The solace of quantum

Cryptography

The solace of quantum

Eavesdropping on secret communications is about to get harder

May 25th 2013 | From the print edition

CRYPTOGRAPHY is an arms race between Alice and Bob, and Eve. These are the names cryptographers give to two people who are trying to communicate privily, and to a third who is trying to intercept and decrypt their conversation. Currently, Alice and Bob are ahead—just. But Eve is catching up. Alice and Bob are therefore looking for a whole new way of keeping things secret. And they may soon have one, courtesy of quantum mechanics.

At the moment cryptography concentrates on making the decrypting part as hard as possible. The industry standard, known as RSA (after its inventors, Ron Rivest, Adi Shamir and Leonard Adleman, of the Massachusetts Institute of Technology), relies on two keys, one public and one private. These keys are very big numbers, each of which is derived from the product of the same

two prime numbers. Anyone can encrypt a message using the public key, but only someone with the private key can decrypt it. To find the private key, you have to work out what the primes are from the public key. Make the primes big enough—and hunting big primes is something of a sport among mathematicians—and the task of factorising the public key to reveal the primes, though possible in theory, would take too long in practice. (About 40 quadrillion years with the primes then available, when the system was introduced in 1977.)

Since the 1970s, though, the computers that do the factorisation have got bigger and faster. Some cryptographers therefore fear for the future of RSA. Hence the interest in quantum cryptography.

Alice, Bob and Werner, too?

The most developed form of quantum cryptography, known as quantum key distribution (QKD), relies on stopping interception, rather than preventing decryption. Once again, the key is a huge number—one with hundreds of digits, if expressed in the decimal system. Alice sends this to Bob as a series of photons (the particles of light) before she sends the encrypted message. For Eve to read this transmission, and thus obtain the key, she must destroy some photons. Since Bob will certainly notice the missing photons, Eve will need to create and send identical ones to Bob to avoid detection. But Alice and Bob (or, rather, the engineers who make their equipment) can stop that by using two different quantum properties, such as the polarities of the photons, to encode the ones and zeros of which the key is composed. According to Werner Heisenberg’s Uncertainty Principle, only one of these two properties can be measured, so Eve cannot reconstruct each photon without making errors. If Bob detects such errors he can tell Alice not to send the actual message until the line has been secured.

One exponent of this approach is ID Quantique, a Swiss firm. In collaboration with Battelle, an American one, it is building a 700km (440-mile) fibre-optic QKD link between Battelle’s headquarters in Columbus,Ohio, and the firm’s facilities in and around Washington, DC. Battelle will use this to protect its own information and the link will also be hired to other firms that want to move sensitive data around.

QuintessenceLabs, an Australian firm, has a different approach to encoding. Instead of tinkering with photons’ polarities, it changes their phases and amplitudes. The effect is the same, though: Eve will necessarily give herself away if she eavesdrops. Using this