It was clear at the beginning therefore that the two message settings
differed only in the settings of the X wheels and further that the settings of
the 3rd and 4th wheels were the same. Moreover the messages were stated in the
clear preambles to be 3rd. and 2nd parts of messages (presumably the same message)
respectively. From experience with the decodes of July and August 1941, and of
March 1942 the clear messages were expected to begin with
DRITTER9TEIL9DES9SPRUCHES9
and
ZWOTER9TEIL9DES9SPRUCHES9
or equivalent phrases, respectively.
The initial problem was to find two such phrases which when added together
gave a result which agreed in the third and fourth impulses with the sum of the
two cipher messages. This problem was solved without difficulty and the wheels
completed. (The screed of the Research Section contains further details of this
job.)
(b) Setting
When the wheel patterns had been obtained the April depths were set, and
then messages whose clear language was unknown were studied. The process of
message setting was carried so far that the indicator system was completely
solved.
At this stage, early in May, 1942, it was possible to draw conclusions about
the periods over which the wheel patterns remained valid. It was found that the
patterns of the motor wheels changed every day, and the X patterns changed at
the beginning of each month. The patterns of the PSI wheels, it was found, had
changed at the beginning of April, and they were constant over each of the months
March and April. But it was remembered that the same PSI patterns were used in
August as in July of 1941 so it was suspected that the PSI patterns were constant
over a period of several months. Three months seemed a likely period, since the
first set of PSI patterns had presumably come into force at the beginning of July
1941.
A curious difficulty arose out of the first letter of each message, which
never seemed to decode according to the rules. This effect was not understood
until the studies described in the next section had been made.
42E THE INDICATOR METHOD
(a) General Tunny position in April 1942.
The Research section had achieved great success with the March and April
messages, The complete decoding of all this traffic would have been possible if
suitable machines had been available at the time. (As a result, while this
analysis was proceeding, it was decided to have such machines made; the
first one came into operation at the beginning of June, 1942).
But the mastery of the problem was not so complete as the March and April
success might seem to indicate. No way of breaking a length of key, without
independent information was