The Zimmermann Telegram
The Zimmerman Telegram, sent from Germany to Mexico during WWI, was intercepted and decoded by the British early 1917. It took them approximately a month to send the decoded message to the U.S.
British intelligence decoded the message in Room 40 of the Birtish Admirality. They found that the message had been coded using the German simple substitution code system 0075, consisting of 10,000 words and phrases correlating to numerals 0000 through 9999.
The British were able to decode the message using the older, similar German simple substitution code system 13040 as reference for the cryptanalysis technique.
Examples of the 13040 code system includes:
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