Dan Brown’s 1998 novel Digital Fortress hinged on the development of unbreakable encryption, something the cipher expert in the book described as ‘a mathematical impossibility’.
This would be a surprise to security agencies, as an unbreakable method was developed 80 years earlier. This is the so-called ‘one-time pad’, where each letter of a message is encrypted with a different value from a random string of numbers as key. Without that key to decrypt it, there is no way to break the cipher.
However, there is a problem with the method – which is why almost all current means of encryption, such as the mechanism behind the internet’s SSL security, don’t use it. A one-time pad will only work if both the sender and receiver have a copy of that random key. It is always possible in principle to intercept the key when it is sent to the person receiving the message, whether it is done electronically or as physical documents.
At least, that was the case before the discovery of a mind-boggling quantum phenomenon known as entanglement. Pairs of quantum particles, such as photons of light, can be put in an entangled state where, as long as they stay entangled, measuring a property of one particle such as polarisation or spin results in the partner particle instantly adopting an associated state. So, for example, if a particle’s spin is measured, the spin of the other particle is immediately known, no matter how far away it is.
This doesn’t make it possible to send instantaneous messages, because the value of this property is genuinely random (unlike the pseudorandom numbers produced by computer programs) – there is no way to predict it. But this apparent defect makes entanglement an ideal means of distributing the random string of characters need for a one-time pad. If the sender of the message has one particle and the receiver has the other, the key is only generated at the time that one of the particles is examined.
Of course, it’s possible that a stream of particles could be intercepted, the value fixed and the no-longer entangled particles sent on to the receiver. But even this can be prevented, as there is a means to check if the particle stream is still entangled, which it would no longer be if intercepted.
This means if you can get a stream of entangled particles to a recipient, you have established truly unbreakable encryption. The approach was first demonstrated in Vienna in 2004 by Austrian physicist Anton Zeilinger, who used an entangled link to make a secure payment from City Hall to the Bank of Austria over a distance of around 500 metres, with optical cables threaded through Vienna’s ancient sewers, previously best known as a location in the Orson Welles movie, The Third Man.
Range of Transmission
Range of transmission initially proved a problem because, while entanglement can remain intact indefinitely if the particles are isolated, it is very fragile if the particles interact with matter. However, in 2017 Chinese researchers launched a satellite called Micius, named after the Latinised name of an ancient philosopher, which can send entangled pairs of photons to ground-based locations 1,200 kilometres apart, a starting point for a long-range quantum encryption network. This ability is under continuous development and such networks are beginning to see the light of day.
Unbreakable encryption will always be of interest to the security services and the military – but could also prove necessary for future internet security. A huge amount of work is going on around the world to develop quantum computers, based on quantum particles rather than bits to produce incredible speed for some applications. One of the first ever algorithms for quantum computers, developed decades ago, would enable them to break internet encryption, making something stronger, like quantum security, necessary. The race is on.