We consider what constitutes identities in cryptography. Typical examples include your name and your social-security number, or your fingerprint/iris-scan, or your address, or your (non-revoked) publickey coming from some trusted public-key infrastructure. In many situations, however, where you are defines your identity. For example, we know the role of a bank-teller behind a bullet-proof bank window not because she shows us her credentials but by merely knowing her location. In this paper, we initiate the study of cryptographic protocols where the identity (or other credentials and inputs) of a party are derived from its geographic location.We start by considering the central task in this setting, i.e., securely verifying the position of a device. Despite much work in this area, we show that in the Vanilla (or standard) model, the above task (i.e., of secure positioning) is impossible to achieve. In light of the above impossibility result, we then turn to the Bounded Storage Model and formalize and construct information theoretically secure protocols for two fundamental tasks:-Secure Positioning; and -Position Based Key Exchange.We then show that these tasks are in fact universal in this setting -we show how we can use them to realize Secure Multi-Party Computation. Our main contribution in this paper is threefold: to place the problem of secure positioning on a sound theoretical footing; to prove a strong impossibility result that simultaneously shows the insecurity of previous attempts at the problem; and to present positive results by showing that the bounded-storage framework is, in fact, one of the "right" frameworks (there may be others) to study the foundations of position-based cryptography.Full version available on eprint.

We consider what constitutes identities in cryptography. Typical examples include your name and your social-security number, or your fingerprint/iris-scan, or your address, or your (non-revoked) publickey coming from some trusted public-key infrastructure. In many situations, however, where you are defines your identity. For example, we know the role of a bank-teller behind a bullet-proof bank window not because she shows us her credentials but by merely knowing her location. In this paper, we initiate the study of cryptographic protocols where the identity (or other credentials and inputs) of a party are derived from its geographic location. We start by considering the central task in this setting, i.e., securely verifying the position of a device. Despite much work in this area, we show that in the Vanilla (or standard) model, the above task (i.e., of secure positioning) is impossible to achieve. In light of the above impossibility result, we then turn to the Bounded Storage Model and formalize and construct information theoretically secure protocols for two fundamental tasks:-Secure Positioning; and-Position Based Key Exchange. We then show that these tasks are in fact universal in this setting-we show how we can use them to realize Secure Multi-Party Computation. Our main contribution in this paper is threefold: to place the problem of secure positioning on a sound theoretical footing; to prove a strong impossibility result that simultaneously shows the insecurity of previous attempts at the problem; and to present positive results by showing that the bounded-storage framework is, in fact, one of the "right" frameworks (there may be others) to study the foundations of position-based cryptography.

Abstract. We address the problem of realizing concurrently composable secure computation without setup assumptions. While provably impossible in the UC framework of [Can01], Prabhakaran and Sahai had recently suggested a relaxed framework called generalized Environmental Security (gES) [PS04], as well as a restriction of it to a "client-server" setting based on monitored functionalities [PS05]. In these settings, the impossibility results do not apply, and they provide secure protocols relying on new non-standard assumptions regarding the existence of hash functions with certain properties. In this paper, we first provide gES protocols for general secure computation, based on a new, concrete number theoretic assumption called the relativized discrete log assumption (rDLA). Second, we provide secure protocols for functionalities in the (limited) client-server framework of [PS05], replacing their hash function assumption with the standard discrete log assumption. Both our results (like previous work) also use (standard) super-polynomially strong trapdoor permutations. We believe this is an important step towards obtaining positive results for efficient secure computation in a concurrent environment based on well studied assumptions. Furthermore, the new assumption we put forward is of independent interest, and may prove useful for other cryptographic applications.

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Abstract. In this paper we show a general transformation from any honest verifier statistical zero-knowledge argument to a concurrent statistical zero-knowledge argument. Our transformation relies only on the existence of one-way functions. It is known that the existence of zeroknowledge systems for any non-trivial language implies one way functions. Hence our transformation unconditionally shows that concurrent statistical zero-knowledge arguments for a non-trivial language exist if and only if standalone secure statistical zero-knowledge arguments for that language exist.Further, applying our transformation to the recent statistical zeroknowledge argument system of Nguyen et al (STOC'06) yields the first concurrent statistical zero-knowledge argument system for all languages in NP from any one way function.