German Enigma operators in WW2 were told always to send a certain number of messages per day to make it harder to perform traffic analysis. One bored operator sent a message composed entirely of "W" repeated 4000 times (or so).
One on-the-ball analyst noticed a message that had no "W"s in it, and deduced what had been sent[0]. That allowed the daily settings to be cracked, and thus all messages for that day.
[0] Enigma has a weakness in that no letter can be encrypted as itself[1].
[1] Enigma is effectively a "one-time-pad" where the pad is a pseudo-random sequence determined by the daily settings.
German Enigma operators in WW2 were told always to send a certain number of messages per day to make it harder to perform traffic analysis. One bored operator sent a message composed entirely of "W" repeated 4000 times (or so).
One on-the-ball analyst noticed a message that had no "W"s in it, and deduced what had been sent[0]. That allowed the daily settings to be cracked, and thus all messages for that day.
[0] Enigma has a weakness in that no letter can be encrypted as itself[1].
[1] Enigma is effectively a "one-time-pad" where the pad is a pseudo-random sequence determined by the daily settings.