After that, the book Groups are filled out to four letters according
to indicator group procedure.
Radio Indicator Groups.
45. The Book Indicator Groups now filled out to four letters are
enciphered according to indicator group procedure. The enciphered
Book Indicator Groups are called Radio Indicator Groups; they are
entered in the column entitled "Radio Groups" (Funkgruppen), as
as first and second Radio Groups (the Beginning Indicator Groups).
and then repeated in identically the same order as the
next-to-last and last Radio Groups (the End Indicator Groups).
46. For every radio message sent according to Enigma General Procedure,
a special Message Cipher (three-letter group) is used.
47. The Message Cipher is obtained as follows. The Cipher of the
Day for enciphering the message according to General Procedure is
set in, and the letters of the three-letter Procedure Indicator
Group (Book Indicator Group, entered as per paragraph 44) are punched
once in succession on the machine. The three letters which light up
one after the other when this is done are the Message Cipher.
48. The Message Cipher is formed in the same way for deciphering
as for enciphering.
49. The Message Cipher is to be noted down on the cipher form.
50. With the Message Cipher thus obtained, which is set in in the
windows of the Enigma from left to right, the ciphering of the
actual messege is begun.