Position-Based Quantum Cryptography

In this work, we initiate the study of position-based cryptography in the quantum setting. The aim is to use the geographical position of a party as its only credential. This has interesting applications, e.g., it enables two military bases to communicate over insecure channels and without having any pre-shared key, with the guarantee that only parties within the bases learn the content of the conversation. We present schemes for several important position-based cryptographic tasks: position-verification, authentication, and key exchange, and we prove them unconditionally secure, i.e., without assuming any restriction on the adversaries (beyond the laws of quantum mechanics). Unlike key-distribution, which is possible under cryptographic hardness assumptions alone, position-based cryptography is impossible under any hardness assumptions. Thus, this is the first example of a cryptographic task that we are aware of which is impossible in the standard complexity-based setting but becomes possible when using quantum methods. We also present schemes for which we can merely conjecture security; proving them secure (or insecure) remains an interesting challenge. Our results open up a fascinating new direction of quantum cryptography where security of protocols is solely based on the laws of physics.