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French encrypted communications in 1811 using simple ciphers known
as petits chiffres. These were designed to be written and
deciphered in haste on the battlefield and were generally short notes
of instruction or orders, based on 50 numbers. In the spring of 1811
they began to write letters with a more robust code based on a combination
of 150 numbers, known as the Army of Portugal Code. George Scovell
cracked it within two days.
At the end of 1811 new cipher tables were sent from Paris to all
French military leaders. Based on 1400 numbers and derived from
a mid-18th century diplomatic code, the tables were sent with cunning
guidelines to trick the enemy, such as adding meaningless figures
to the end of letters (codebreakers would often try to tackle the
end of a letter first, looking for the standard phrases which close
formal correspondence). For the next year Scovell pored over intercepted documents. He
made gradual progress using letters that contained uncoded words
and phrases, so that the meaning of coded sections could be inferred
from the context. The information on troop movements gathered by
Scovell’s Army Guides was also crucial when making informed
guesses about the identity of a person or place mentioned in coded
letters, solving one more piece of the puzzle. When a letter from Joseph
to Napoleon
was intercepted in December 1812, Scovell had cracked enough of
the code to decipher most of Joseph’s explicit account of
French operations and plans. This information allowed Wellington
to prepare for the final battle for control in Spain (Vittoria)
on 21 June 1813. That night British troops seized Joseph Bonaparte’s
coaches and discovered his copy of the Great Paris Cipher table.
The code was broken.
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