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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.

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.

An early weakness was identified in German army and air force (but not navy) procedures.  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 plugboard 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.)

*Set the rotors to KES, and encrypt the rest of the message.

*Set the rotors to KES, and encrypt the rest of the message.

*Transmit HRMPLB followed by the encrypted message.

*Transmit HRMPLB followed by the encrypted message.

When Bletchley Park became aware of this they realised that because they had the same three-letter group encoded twice but with a three-space difference in the position of the fastest moving rotor, it was often possible to work out which rotors were fitted and in which order. However the Germans stopped using this procedure in May 1940, apparently realising that it was dangerous. Various systems were later developed by each service to determine rotor start positions.

Now the part of the message that is encrypted the same way by all submarines is only 6 letters long; too short for frequency analysis.

 

At the receiving end, one sets the rotors as per the daily key, types in the first 6 letters of the received message, HRMPLB, and gets back KESKES. After noting that the same 3 letters are repeated (the reason for sending it twice is for extra reliability; a mistake here would make the whole message unreadable) set the rotors to KES and decrypt the rest of the message.

=== The 4^th Rotor ===

=== The 4^th Rotor ===

=== Breaking the Code ===

=== Breaking the Code ===

Breaking this code began before the war, in Poland.  Mathematician Marian Rejewski was able to figure out how the initial three rotors (I, II and III) were wired, apparently just by analyzing encrypted messages.  The Polish intelligence service was later able to steal a machine from Germany and "reverse engineer" it in detail.  After Poland was overrun by Nazi Germany in the early stages of the war, the decrypting effort moved to England.

Breaking this code began before the war, in Poland.  Mathematician Marian Rejewski was able to figure out how the rotors were wired, apparently just by analyzing encrypted messages.  The Polish intelligence service was later able to steal a machine from Germany and "reverse engineer" it in detail.  After Poland was overrun by Nazi Germany in the early stages of the war, the decrypting effort moved to England.

Breaking this code was a life-or-death matter for the British.  They gathered the brightest mathematicians and puzzle solvers, including, famously, [[Alan Turing]] and Gordon Welchman, into a facility at Bletchley Park.  They built the most sophisticated computing devices that were available at the time, which were electromechanical.  Completely electronic computers would have been enormously faster, but the technology at the time could not provide electronic computers that would have worked reliably enough to do the job.  The computers were called "bombes", and were basically devices to spin Enigma rotors as fast as possible, trying out different keys while working on "cribs".

Breaking this code was a life-or-death matter for the British.  They gathered the brightest mathematicians and puzzle solvers, including, famously, [[Alan Turing]] and Gordon Welchman, into a facility at Bletchley Park.  They built the most sophisticated computing devices that were available at the time, which were electromechanical.  Completely electronic computers would have been enormously faster, but the technology at the time could not provide electronic computers that would have worked reliably enough to do the job.  The computers were called "bombes", and were basically devices to spin Enigma rotors as fast as possible, trying out different keys while working on "cribs".