Quantum Key Distribution: The Unbreakable Safeguard in the Age of Quantum Computing

In today’s digital world the criticality of secure communication cannot be exaggerated. Most of the financial transactions take place in digital world. E-commerce which has revolutionized the way business is done, is totally dependent on secure communication. We use secure communication to make online purchases, pay for those and receive funds in our accounts. Governments rely on secure communication channels to keep their exchanges confidential. Defence and strategic sectors can’t do their job at all without secure digital communications.

This exchange of confidential information relies upon different cryptographic algorithms using different key sizes. The public key cryptography algorithms like RSA, ECDH are backbone of digital communication and ensure that the private communication remains private. Even if an attacker manages to access the public key which is readily accessible for any communication, breaking it to identify the private key and then decrypting communication just won’t be feasible on today’s super computers (time taken would be in thousands of years). However, with advent of quantum computers it will be possible to break the key combination and then decrypt all subsequent communication. Hence more secure mechanisms for key exchange are required, the kind which would alert the sender & receiver about any intermediate attack on the key exchange.

This has led to the development of Quantum Key Distribution (QKD) that uses the principles of quantum mechanics to create quantum safe communication. Instead of relying upon the mathematical complexity, QKD uses the basic nature of quantum mechanics to protect the key. One of the principles used is the Heisenberg’s Uncertainty Principle, named after its author Werner Heisenberg, a German physicist and Nobel laureate. In the year 1927, Werner Heisenberg proposed that precise, simultaneous measurement of two complementary values e.g. the position and momentum of a subatomic particle - is impossible. Contrary to the principles of classical physics, their simultaneous measurement is inescapably flawed; the more precisely one value is measured, the more flawed will be the measurement of the other value. This theory became known as the “Uncertainty Principle” and led to the famous statement from Albert Einstein that “God does not play dice”. Einstein believed that the laws of nature can’t have such randomness (like throw of dice) and hence could not accept the Heisenberg theory. However subsequent experiments conducted over the decades have conclusively established the correctness of Heisenberg theory and this theory is used as one of the methods for development of Quantum Key Distribution.

Following from the Uncertainty Principle, Wooters, Zurek and Dieks in 1982 stated that it is impossible to create the exact copy of an unknown quantum state. This theorem became knowns as “No Cloning Theorem”. It is clear that no cloning theorem is a natural follow-on from the Uncertainty principle, else it would be possible to create multiple copies of the unknown quantum state and then perform different measurements on those copies, thereby negating the Uncertainty principle.

Using these principles Charles Bennett and Gilles Brassard published a protocol in 1984. The protocol is called BB84 (after the name of authors and the year when it was published). The basic idea for the protocol is that sender can transmit a random secret key by using quantum bits where the keys are part of the polarization of bits. If any eavesdropper tries to measure the bits to find the secret key, the state of quantum bits will change which would subsequently alert the receiver about intrusion. While this implementation faces technical challenges due to losses in transmission medium and problems with measurement, there have been practical implementations, with some solutions being available in market. A joint team of DRDO and IIT Delhi scientists has successfully demonstrated Quantum Key Distribution over a distance of 100 km.

We can see as quantum computing ushers in the unimaginable processing power which can break the traditional methods of encrypted communication, the nature of quantum mechanics also provides the capability to use its randomness to at least detect when someone is trying to eavesdrop on the communication. Going forward we are going to see many breakthrough in quantum communication as well as hackers getting ready to figure out ways for breaking the QKD security.

Authored Article by

Sudhanshu Mittal

Head & Director Technical Solutions,

NASSCOM Center of Excellence – IoT & AI, Gurugram