Peer-to-peer (P2P) and grid systems allow their users to exchange information and share resources, with little centralised or hierarchical control, instead relying on the fairness of the users to make roughly as much resources available as they use. To enforce this balance, some kind of currency or barter (called karma) is needed that must be exchanged for resources thus limiting abuse. We present a completely decentralised, o#-line karma implementation for P2P and grid systems, that detects double-spending and other types of fraud under varying adversarial scenarios. The system is based on tracing the spending pattern of coins, and distributing the normally central role of a bank over a predetermined, but random, selection of nodes. The system is designed to allow nodes to join and leave the system at arbitrary times.

Peer-to-peer (P2P) and grid systems allow their users to exchange information and share resources in a uniform and reliable manner. In an ideal world, users make roughly as much resources available as they use. In reality, this is not always the case, and some kind of currency or barter (called karma) is needed that can be exchanged for resources to limit abuse. Previous proposals for such systems have not taken the decentralised and non-hierarchical nature of P2P and grid systems into account.

In this paper, we show how secret sharing can be used to address a number of shortcomings in state-of-the-art publickey-based inter-provider roaming. In particular, the new concept does not require costly operations for certificate validation by the mobile device. It furthermore eliminates the need for a secure channel between providers upon roaming. We demonstrate the new approach by introducing a new protocol, EAP-TLS-KS, for roaming between 802.11i-protected WLANs. In addition, we show that the properties of EAP-TLS-KS allow for an efficient integration of a micropayment scheme.

A model of a reputation system for incentive

This thesis is typeset using LATEX. Cover: Zimmermann Telegram, written by German Foreign Secretary Arthur Zimmermann, is an encrypted message sent to Mexico, proposing a military alliance against the United States. It was intercepted and decrypted by British cryptographers. The telegram inflamed American public opinion and draw the United States to declare war against Germany in 1917. The bookmarker contains the telegram as partially decrypted by the British cryptographers of Room 40.

Abstract. RFID can be used for a variety of applications, e.g., to conveniently pay for public transportation. However, achieving security and privacy of payment is challenging due to the extreme resource restrictions of RFID tags. In this paper, we propose PSP – a secure, RFID-based protocol for privacy-preserving payment. Similar to traditional electronic cash, the user of a tag can pay access to a metro using his tag and so called coins of a virtual currency. With PSP, tags do not need to store valid coins, but generate them on the fly. Using Bloom filters, readers can verify the validity of generated coins offline. PSP guarantees privacy such that neither the metro nor an adversary can reveal the identity of a user or link subsequent payments. PSP is secure against invention and overspending of coins, and can reveal the identity of users trying to doublespend coins. Still, PSP is lightweight: it requires only a hash-function and few bytes of non-volatile memory on the tag. 1