Rita Pizzi Department of Information Technology Università degli Studi di Milano

PROTOTYPE OF A QUANTUM CRYPTOGRAPHY SYSTEM FOR THE END USER

QUANTUM INFORMATION The quantum computer does non exist yet But a real world application based on quantum information exists: QUANTUM CRYPTOGRAPHY It allows the secure transmission of data, independent from algorithms and computing power of the attacker It is possible to detect any intrusion immediately Nowadays optical fiber systems exist that reach distances of 100 km Methods to increase distances and usability are underway (quantum repeaters for optical fibers / satellite transmissions)

QUANTUM CRYPTOGRAPHY TODAY Quantum cryptography performances captured the interest of banks, big companies and institutions. Systems already on sale: MagiQ Technologies New York idQuantique Geneve SmartQuantum York QinetiQ UK (defence) Toshiba Corp Tokio National Institute of Standards and Technology (US government agency ) are acquiring this technology

QUANTUM CRYPTOGRAPHY TODAY Today the cost of a system is around $ Less expensive applications are interesting, affordable for the end user: ATM terminals, online internet transactions We developed our prototype to this purpose A compact and cheap system that could be embedded in a smartphone

THE BB84 PROTOCOL (Bennet Brassard 1984) In quantum physics the act of observation modifies in an unpredictable way the observed system Thus any external action in the system will corrupt the flow of information, revealing the intrusion The BB84 protocol is based on the polarization properties of the photons

THE BB84 PROTOCOL (Bennet Brassard 1984) Alice chooses rendomly a sequence of 1 and 0 bits, turns them into photons, applies to each bit one of the possible polarizations, then sends them to Bob. Bob chooses randomly a polarization to examine each of the received photons, turns them into bits and records the results of his observations.

THE BB84 PROTOCOL (Bennet Brassard 1984) Now Bob sends to Alice on a public channel (e.g. Internet) his polarization sequence (but NOT the result of his measures) Alice selects the positions in the sequence that Bob sent correctly and sends them back to Bob on the public channel

THE BB84 PROTOCOL (Bennet Brassard 1984) Both Alice and Bob share now an identical sequence of bits, i.e. they possess a shared key that is definitely secret.

BB84 – THE INTRUSION In this kind of transaction an intrinsic error rate exists, that can be minimized by means of error correction and privacy amplification techniques If an eardropper E interposes to intercept the sequence of bits, for the quantum physics laws he corrupts the sequence and sends back to Bob a sequence with a much higher error rate This reveals immediately the presence of the intruder and the transaction can be stopped without damage

OUR SYSTEM Our system is based on two custom cards: the transmitter and the receiver. TRANSMITTER It is an electronic circuit that drives four high-performances LEDs The LEDS are endowed with polarizing filters and their intensity is suitably attenuated. Random logical signals are generated that turn on the four LEDs in sequence

OUR SYSTEM RECEIVER The receiving circuit must re-establish a sequence of data starting from the received photons. Four high-sensitivity photodiodes turn the photons (passed through four polarizing filters) into electrical signals, then into bits. This is made possible by a logic state analyzer that detects the voltage peaks coming from the photodiodes.

THE FIRMWARE A C-written software drives the whole process on two separated PCs. In the first PC the software, using the BlumBlumShub pseudorandom number generator, generates the sequence of bits and synchronizes it This is acquired by the transmitter through the parallel port.

THE FIRMWARE On the second PC the software reads the signals reconstructed by the logic state analyzer and syncronizes them We also simulated the comparison on public channel between sequences generated by transmitter and receiver At the end of simulation we obtain the secure key.

FUTURE DEVELOPMENTS At the moment our system is a prototype on optical bench In the future it can be adapted to work on optical fibers or directly on ATM terminals. The system performances are improvable with more effective components and with more powerful software algorithms

FUTURE DEVELOPMENTS We are acquiring avalanche photodiodes that will ensure single-photon performances The software random number generator will be substituted by a portable and affordable hardware generator (IdQuantique o custom) Robust algorithms of error correction and privacy amplification will be developed.