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In Bitcoin network, the distributed storage of multiple copies of the block chain opens up possibilities for double-spending, i.e., a payer issues two separate transactions to two different payees transferring the same coins. While Bitcoin has inherent security mechanism to prevent double-spending attacks, it requires a certain amount of time to detect the double-spending attacks after the transaction has been initiated. Therefore, it is impractical to protect the payees from suffering in double-spending attacks in fast payment scenarios where the time between the exchange of currency and goods or services is shorten to few seconds. Although we cannot prevent double-spending attacks immediately for fast payments, decentralized non-equivocation contracts have been proposed to penalize the malicious payer after the attacks have been detected. The basic idea of these contracts is that the payer locks some coins in a deposit when he initiates a transaction with the payee. If the payer double-spends, a cryptographic primitive called accountable assertions can be used to reveal his Bitcoin credentials for the deposit. Thus, the malicious payer could be penalized by the loss of deposit coins. However, such decentralized non-equivocation contracts are subjected to collusion attacks where the payer colludes with the beneficiary of the depoist and transfers the Bitcoin deposit back to himself when he double-spends, resulting in no penalties. On the other hand, even if the beneficiary behaves honestly, the victim payee cannot get any compensation directly from the deposit in the original design. To prevent such collusion attacks, we design fair time-locked deposits for Bitcoin transactions to defend against double-spending. The fair deposits ensure that the payer will be penalized by the loss of his deposit coins if he double-spends and the victim payee's loss will be compensated within a locked time period. We start with the protocols of making a deposit for one transaction. In particular, for the transaction with single input and output and the transaction with multiple inputs and outputs, we provide different designs of the deposits. We analyze the performance of deposits made for one transaction and show how the fair deposits work efficiently in Bitcoin. We also provide protocols of making a deposit for multiple transactions, which can reduce the burdens of a honest payer. In the end, we extend the fair deposits to non-equivocation contracts for other distributed systems. [ABSTRACT FROM AUTHOR]

Formal RFID security and privacy frameworks are fundamental to the design and analysis of robust RFID systems. In this paper, we develop a new definitional framework for RFID privacy in a rigorous and precise manner. Our framework is based on a zero-knowledge (ZK) formulation [The Foundations of Cryptography, Cambridge Univ. Press, Cambridge, 2001; ACM Symposium on Theory of Computing, 1985, pp. 291-304] and incorporates the notions of adaptive completeness and mutual authentication. We provide meticulous justification of the new framework and contrast it with existing ones in the literature. In particular, we prove that our framework is strictly stronger than the ind-privacy model in International Conference on Pervasive Computing and Communications, 2007, which answers an open question posed in International Conference on Pervasive Computing and Communications, 2007, for developing stronger RFID privacy models. We also clarify certain confusions and rectify several defects in the existing frameworks. Finally, based on the protocol in Conference on Computer and Communications Security, 2009, we propose an efficient RFID mutual authentication protocol and analyze its security and privacy. The methodology used in our analysis can also be applied to analyze other RFID protocols within the new framework. [ABSTRACT FROM AUTHOR]

In Bitcoin network, the distributed storage of multiple copies of the block chain opens up possibilities for double-spending, i.e., a payer issues two separate transactions to two different payees transferring the same coins. While Bitcoin has inherent security mechanism to prevent double-spending attacks, it requires a certain amount of time to detect the double-spending attacks after the transaction has been initiated. Therefore, it is impractical to protect the payees from suffering in double-spending attacks in fast payment scenarios where the time between the exchange of currency and goods or services is shorten to few seconds. Although we cannot prevent double-spending attacks immediately for fast payments, decentralized non-equivocation contracts have been proposed to penalize the malicious payer after the attacks have been detected. The basic idea of these contracts is that the payer locks some coins in a deposit when he initiates a transaction with the payee. If the payer double-spends, a cryptographic primitive called accountable assertions can be used to reveal his Bitcoin credentials for the deposit. Thus, the malicious payer could be penalized by the loss of deposit coins. However, such decentralized non-equivocation contracts are subjected to collusion attacks where the payer colludes with the beneficiary of the depoist and transfers the Bitcoin deposit back to himself when he double-spends, resulting in no penalties. On the other hand, even if the beneficiary behaves honestly, the victim payee cannot get any compensation directly from the deposit in the original design. To prevent such collusion attacks, we design fair time-locked deposits for Bitcoin transactions to defend against double-spending. The fair deposits ensure that the payer will be penalized by the loss of his deposit coins if he double-spends and the victim payee's loss will be compensated within a locked time period. We start with the protocols of making a deposit for one transaction. In particular, for the transaction with single input and output and the transaction with multiple inputs and outputs, we provide different designs of the deposits. We analyze the performance of deposits made for one transaction and show how the fair deposits work efficiently in Bitcoin. We also provide protocols of making a deposit for multiple transactions, which can reduce the burdens of a honest payer. In the end, we extend the fair deposits to non-equivocation contracts for other distributed systems. [ABSTRACT FROM AUTHOR]