For thousands of years, it had been assumed that there was no solution to the so-called key distribution problem – if you want to scramble a message according to a recipe, then surely the unscrambling recipe had to be given to the receiver in advance. But in the early 1970s, there was a revolution in cryptography known as public key cryptography, which destroyed the key distribution problem. This was a technology that was tailor-made for the Internet. Customers could encrypt their credit card details and send them to retailers on the other side of the world. Penpals who had never met could encrypt emails. Related technologies, based on similar mathematics, also enabled digital signatures, integrity checks and non-repudiation.

These are valuable technologies. For example, if I vote electronically, then I certainly want my vote to be secret, but the polling station needs to know it was me who voted, so that I cannot vote twice – a digital signature guarantees this. An integrity check stops my vote being changed, and non-repudiation guarantees that my vote has been registered.