Quantum Keys Open a New Era of Unbreakable Security
The future of encryption lies in physics, not mathematics.
When most people think about space they picture the big things - like colonising Mars or building Moon bases. These are admirable goals, but they are also ones that are unlikely to happen for a long time. The reason why is simple: there is no economic drive to do them.
Sending people to other worlds is an expensive endeavour. Any payback will be decades, at least, from now. Those who need to justify such things - governments and ambitious CEOs - struggle to do so in economically rational ways. The result is an absence of sufficient funding, and slow progress towards such big goals.
Not everything in space faces the same problem. Some things - like weather monitoring or global positioning satellites - can provide immediate benefits and profits. These are fields that have seen a lot of progress since the dawn of the space age, and will likely see more as technology advances.
The capitalist instinct is driving other developments in space as well. These may not have the glamour of asteroids or Mars, but they do represent steps towards a space-based economy. Quantum key constellations, as described below, may be the next big development in this arena.
The laser flickers to life, a narrow beam of light shooting far into the star-strewn sky. Up here, high in the Chinese mountains, the night sky is clear and vibrant, a stark reminder of our place in the universe. For ten minutes the laser beam dances through the darkness, steadily moving from one side of the sky to the other.
Encoded in the stream of photons is a message, rushing towards a satellite a few hundred miles above the Earth. When they arrive, the message is decoded and read; the hidden quantum information revealed and stored for later use.
Inside the message is a quantum key: a new approach to cryptography that promises an era of true secrecy. The satellite, QUESS, forms the first step in the road towards a new quantum infrastructure that will one day encircle the world. This is not the first time a quantum key has been sent, but it is the first time one will be sent thousands of miles, from one continent to another.
Sometime later, as QUESS flies over the city of Vienna, Austria, the satellite activates its own laser. Once again the photons are encoded with a quantum message, and a waiting receiver captures the stream. Researchers celebrate, and then use the key to open an ultra-secure communication channel, marking the beginning of a new age of guaranteed privacy.
For as long as people have communicated, they have sought to conceal what is said. Evidence of cryptography stretches back to the very earliest known writing. Today it provides the foundation of the economy, from banking to bitcoin, powers revolutions and coups across the planet, and underpins the Internet, the greatest mass communication device ever created.
All encryption, however, has a weakness. Coded messages are only secure as long as the encryption keys are kept hidden. And for most methods of encryption the keys must be shared before messages can be sent. That initial exchange of keys — which must be done secretly and in clear text — is a challenge, and potentially, if the keys are intercepted, a flaw in the whole system.
To illustrate, imagine a person, Alice, wants to send a message to Bob. She’s worried that a third person, Eve, is trying to intercept and read the message. To avoid this, she writes the message in code. That works to keep the message hidden, but now Bob has a problem. He cannot read Alice’s message without knowing how to decode it.
Somehow Alice needs to provide the key to Bob. She can’t send the key with the message, since if Eve were to intercept it she would have no problem decoding it either. She can send the key and message separately, but there’s still a chance Eve manages to intercept both. The only option is to find another secure way to send the key.
This situation is known as the key exchange problem, and over the years various inventive solutions have been tried. Modern computer methods use an approach called public key cryptography, a technique relying on mathematical tricks to avoid the need to share your whole key with another person.
But though public key cryptography is secure against modern computing power, it is highly vulnerable to quantum computing. Where a supercomputer might take a billion years to crack the key, a reasonably powerful quantum computer could do it in minutes.
That is a problem. Quantum computers are, perhaps fortunately, several years, even decades, from mainstream use. But they will eventually be here, and with them the entire infrastructure underlying the modern Internet, economy and democracy will be under threat.
Throughout history, code makers and code breakers have fought a long running battle. Every step forward in cracking codes has, sooner or later, been met by a new advance in secrecy. Quantum computing will be no exception. Indeed, researchers have already started working on quantum cryptography, secure against any form of attack.
Almost all forms of encryption depend on mathematics for their security. Modern codes are hard to break because they involve difficult to solve calculations. Quantum cryptography takes a different approach, using the physical nature of photons. Instead of relying on math, quantum codes rely on physics.
To understand the benefit, let’s return to the key exchange problem. If you recall, Alice was trying to find a way to send her key to Bob without Eve intercepting it. Luckily she knows about quantum cryptography, so she decides to encode the key using the quantum states of photons.
The details are complex, but you can imagine that photons spin in two possible directions. By making the photons spin in one direction or the other you create a series of (quantum) 1s and 0s, allowing data transmission. These photon spins are quantum states, and they obey quantum laws.
Alice sends her series of photons to Bob, but Eve, aware they are coming, manages to intercept them. Now, though, she has a problem. Thanks to the laws of quantum physics she cannot copy or read the quantum state without destroying the original. There is, in short, no way for her to read the key without Bob knowing.
This is the magic of quantum key distribution, or QKD. Eve can intercept the key as much as she wants, but she cannot do so secretly. Alice and Bob will always know, thanks to the quantum laws of nature, that she has stolen the key. Once they do get a key through without interception, they are free to start communicating without risk of being overheard.
In practice this key distribution would form the first part of a communication exchange. Two people would swap their key securely using quantum technology, and then switch to a more standard form of encryption, such as AES-256, for the rest of the conversation.
The idea behind QKD has already been demonstrated in practice. In 2006 teams of researchers exchanged keys between two Spanish islands, establishing secure communication over a few dozen miles. But the technology does have some rather severe limitations. Quantum states are hard to maintain, and transmitting the keys over long distances was, until recently, impossible.
As any futuristic technology surely should, QKD will one day involve networks of lasers and satellites. The lasers transmit the encoded photons while the satellites solve the problem of carrying the keys over long distances. In recent years the tentative first steps towards building such a global QKD network have taken place.
The first long distance quantum key distribution took place between Vienna and Beijing in 2016, a distance of almost 5,000 miles. Researchers first created the secret key in China, encoded it into photons and then fired them in a laser beam towards the orbiting QUESS satellite. When QUESS flew over Vienna some time later, the satellite shot a second laser beam back down to Earth, carrying the key to its intended destination.
China holds the lead in QKD, a fact that has not gone ignored in Western capitals. Companies and space agencies around the world are now racing to develop the technology, and to put QKD satellites in orbit. So far these projects are purely experimental, but it is probably only a few years until commercial networks are put in place.
Once they are, an era of truly unbreakable encryption looms. In future wars the military need not fear losing their code books, as both sides did in World War II. Instead the battle will inevitably be fought in space, as each side seeks to gain an advantage over the other.
Constellations of satellites already provide vital services like GPS and weather data to anyone who needs them. It will not be long until QKD constellations join them, forever changing the nature of information security.