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 Vigenère cipher was created in the 16th-century by French cryptographer Blaise de Vigenère. It was revered as "le chiffre indéchiffrable" or "the unbreakable cipher" for many years after its invention.
The WWII Pigeon Cipher describes a coded message found on the remains of a carrier pigeon discovered in 1982. This message was sent to the curator of the Pigeons at War exhibit at Bletchly Park, however he found it impossible to crack.