Cabinet Noir>History / Beaufort cipher

Not sure which cipher? Identify it
The Vigenère's mirror twin — same repeating key, subtraction the other way round, self-reciprocal.
Typepolyalphabetic substitution cipher
Breaking ithard to break by hand
EraRenaissance family, 19th-century name
NameRoyal Navy, named for Sir Francis Beaufort
Modesencrypt · decrypt · cryptoanalysis

History

Beaufort cipher

History of the Beaufort cipher

The table was old, the name is young

The cipher's arithmetic — subtracting the plaintext from the key instead of adding — appears in Giovanni Sestri's 1710 treatise, almost a hundred and fifty years before the name everyone uses. Like so many ciphers, it was reinvented independently and the most famous name won.

Admiral Beaufort and his card

Sir Francis Beaufort (1774–1857) was a Royal Navy hydrographer, the officer who gave the world the Beaufort scale of wind force — the "force 8 gale" scale still used in shipping forecasts. The cipher published under his name (posthumously, in 1857, by his brother) was something he apparently used privately for family correspondence. There is no evidence he claimed originality; the cipher itself was likely picked up from published sources of his time.

The variant's other parent

The variant Beaufort (cipher = plaintext − key) has an even more distinguished career: it is essentially the arithmetic the German army's M-209 converter of the 1930s–1940s used inside its more elaborate mechanical construction. A discarded variant of one century became a component of a field machine in the next.

The mirror family in practice

For a working cryptographer of the 1800s the three siblings — Vigenère, Beaufort, variant Beaufort — were one tool with three settings. Manual encyclopedias listed them together, and the choice between them was a matter of taste: only classic Beaufort's self-reciprocity (one table lookup serves both encryption and decryption in the field) was a practical argument. Against a determined cryptanalyst all three fell together, once Kasiski published the periodicity attack in 1863.

1553Giovan Battista Bellaso publishes the key-phrase cipher family the Beaufort variants belong to. Source
1857The cipher is published posthumously under the name of Sir Francis Beaufort, the Royal Navy hydrographer of the wind-scale fame. Source
1930sThe variant Beaufort arithmetic (C = P − K) works inside the American M-209 field cipher machine. Source