BB84's new approach to encryption
Quantum encryption emerged as a concept in the 1970s, but it wasn't until 1984 that a workable protocol describing its operation emerged. That protocol, called BB84, was developed by Charles Bennett and Gilles Brassard, provided a framework for quantum key distribution that would enable foolproof cryptography that even allows the sender and recipient to check whether the data stream had been snooped upon.
Distance has always been a factor in the efficacy of light-based transmission, and BB84 was no exception. When it was first demonstrated in 1991, BB84 ran over just 32cm of fibre-optic cablingââ,¬"far too little to be of real commercial value. Improvements over the years, however, have gradually extended this to the point that today's equipment can work over fibre-optic cable runs of up to 60km.
In a BB84 implementation, a sender and receiver, usually called Alice and Bob for simplicity's sake, want to securely exchange an encryption key. Alice uses a laser or LED to produce short bursts of low-intensity light. This light is randomly polarised so it aligned either rectilinearly (horizontal and vertical) or circularly (left-circular or right-circular), and transmitted along the fibre.
Bob randomly samples the transmitted beam of light, randomly choosing his own polarisation so he only receives data when the polarisation matches that sent by Alice. Bob then tells Alice what sequence of polarisations he used. Alice compares this sequence to the actual sequence she used to encode the beam.
Alice and Bob then discard any observations of the data that were conducted where the polarisations did not match (and therefore did not produce the right result). Once Bob has confirmed he has an accurate copy of the data sent, it is translated back into computer data: left-circular or horizontally polarised light equals a 0 bit, while right-circular or vertically polarised light corresponds to a 1 bit.
By adding some additional steps, Alice and Bob can check whether their transmission has been affected by noise or eavesdropping. This is done by splitting the stream into blocks small enough that each block would not normally be subject to many errors. A parity bit is calculated for each block, and the last bit of each block is discarded. For every block for which calculated parities are different between Alice and Bob, each side searches the block to spot the error.
Because the data is embedded in what is effectively random noise, only Alice and Bob know the correct sequence necessary to identify the real data. BB84's high level of communication between nodes limits its speed significantly, however: contemporary systems only offer around 1000 bits per second of transmission speed. That's enough to support extremely low-volume research work but is far too slow for commercial-grade applications.
The concept becomes clearer when illustrated graphically. This explanation was adapted from a helpful tutorial by Dartmouth College PhD student Jamie Ford, located at www.cs.dartmouth.edu/~jford/crypto.html. Frederick Henle, a peer of Ford's, offers an interactive demonstration of the BB84 protocol at www.cs.dartmouth.edu/~henle/Quan tum. Both are worth a look to better understand the concept.
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