Sir Charles Wheatstone (1802-1875) was the Professor of Experimental Philosophy at Kings College London and was renowned for his many inventions, including the English Concertina, an early electrical telegraph, the stereoscope, and a ‘Magic Harp’ that later inspired Alexander Graham Bell’s telephone. Wheatstone was also a cryptographer.

Developed from his work in telegraphy to secure telegraph messages, Wheatstone created the Playfair Cipher, a digraph cipher that encrypts pairs of letters rather than single letters. This is achieved using a keyword, determined by the individual encrypting the message, and a five by five grid populated by the alphabet – omitting the letters already found in the keyword.

Wheatstone named the cipher after his friend, Lord Lyon Playfair, who promoted and demonstrated the cipher on Wheatstone’s behalf. Although the Playfair Cipher proved to be difficult to break, it was also deemed too complicated by the Foreign Office who chose not to adopt it. However, it would later be used by the British during the Second Boer War and the First World War.

Wheatstone also created the Cryptograph, a device that consisted of an inner and outer dial, and an arrow that would be used to form a code that was only known to the recipient and sender. Unlike the Playfair Cipher, Wheatstone’s Cryptograph was easy to break due to the ratio of letters, 26 to 27, making it easy to determine the patterns in the cipher.

Wheatstone’s experimentations in cryptography helped pave the way to the development of enciphering machines in the 20th century, but also highlights that protecting privacy was a concern since the infancy of telecommunication.

The IET Archives holds some of Wheatstone’s correspondence and diagrams that can be consulted by appointment.