A method of concealing a message by transposing or substituting individual letters.
The CIA glossary distinguishes a cipher from a code: a cipher conceals a message by scrambling or transposing its letters according to an algorithm, whereas a code replaces whole words or phrases with other words, numbers or symbols. The output of a cipher is determined by the key.1,2
In the 20th century encryption was mechanised. The German Enigma was a rotor machine, and its cipher was first broken by Polish cryptologists of the Cipher Bureau (Biuro Szyfrów). According to the British Foreign Office historian Gill Bennett, on 26 July 1939 at Pyry near Warsaw the Poles revealed to the British and the French that they had been reading German Enigma-enciphered traffic since 1933.3,4
The only cipher that is mathematically unbreakable when used correctly is the one-time pad.5
Sources
- CIA, Spy Speak Glossary
- Crypto Museum, Crypto glossary
- Foreign Intelligence Agency, Enigma decryption
- Gill Bennett, blog of the UK Foreign Office historians (history.blog.gov.uk), What’s the context? Polish cryptologists reveal they have cracked the Enigma code, 26 July 1939 (26.07.2019)
- International Spy Museum (Washington, DC), Language of Espionage (glossary)
Checked: 4 November 2026