Low cost secure key exchange for consumer protection

We have developed a low cost free-space quantum cryptography system, complete with purpose-built software that can operate in daylight conditions. The system uses the BB84 protocol. The transmitter (Alice) consists of an HP handheld computer interfaced to a field programmable gate array (FPGA). Pulses from the FPGA drive four polarised light emitting diodes which are combined in a diffractive optical element (fig 1), spatially filtered and sent to the receiver (Bob). The receiver is run from a laptop computer which controls a specially developed low cost timing system based on an FPGA which measures the arrival times of he photodetections. The optical system uses a standard beamsplitter based analyser with four photon detectors (fig 2). The system works with up to 0.3 photons per pulse and with a pulse repetition rate up to 5 MHz allowing the generation of renewed shared secrets of order 10 kilobits to be generated in less than one second. The main error source is background light and error correction is through low density parity check codes followed by privacy amplification to ensure security.



Fig 1: Assembly of the diffractive optical combined. Four LED's are mounted in a mount angled to illuminate a four way diffractive combiner at the correct diffraction angles



Fig 2: Four way detector unit. The beam from the transmitter passes through a narrow band filter then a focusing lens. The beam is then split in a 'coin toss' beamsplitter and directed to polarizing beamsplitters followed by four detectors. The undeviated beam polarization is rotated by 45 degrees in a waveplate to make detections in this arm correspond to the 45 degree measurement basis.

The system is designed to eventually work in applications where a consumer can regularly 'top up' a store of secrets for use in a variety of one-time-pad (OTP) and authentication protocols. An intended application is the use of quantum key distribution (QKD) in the secure exchange of PIN and transaction information between a handheld transmitter and a stationary receiver unit. In this case, the transmitter (Alice) module could be incorporated within a mobile phone and the receiver (Bob) module within a bank Automated Teller Machine (ATM) (fig 3). The assembled system is shown in fig 4.



Fig 3: The key exchange system with transmitter (Alice) on the left and receiver (Bob) on the right. Alice is interfaced to an HP IPAQ handheld computer and Bob to a timing card and laptop PC. The classical public channel is through wifi.



Fig 4 The use model where keys are exchanged between a handheld device and a fixed terminal such as an ATM

1. J L Duligall, M S Godfrey, K A Harrison, W J Munro and J G Rarity, New Journal of Physics 8 (2006) 249