Clifford Cocks, a mathematician at GCHQ (Government Communications Headquarters) in Cheltenham, develops in November 1973 a public-key encryption system based on the factorization of large prime numbers — the same system Rivest, Shamir, and Adleman would publish as RSA in 1977. Cocks's work was classified as a state secret and not declassified until 1997. James Ellis (GCHQ, 1970) had established the theoretical concept of public-key cryptography two years earlier; Malcolm Williamson (GCHQ, 1974) independently developed the equivalent of the Diffie-Hellman protocol. The three GCHQ works — Ellis, Cocks, Williamson — precede their equivalents published in the open literature by 3–7 years. State secrecy prevented the civilian scientific community from building on these results, delaying the development of public-key cryptography.