Wheatstone's remarkable ingenuity was also displayed in the invention of ciphers. He was responsible for the then unusual Playfair cipher, named after his friend Lord Playfair. It was used by the militaries of several nations through at least World War I, and is known to have been used during World War II by British intelligence services.
It was initially resistant to cryptanalysis, but methods were eventually developed to break it. He also became involved in the interpretation of cipher manuscripts in the British Museum. He devised a cryptograph or machine for turning a message into cipher which could only be interpreted by putting the cipher into a corresponding machine adjusted to decrypt it.
As an amateur mathematician, Wheatstone published a mathematical proof in 1854 (see Cube (algebra)).