Then he turns to the bigram table for the day and taking the vertical pairs of letters LA, YR, VQ, TN replaces them by their equivalents in the table day IJ, TV, US, YX. He then sends off IJTV USYX as indicator.
28. It may help to make this rather complicated process clearer if we trace it back i.e. see how the recipient deals with IJTV USYX when he receives the message.
29. First he looks up IJ, TV, US, YX and replaces them by their bigram equivalents LA, YR, VQ, TN. Then he writes the latter as
and finds out from the Gruppenliste which column YVT belongs to and therefore what key the message is on [Often he will know this and can omit this step.] Next he sets his machine up at the Grund (JNY in our example) decodes ARQ, gets LVN, sets his machine to LVN and decodes the message. The procedure on 4 wheel keys was the same as the above, except that all 4 letters ARQN would be used, not just ARQ.
The "Boxing" or "Throw-on" System.
30. Besides this system an earlier and much loss cryptographically secure method was used on a number of keys. Here there was no K book or Bigram Tables and the operator chose the message setting out of his head. Suppose this was ASD. Then setting up to the Grund he encoded ASD twice i.e. he encoded ASD ASD giving QJVRTS. This was then sent off generally with dummy letters inserted (in Mediterranean traffic the 2 extra letters inserted indicated what particular key was being used). So the groups XQJV ARTS might be sent over the air.
31. It is easy to spot when this system is being used because if in two six letter indicators (ignoring the dummy letters) the 1st 2nd or 3rd letters agree so must the 4th 5th or 6th respectively and vice versa. In our example ASD ASD encoded as QJV RTS; now suppose we have another indicator group beginning with Q - then this implies that the message setting must be A.. and therefore A..A will be encoded at the Grund which must produce an R in the fourth place to agree with the R of RTS. This "throw-on" effect from Q to R is what gives the system its name.
32. The "throw-on" effect produces very serious weakness (see Chap. )