Quantum Information Group, Security from Eavesdropping
The security of quantum cryptography derives from fundamental laws of nature. Quantum Mechanics was devised in the early part of the 20th century and is widely used today to describe many processes and phenomena, such as the energy level of atoms and solids and the operation of Si chips and lasers. Indeed, it describes every physical process known! Despite its universal applicability, many of the basic rules of quantum mechanics appear counter-intuitive to our ‘classical’ understanding of the world. A good example of this, which is also the basis for quantum cryptography, is that (in general) measurement alters the state of an unknown single quantum. This fact is the essential reason why it is impossible for a third party to copy faithfully a key formed by quantum cryptography.
Using single photons to carry the bit material for the key prevents undetected eavesdropping. Because each bit is carried by a single photon, it is not possible for a hacker to tap in and remove part of the signal, as shown in the illustration. Single photons do not split, so if the hacker (Eve) measures the photons on the fibre, they will not reach the intended recipient (Bob). Only the photons that arrive at Bob are used to form the key, so Eve cannot gain any useful information by this crude ‘tapping’ attack
In order to conceal her presence, a hacker could of course try to measure the state of the photons and then retransmit them in the same state to Bob. This type of ‘detect and resend’ attack will also fail, because the laws of quantum mechanics dictate that encoded single photons cannot be copied faithfully. Thus if a hacker tries to ‘read’ and copy the single photons, she will inevitably introduce errors to their encoded values. This fact allows Alice and Bob to test whether their communication has been intercepted or altered by a hacker on the channel.
We have illustrated a few (very crude) attacks that could be launched by a hacker. More generally, it is physically impossible for Eve to gain any information about the encoded single photons without causing a detectable disturbance. The keys sent by quantum cryptography are rather like the assignment instructions in ‘Mission Impossible’ — once read, their information content self-destructs.