For its time, the Enigma machine (see Section 2.7) was an excellent system, and the German troops’ belief in this led to security breaches. After all, if your post is being overrun or your ship is sinking, are you going to try to escape, or are you going to risk your life to destroy a crypto machine that you’ve been told is so good that no one can break the system?

But there were also other lapses in its use. One of the most famous took advantage of an inherent “feature” of Enigma. The design, in particular the reflector (see Section 2.7), meant that a letter could not be encrypted as itself. So an $A$ would never encrypt as an $A$, and a $B$ would never encrypt as a $B$, etc. This consequence of the internal wiring probably looked great to people unfamiliar with cryptography. After all, there would always be “complete encryption." The plaintext would always be completely changed. But in practice it meant that certain guesses for the plaintext could immediately be discarded as impossible.

One day in 1941, British cryptographers intercepted an Enigma message sent by the Italian navy, and Mavis Batey, who worked at Bletchley Park, noticed that the ciphertext did not contain the letter $L$. Knowing the feature of Enigma, she guessed that the original message was simply many repetitions of $L$ (maybe a bored radio operator was tapping his finger on the keyboard while waiting for the next message). Using this guess, it was possible to determine the day’s Enigma key. One message was “Today’s the day minus three.” This alerted the British to a surprise attack on their Mediterranean fleet by the Italian navy. The resulting Battle of Cape Matapan established the dominance of the British fleet in the eastern Mediterranean.