Enhancements to prepare-and-measure based QKD protocols

We propose some simple changes to a class of Quantum Key Distribution protocols. The first enhancement ensures early detection of any attempted Man-in-the-Middle attack and results in less leakage of key material to any eavesdropping attacker. We argue that this version is at least as secure as the original BB'84 scheme, but ensures a closer binding of the key establishment and authentication components of the protocol. Further proposed enhancements lead to a doubling of the key rate, but the security arguments become more delicate. We also touch on the need to enhance the models used to analyze both the classical and quantum aspects of QKD protocols. This is prompted by the observation that existing analyses treat the quantum (key-establishment) and classical (authentication etc) phases separately and then combine them in a simple-minded fashion.