From the beginning of recorded history, codes and ciphers have been used to carry secret messages in times of war and unrest, protecting military and political information should documents fall into the hands of the enemy.

The National Archives holds many secret documents. These span the centuries that mark the development of codes and ciphers - from the simple encrypted messages used in plots against Elizabeth I to the sophisticated system used by the Enigma machine during the Second World War. The race to design ever more complicated codes, and the parallel efforts to break them, continues as computers create ‘unbreakable’ complex encryptions of sensitive data today.

Sometimes codes and ciphers have been broken when the codebook or key was captured or the message contained errors. Sometimes they have been broken through patient examination of mathematical frequencies and an understanding of linguistic patterns. Once a code is broken, all secret communication is revealed - plots are uncovered, plans are thwarted. Codebreakers have sealed the fate of queens and of wars.

Read on to find out more about the importance of codes and ciphers in British history.