Comment on "Unconditionally secure commitment in position-based quantum cryptography"

We show that a recently proposed relativistic commitment scheme [Sci. Rep. 4, 6774 (2014). arXiv:1406.6679] is insecure against the cheating of the committer.Comment: 1 pag

Insecurity of position-based quantum cryptography protocols against entanglement attacks

Recently, position-based quantum cryptography has been claimed to be unconditionally secure. In contrary, here we show that the existing proposals for position-based quantum cryptography are, in fact, insecure if entanglement is shared among two adversaries. Specifically, we demonstrate how the adversaries can incorporate ideas of quantum teleportation and quantum secret sharing to compromise the security with certainty. The common flaw to all current protocols is that the Pauli operators always map a codeword to a codeword (up to an irrelevant overall phase). We propose a modified scheme lacking this property in which the same cheating strategy used to undermine the previous protocols can succeed with a rate at most 85%. We conjecture that the modified protocol is unconditionally secure and prove this to be true when the shared quantum resource between the adversaries is a two- or three- level system

Position Based Cryptography

We consider what constitutes {\em 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) public-key coming from some trusted public-key infrastructure. In many situations, however, {\bf 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 \emph{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 Retrieval Model (a variant of the Bounded Storage Model) and formalize and construct information theoretically secure protocols for two fundamental tasks: \begin{itemize} \item Secure Positioning; and \item Position Based Key Exchange. \end{itemize} We then show that these tasks are in fact {\em 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-retrieval framework is, in fact, one of the ``right frameworks (there may be others) to study the foundations of position-based cryptography