2. PROTOTYPE OF A
QUANTUM
CRYPTOGRAPHY SYSTEM
FOR THE END USER
3. 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)
4. 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
5. QUANTUM CRYPTOGRAPHY TODAY
Some cities (Durban, Madrid, London) are going to be
completely cabled to apply quantum cryptography
Today the cost of a system is around 100.000 $
Less expensive applications are interesting,
affordable for the end user:
ATM terminals, online internet transactions
We developed our prototype for this purpose:
a compact and cheap system that could be
embedded in a smartphone
6. 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
7. THE BB84 PROTOCOL
(Bennet Brassard 1984)
Alice chooses randomly 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.
8. 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
9. 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.
10. 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
11. 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
12. 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.
13. 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.
14. 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.
15. 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
16. 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.
Notas do Editor
It is well-known that quantum computing is a work in progress at the moment. However, a real-world application of quantum information exists and it is quantum cryptography. QC allows secure data transmission independent form the computing power of the attacker. Nowadays qc works up to 100 km, something more, on optical fibers. I saw some news abot Durban, south africa, that has been completly cabled for qc, and London will be the next city. But the problem of effective quantum repeaters must be solved. Many reaserchers work on the possibility to transmit on the air up to satellites and back to the earth.
QC performances captured the interest of banks, big companies and institutions
The cost of this kind of system is nowadays around 100000 dollars. It is expected to get less expensive and reach longer distances. But the need of secure transactions is not only from big companies, but also from home users and normal citizens that use internet for online commercial transactions and ATM terminals evry day. This is why a compact and low-cost qc system is an important achievement. We aim to insert the system into a smartphone, an iphone or blakberry like.
The basic idea of qc goes back to 1969, when a PhD studnet proposed the protocol to Charles Bennet. Bennet formilized it with Gilles Brassard many years later in 1984. It is known that in qm the act of observation affects any quantum system and corrupts its information content. So if Alice (the transmitter ) sends a quantum information to Bob (the receiver), any eardropper E (Eve) will corrupt the information sequence, whatever his computing power. The protocol is based on the polarization properties of photons. You see here two polarized filters that allow photons to pass or stop them depending on the photon polarization of the photons.
Let’s imagina now that A and B can communicate through a quantum channel (photons) and aa public channel too (e.g. internet or phone). A chooses a random series of polarized photons and records them and send them to B. They are polarized with 4 different polarizations: vertical, hrizontal, +45°, -45° Bob usese an analyzer at random to analyze the photons, and records the seqquence and the polarizations used.
On the public channel B communicates A his polarizations sequence. A checks the sequence and on the public channel communicates B which bits are to be deleted because their polarization does not match, but does not communicates any information about the polarization itself.
A and B share now a correct sequence of polarizations, i.e. the secret key. Really bright (smart)
You can also settle if E the eardropper has tried to intercept the message, A and B via public channel share a portion of the sequence and see if it is coincident. There is a specific error statistics for this kind of transmission. For the mq laws , if E intercepts the photn sequence, the error statistics changes dramatically and the transaction can be immediately stopped. Errors can be further minimized by means of standard or quantum error correction and privacy amplification techniques.
Our syste cconsists of two custom circuit boards, transmitter and receiver. The transmitter is an electronic circuit that drives four high-performance LEDs. Light is suitably attenuated to obtain “raindrops” photons. LEDs are covered by 4 polarized filters. The circuit generates random casual logical signals. The random logic signals are applied to two pins of the parallel port, then sent to the transmission circuit This generates four bits that pilot a transistor that turns on the LEDs in sequence. The synchronism signal allows the bits to last the same time. An acquisition card sends to the second PC the bits after amplification and level adaptation.
The four channel receiver is equipped with high sensitivity photodiodes. It must establish the s equence of bits starting from the received photons. The photodiodes transform photons into electrical signals, then into bits. We used a logic state analyzer that revelas the voltage peaks coming from the photodiodes. The current intensity of the signal coming ffrom the photodiodes is extremely low, so they have to be amplified The receiver front-end uses the IVC102 integrated that is endowed with high-impedance inputs that can detect fA currents. We use four single channels on the same card, with four IVC102 Il front-end del ricevitore utilizza l'integrato IVC102 il quale è dotato di ingressi ad alta impedenza a FET che consentono di rilevare correnti a livello di fA , segue una seconda amplificazione e un circuito di trigger. After the integration process, the output voltage of the IVC102 is proportional to the input current. The integration value adopted is 100pF.
The two cards are driven by two separated computers equipped with a software written in C, that generates and decodes the signals. In the first PC the sogftware generates psudo-casual numbers, generates and synchronizes the sequence of bits Through the paralle port the second PC acquires the sequence of bits.
On the second PC the logic state analyzer reconstructs the signals and the software reads and synchronize them at the same clock frequency as in the transmitter We also built a software that simulates the comparison on public channel between generated sequences. In this way we obtain the in-clear secret secure key. A logic analyzer is the digital counterpart of an analog oscilloscope. It allows a number of digital input signals to be sampled and stored sequentially in a high-speed memory or buffer. A logic analyzer can also recognize a condition, or sequence of conditions, on the input data and use that combination of events to trigger data storage. The information acquired is displayed as oscilloscope-like waveforms or as list of numbers representing a sequence of logic states
At the moment the system is a prototype on optical bench, but in the futuro it can be adapted to work on optical fibers or directly on ATM terminals. The adopted optics can vary depending on the applications. Currently we carry out experiments if free air under darkness conditions, placing TX and RX in front one to each otehr at a distance between 30 and 100 cm. The performances of the system can be improved by substituting components and software with more effective models.
We are already acquiring new avalanche photodiodes to ensure single-photon performances. Of course the best performances are ensured at -20°C reducing thermal noise. The random number generator will be replaced with the portable IdQuantique hardware device or a custom hardware, both at a low cost. Finally, robust error correction and privacy amplification algorithms will be applied. The