applied: (1) The quorum peers must make there membership decision unanimously. This is sufficient to guarantee a correct membership decision, if a fraction of the peers in the P2P system is honest and if the quorum peers are selected in a random manner. (2) In order to ensure a random selection of the quorum peer a random value is created in a distributed way that is used to select the quorum. It can be verified, that the quorum was correctly selected. The presented mechanism is evaluated in terms of the offered security and the created traffic overhead.
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