Changes

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.

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:

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:

*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.

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.

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.

 

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 ===