Non-secret encryption

Until relatively recently cryptosystems were always symmetric. They relied on the use of a shared secret key known to both sender and receiver.

This all changed in the 1970s. Public key cryptosystems, as they are now called, revolutionised the theory and practice of cryptography by relying for their impenetrability on the existence of a special type of one-way function known as a trapdoor function. Using these the need for a shared secret key was removed. Hence James Ellis and Clifford Cocks of the Government Communication Headquarters (GCHQ), Cheltenham in the UK, who first discovered this technique, named it ‘non-secret encryption’.

For a fascinating account of how this discovery was made see Chapter 5 of Singh (2000). He recounts how key distribution was a major problem for the UK military in the late 1960s. In 1969 Ellis came up with the idea of what we now call a ‘trapdoor function’. Informally this is a one-way function which can be inverted easily by anyone in possession of a special piece of information: the trapdoor.

This was exactly the same idea as Diffie, Hellman and Merkle came up with several years later, but like them Ellis was unable to find a way of implementing it.

It was three years later in November 1973 that Cocks, a young recruit to GCHQ, came up with the very simple solution (essentially the RSA cryptosystem) which was rediscovered several years later by Rivest, Shamir and Adleman (1978).