Grupo AIA develops algorithms for Quantum Cryptography

Modern encrypted communications prevent eavesdropping by means of cryptographic protocols and algorithms whose strength relies on the difficulty of solving certain mathematical puzzles with current computers. However, all this algorithmic infrastructure is facing an uncertain risk, as several key encryption algorithms have been shown to be easily broken using Quantum Computers. Nobody knows for sure when Quantum Computers will be able to successfully break current communication crypto, but nobody wants to take chances.

Quantum Key Distribution (QKD) is based on a classic encryption method that has existed for a long time, is absolutely bug-proof, and is resistant against any quantum computer, no matter how powerful: the One-Time Pad. The direct telex wire between the American and Soviet presidents during the Cold War already used this procedure: A message is encrypted using a random key, previously known to both parties, of the same length as the message. However, ensuring this level of security has its price: the key can only be used once. Since both parties need to securely exchange the keys beforehand, this makes it an inconvenient method in practice. Not so with QKD. Thanks to the quantum process known as entanglement, it is possible for both parties—traditionally called “Alice” and “Bob”—to securely exchange a random key, as long as necessary. Then the sender can encrypt the message with his/her key (using a One-Time Pad) and transmit it over a normal Internet connection.

The advantage of exchanging the key using this quantum mechanism is that it is physically impossible to intercept or even observe the quantum transmission without altering it. When Alice or Bob detect an intrusion, they just discard the key (they haven’t sent the message yet!), and just attempt to generate a new one.

The common version of this procedure uses single photon sources and sophisticated detectors, but it is possible to build something similar using the equipment we can find in conventional communications. In this setup, called Continuous-Variable QKD, the entangled quantum system consists of coherent laser pulses generated at Alice’s lab and transmitted to Bob using a conventional optical fiber.

When using laser pulses, instead of single photons, the quantum system is not the key itself, but a kind of noisy channel, built over a classical communications channel (for instance, the Internet). In the so-called reverse reconciliation, Bob generates a random key (which, incidentally, uses a Quantum Random Generator to guarantee true randomness) and sends a noisy version of it to Alice.

Then, here comes the magic: it has been known since the 60s that in every noisy channel there is a certain maximum amount of information that can be transmitted, and also that there are ways to send the information with the precise degree of redundancy that will allow for the correction of all errors induced by the channel. The trick is to work at the very edge of the channel capacity (Shannon limit), allowing Alice to correct the errors. Any attempt by Eve to eavesdrop or tamper with the channel will slightly increase the virtual channel noise, and therefore prevent Alice from successfully decoding the key sent by Bob.

Some of the error correcting codes that allow us to work at the channel capacity are Multi-Edge Low Density Parity Codes (MET-LDPC).  AIA has developed a toolchain for designing adequate LDPCs for a given setup (transmission distance, detector efficiency), plus a full stack for real-time key processing: Parameter estimation, authentication, key reconciliation (error correction), and privacy amplification. All these modules were implemented to fulfill the so-called “security proof”, a mathematical validation that an eavesdropper cannot obtain a single bit of the key. The security proof for QKD (and also the LDPC property of maxing out the channel capacity) requires key lengths to be as large as possible. Here we reached keys of about 1 million bits, which is a challenge from the algorithmic point of view.

After being awarded a public tender published by ICFO, AIA has provided this software that enables ICFO to test their CV-QKD implementations for secure encrypted communications.

AIA is an industrial Associated Member of the QuantumCAT Hub, a consortium of research institutions in Catalonia and industrial actors devoted to promoting quantum tech transfer projects and innovation with a short-term or mid-term industrial and social impact.

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