Need to share a secret? A code that's as strong as the laws of physics is within our grasp and could finally make sure your data can never leak

THE Romans had a saying in praise of a reliable man: “You can trust him in the dark.” But as Julius Caesar realised when several members of his inner circle stabbed him to death, sometimes the best course of action is to trust no one.

Throughout history, people have been burned by misplaced trust. Users of the extramarital affairs website Ashley Madison, whose details were leaked in August, are a good example. Their spouses are another. But as far as cybersecurity is concerned, we are finally poised to create a world in which trust is optional. The development taking us there is called device-independent quantum cryptography. Once it is perfected, you will be able to buy a secure device from your worst enemy and still be certain that no one is spying on the messages you send using it. “You don’t have to trust anyone,” says Artur Ekert, the University of Oxford physicist whose innovations in cryptography led to the idea.

This perfectly secure future can’t arrive quickly enough, as present-day cryptographic systems are in a precarious state. The security of all of our online purchases, bank transactions and personas rely on a single shaky assumption: that certain mathematical operations are hard to do. The best known of our modern encryption systems is called RSA. To encode data, it builds a key from two very large prime numbers. These are kept secret, but their product – a number thousands of binary digits long – is public knowledge. Data can be encoded using this public key, but only those with knowledge of the original numbers can decrypt it. RSA’s security relies on the fact that there is …