Quantum Cryptography
An essential part of any cryptographic protocol is the key – a string of random bits that are used to encode the data to be exchanged between parties. The key is communicated by transmitting the information (light) via a fiber optic cable. This is open to security loopholes because keys can be easily compromised by the use of fiber tappers and sniffer software to gather fragments of data as it passes through the fiber. In effect, an eavesdropper wishing to steal information can take some of the transmitted light particles, while allowing the rest to pass through. In so doing, neither communicating party is aware eavesdropping has occurred. With a complete stream of encrypted data, powerful computers can attempt to crack the code.
Quantum cryptography introduces a new way to transmit data, which is absolutely guaranteed to be secure by some of the fundamental laws of quantum mechanics. Instead of sending information using a large quantity of light, only single light particles are used to distribute a quantum secure key. Quantum Mechanics guarantees that the act of an eavesdropper intercepting a single photon, even if it is just to observe it, irreversibly changes the information encoded on that photon. Furthermore, an eavesdropper is easily noticed given that they cannot replicate the information encoded on the photon without modifying it.
For quantum cryptosystems to offer absolute security, a true single photon source (SPS) is required. Current quantum cryptosystems rely on heavily filtered laser light to provide single photons. Such a scheme generates single photons with a reliability of 85%. This can be detrimental to the key generation rate and may comprise the practicality of quantum cryptography in high demand communication channels. The technology currently being prototyped by QCV will resolve the existing issues of single photon production ... read more
Quantum Key Distribution
The diagram below is an example of how quantum cryptography can be used to securely distribute keys.This scheme includes a sender Alice and a receiver Bob.
Alice sends encoded single photons (key) to Bob via a fibre optic channel. To encode the single photons Alice randomly chooses to transmit the photons in one of four orientations (0, 45 90 and 135 degrees). For each individual photon, Bob will randomly choose a filter to measure the orientation which is either rectilinear (0 or 90 degrees) or diagonal (45 or 135 degrees).
Bob will then publicly inform Alice to the type of filter he used without mentioning the actual results. Alice then conveys which filter orientations are correct. The photons that were incorrectly measured will be discarded, while the correctly measured photons are translated into bits (0 or 1) based on their orientation.These photons are used to form the basis of a one-time pad for sending encrypted information.It is important to note that neither Alice nor Bob are able to determine what the key will be in advance because the key is the product of both their random choices. Hence, quantum cryptography enables the distribution of a one-time key exchanged securely.
Let us suppose that an eavesdropper "Eve" tries to intercept the key being sent. She too must randomly select either a rectilinear or diagonal filter to measure each photon sent by Alice. Hence, Eve will have an equal chance of selecting the right and wrong filter, however she will not be able to confirm with Alice the type of filter used. Even if Eve was to eavesdrop while Bob confirms with Alice the photons he received, this information will be useless unless she knows the correct orientation of each photon. Consequently, Eve will not correctly interpret the photons that form the final key.