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Sophisticated hardware is only one part of a secure cryptographic system.  The manner in which it is used is crucial.  The Nazis made some serious mistakes.

Sophisticated hardware is only one part of a secure cryptographic system.  The manner in which it is used is crucial.  The Nazis made some serious mistakes.

The most secure way of handling keys is the "one-time-pad", commonly used in low traffic systems such as espionage.  In this system, each submarine would have been given a different list of keys, randomly generated in advance, with headquarter keeping a copy.  Each key would be used only once.  That is, the code book would effectively say "For the 43^rd message you send use key XYZ; for the 44^rd message, use key FUO, ..."  After every message is sent, that key is crossed off the list.

The most secure way of handling keys is the "one-time-pad", commonly used in low traffic systems such as espionage.  In this system, each submarine would have been given a different list of keys, randomly generated in advance, with headquarter keeping a copy.  Each key would be used only once.  That is, the code book would effectively say "For the 43^rd message you send, use key XYZ; for the 44^rd message, use key FUO, ..."  After every message is sent, that key is crossed off the list.

This one-time-pad system is too unwieldy for a large operation such as general military communication, so a common daily key was used instead.  Each submarine had a copy of the list, giving the key for each day; headquarters had that list also.  This had two weaknesses:

This one-time-pad system is too unwieldy for a large operation such as general military communication, so a common daily key was used instead.  Each submarine had a copy of the list, giving the key for each day; headquarters had that list also.  This had two weaknesses:

The Nazis used word salad, but they also made a costly mistake.  They routinely ended each message with "HH", for "Heil Hitler".  This foolish bit of vanity was helpful to the British.

The Nazis used word salad, but they also made a costly mistake.  They routinely ended each message with "HH", for "Heil Hitler".  This foolish bit of vanity was helpful to the British.

There is another problem.  The movement of the rotors meant that simple letter frequency analysis, which plays an important part in solving cryptograms in magazines ("This message has a lot of m's; let's guess that m corresponds to e.") won't work.  But one can still do frequency analysis on the letters in a corresponding position in the massages, since they all used the same key.  That is, if today's messages often have m appearing in the 15^th position, one could guess that those were e's in the 15^th position, since everyone's rotors would have been in the same position at that point.  While utilizing this information would have been much harder than ordinary substitution ciphers, it was still a weakness.

There is another problem.  The movement of the rotors meant that simple letter frequency analysis, which plays an important part in solving cryptograms in magazines ("This message has a lot of m's; let's guess that m corresponds to e.") won't work.  But one can still do frequency analysis on the letters in a corresponding position in the messages, since they all used the same key.  That is, if today's messages often have m appearing in the 15^th position, one could guess that those were e's in the 15^th position, since everyone's rotors would have been in the same position at that point.  While utilizing this information would have been much harder than ordinary substitution ciphers, it was still a weakness.

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: