Sending Sensitive Messages in Quantum Packages

Abstract

A communication protocol is introduced that allows the receiver of a message to place an a posteriori bound on the amount of information that an eavesdropper could have obtained during transmission of that message. This quantum cryptographic protocol is distinct from quantum key distribution. The quantum states and measurements required by this protocol are simple enough that it can be implemented using existing technology.A major concern when transmitting a secret message over a public communication channel is that an eavesdropper might intercept the message and learn its meaning. This is combatted by encrypting the message so that an eavesdropper cannot decipher the meaning of the intercepted message. [1,2] In its most secure form, this relies upon the communicating parties sharing a single-use private key which is as long as the message.[1] Recently, great strides have been made at generating and distributing private keys using quantum mechanical systems sent through insecure channels.[3] Such quantum key generation (QKD) protocols have the distinct feature that an eavesdropper leaves evidence of her activity. However, due to their design, these QKD protocols do not convey messages.In this Letter we introduce a quantum seal protocol, which is a method for two parties to communicate a message, not simply to generate a random key. [4,5] Its utility comes from the fact that it allows the receiver to place an a posteriori bound on the amount of information an eavesdropper could have obtained over the course of the transmission. Each time a message is sent, there is a straightforward way in which an eavesdropper can attempt to intercept the message. Yet if this occurs, the receiver will be given an unambiguous indication that there was a disturbance in the system and an eavesdropper may have been tapping into the communication channel. If there is no eavesdropper and the quantum channel is relatively noise-free, then the receiver will have confidence that no other parties have read the message.This protocol can be used to seal up and communicate an encrypted message, thereby providing an extra layer of assurance while sending sensitive material, on top of any sophisticated encryption technique that may currently be used.The protocol involves a message sender, named Alice, trying to get a message bit b (whose values can be zero or one) to a receiver, called Bob. The entire process is initiated by Alice announcing that she has a message she wants to communicate to Bob. Then a routine is repeated many times. Each repetition is referred to as a single "shot", and proceeds as follows.Bob starts the routine by preparing some physical sys- Bit-Announcements Result-Announcementstem (which we refer to as a particle) in one of four quantum states, represented by density matrices(Here we have used the standard notation for a qubit Hilbert space that is spanned by basis vectors |0 and |1 and |± ≡ 2Bob employs a randomized method of preparation so that he is equally likely to prepare the particle in any of these four states. B...

The effectiveness of quantum operations for eavesdropping on sealed messages

“…However, this example also demonstrates that such attacks cause a disturbance in the system, which can be quantified by the number of mismatches found by the message receiver. A more general analysis a message receiver's bound on the amount of information an eavesdropper could have learned during a particular transmission is taken up elsewhere [2].…”

“…Here, one party starts with a particular string of bits -the message -and when the process ends the other party will (hopefully) have the message as well. (There is a tunably small probability that the process will be unsuccessful [2]). Of course, in QKD the random string of bits can later be used to encrypt a message (which can be sent on a classical channel), but the QKD process itself transfers no information.…”

“…It is worthwhile to emphasize that while using this type of eavesdropping activity is not optimal [2], it provides us with some intuition as to how this protocol can be expected to work.…”

“…The analysis that a message receiver undertakes to place a bound on what an eavesdropper could have learned during a particular message transmission is not undertaken here. This analysis can be found elsewhere [2].…”

“…This analysis can be found elsewhere. [2] The goal of this manuscript is to examine a certain class of strategies for eavesdropping on these sealed messages, and it is divided into four parts: First, the quantum message sealing protocol is introduced. Following this, we examine a certain class of eavesdropping strategies and describe what an eavesdropper expects to learn by employing such strategies.…”