Electronic Cash with Open-Source Observers

Electronic cash (e-cash) systems need to satisfy the property of anonymity, unforgeability, and transparency which prevent criminal activities from abuse of anonymity. Observers proposed by Chaum et al. and Brands in the ’90 s are one of the cleverest solutions, where observers deployed by the authorities to the user’s device only engage in legitimate anonymous payments, but never cooperate with illegal activities. In other words, observers enforce payers’ legitimate behavior while the cryptographic protocols ensure the observers do not obtain private information about individual payments. However, ever-proposed observers only prevent double-spending as legitimate behavior. There had been no contributions to extend the role of observers from only double-spend prevention to general transparency enforcement required by society without sacrificing privacy. In this paper, we propose a novel concept of open-source observers which can potentially achieve both cryptographic anonymity and highly flexible transparency at the same time. In our setting, observers are published in the cloud as open-source programs and executed within a tamper-proof device with an anonymous attested execution capability, in which all outputs of the execution, with the hash value of the executed open-source program, are signed by the secure processor as formulated by Pass et al. recently. The contribution of this paper is two-fold: (1) we first defined the concept of the open-source observers and (2) we showed a construction of Brands’ type provably-secure offline electronic cash scheme with observers extending the scheme by Baldimtsi and Lysyanskaya.