Changes

To avoid this weakness, an extra trick was developed.  Instead of just setting the rotors and plugboard according to that day's instructions in the code book, the procedure was this:

To avoid this weakness, an extra trick was developed.  Instead of just setting the rotors and plugboard according to that day's instructions in the code book, the procedure was this:

*Insert the rotors, and set them and the pugboard according to today's key.

*Insert the rotors, and set them and the plugboard according to today's key.

*Make up a random 3-letter sequence, say, KES.  (This is called a "session key", and is routinely used in modern internet security.)

*Make up a random 3-letter sequence, say, KES.  (This is called a "session key", and is routinely used in modern internet security.)

*Type that message twice (KESKES) into the Enigma, getting say, HRMPLB.

*Type that message twice (KESKES) into the Enigma, getting say, HRMPLB.

Later in the war, the Nazis inceased security by upgrading the machines to use a 4^th rotor, but they committed an incredible blunder.  It took some time to equip the entire fleet with the new machines, since individual submarines had to be given one the next time they were in port.  During this transition period, the operational instructions were for those sumarines with the new machine to leave the 4^th rotor in position zero.  This made it compatible with the older 3-rotor machines.  On one occasion, a submarine sent a message with the 4^th rotor in a non-zero position.  When the operator realized the mistake, he set the rotor back to zero and ''sent the exact same message again''.

Later in the war, the Nazis inceased security by upgrading the machines to use a 4^th rotor, but they committed an incredible blunder.  It took some time to equip the entire fleet with the new machines, since individual submarines had to be given one the next time they were in port.  During this transition period, the operational instructions were for those sumarines with the new machine to leave the 4^th rotor in position zero.  This made it compatible with the older 3-rotor machines.  On one occasion, a submarine sent a message with the 4^th rotor in a non-zero position.  When the operator realized the mistake, he set the rotor back to zero and ''sent the exact same message again''.

The British knew that a 4^th rotor was coming, but did not know how it was wired.  This would have been an extremely serious problem, and might have shut down their decryption efforts for some time.  But, when they were engaged in their routine breaking of that day's traffic, they noticed that a message was gibberish.  They also noticed that it was of exactly the same length as the following messaged.  They correctly guessed what had happened, and, by comparing the messages, were able to deduce how the 4^th rotor was wired.

The British knew that a 4^th rotor was coming, but did not know how it was wired.  This would have been an extremely serious problem, and might have shut down their decryption efforts for some time.  But, when they were engaged in their routine breaking of that day's traffic, they noticed that a message was gibberish.  They also noticed that it was of exactly the same length as the following message.  They correctly guessed what had happened, and, by comparing the messages, were able to deduce how the 4^th rotor was wired.

=== Breaking the Code ===

=== Breaking the Code ===