We consider repeated communication sessions between a RFID Tag (e.g., Radio Frequency Identification, RFID Tag) and a RFID Verifier. A proactive information theoretic security scheme is proposed. The scheme is based on the assumption that the information exchanged during at least one of every n successive communication sessions is not exposed to an adversary. The Tag and the Verifier maintain a vector of n entries that is repeatedly refreshed by pairwise xoring entries, with a new vector of n entries that is randomly chosen by the Tag and sent to the Verifier as a part of each communication session. The general case in which the adversary does not listen in k ≥ 1 sessions among any n successive communication sessions is also considered. A lower bound of n·(k +1) for the number of random numbers used during any n successive communication sessions is proven. In other words, we prove that an algorithm must use at least n·(k +1) new random numbers during any n successive communication sessions. Then a randomized scheme that uses only O(n log n) new random numbers is presented. A computational secure scheme which is based on the information theoretic secure scheme is used to ensure that even in the case that the adversary listens in all the information exchanges, the communication between the Tag and the Verifier is secure.

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Schemes for multi-party trust computation are presented. The schemes do not make use of a Trusted Authority. The schemes are more efficient than previous schemes in terms of the number of messages exchanged, which is proportional to the number of participants rather than to its square. We note that in our schemes the length of each message may be larger than the message length typically found in previously published schemes. The calculation of a trust, in a specific user by a group of community members starts following a request by an initiator. The trust computation is provided in a completely distributed manner, where each user calculates its trust value privately and independently. Given a community C and its members (users) U 1 , . . . , U n , we present computationally secure schemes for trust computation. The first scheme, Accumulated Protocol AP computes the average trust attributed to a specific user, U t following a trust evaluation request initiated by a user U n . The exact trust values of each queried user are not disclosed to U n . The next scheme, Weighted Accumulated Protocol WAP generates the average weighted trust in a specific user U t taking into consideration the unrevealed trust that U n has in each user participating in the trust evaluation process. The Public Key Encryption Protocol PKEP outputs a set of the exact trust values given by the users without linking the user that contributed a specific trust value to the trust this user contributed. The obtained vector of trust values assists in removing outliers. Given the set of trust values, the outliers that provide extremely low or high trust values can be removed from the trust evaluation process. We extend our schemes to the case when the initiator, U n , can be compromised by the adversary, and we introduce the Multiple Private Keys and the Weighted protocols (MPKP and MPWP) for computing average unweighted and weighted trust, respectively. Moreover, the Commutative Encryption Based Protocol (CEBP) extends the PKEBP in this case. The computation of all our algorithms requires the transmission of O(n) (possibly large) messages. Keywords: Private trust computations; Multi-party computations; Anonymity Our contributionThe purpose of this paper is to introduce new schemes for decentralized reputation systems. These schemes do not make use of a Trusted Authority to compute the trust in a particular user that is attributed by a community of users. Our objective is to compute trust while preserving user privacy.We present new efficient schemes for calculating the trust in a specific user by a group of community members upon the request of an initiator. The trust computation is © 2014 Dolev et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dolev et al. Journal of Trust Management2014, 1:8 Pa...

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We propose efficient schemes for information-theoretically secure key exchange in the Bounded Storage Model (BSM), where the adversary is assumed to have limited storage. Our schemes generate a secret One Time Pad (OTP) shared by the sender and the receiver, from a large number of public random bits produced by the sender or by an external source. Our schemes initially generate a small number of shared secret bits, using known techniques. We introduce a new method to expand a small number of shared bits to a much longer, shared key.Our schemes are tailored to the requirements of sensor nodes and wireless networks. They are simple, efficient to implement and take advantage of the fact that practical wireless protocols transmit data in frames, unlike previous protocols, which assume access to specific bits in a stream of data. Indeed, our main contribution is twofold.On the one hand, we construct schemes that are attractive in terms of simplicity, computational complexity, number of bits read from the shared random source and expansion factor of the initial key to the final shared key.On the other hand, we show how to transform any existing scheme for key exchange in BSM into a more efficient scheme in the number of bits it reads from the shared source, given that the source is transmitted in frames. * An extended abstract of this paper was submitted to DIALM-POMC 2010.

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a b s t r a c tWe propose efficient schemes for information-theoretically secure key exchange in the Bounded Storage Model (BSM), where the adversary is assumed to have limited storage. Our schemes generate a secret One Time Pad (OTP) shared by the sender and the receiver, from a large number of public random bits produced by the sender or by an external source. Our schemes initially generate a small number of shared secret bits, using known techniques. We introduce a new method to expand a small number of shared bits to a much longer, shared key.Our schemes are tailored to the requirements of sensor nodes and wireless networks. They are simple, efficient to implement and take advantage of the fact that practical wireless protocols transmit data in frames, unlike previous protocols, which assume access to specific bits in a stream of data.Indeed, our main contribution is twofold. On the one hand, we construct schemes that are attractive in terms of simplicity, computational complexity, number of bits read from the shared random source and expansion factor of the initial key to the final shared key. On the other hand, we show how to transform any existing scheme for key exchange in BSM into a more efficient scheme in the number of bits it reads from the shared source, given that the source is transmitted in frames.