controlled by a single letter of the indicator, that the first five letters of the
indicator corresponded to the five PSI wheels, in order, and the last five letters
corresponded to the five X wheels, in order. The obvious assumption that the
same indicator letter in the same place for two messages meant that the
corresponding wheel had the same setting in both messages was also made.
Justification for it could be found in the fact that the last indicator letter was
restricted to the same 23 values over the whole of any one month, which seemed to
show that there was no change in the indication of the fifth X wheel over this
The message setters therefore restricted themselves to messages which had
for two or more of their X and PSI indicators values which had appeared in
messages which were already set. The settings of the corresponding wheels could be
assumed known, and this greatly simplified the process of message setting
described above. In impulses in which the setting of a X wheel was known, the
crib, usually SPRUCHNUMMER9++ could be tried in many different positions, and
rejected at once in some of them. When the X setting was known for two impulses,
most of the false crib positions could be rejected.
The process of message setting was very successful, and with each success it
became more powerful, since the meanings of more indicator letters were known. In
its later stages the settings of the majority of the wheels for the message
attacked were known, and the process differed but little from ordinary decoding.
The theory of the indicators was completely confirmed. The results,
together with those for April - the two months were soon being attacked
simultaneously - also gave new information about the motor wheels. It was found
that their patterns changed every day but that the corresponding indicator system,
that is the correlation of the indicator letters with wheel positions, was fixed
over each month. The 6th indicator letter controlled the 37 wheel and the 7th
controlled the 61 wheel.
It should be noted that the cyclic order of the wheel settings corresponding
to the indicator letters was not correlated with the order of those indicator
letters in the alphabet.
It was found that the X and PSI wheel patterns remained constant over each
of the months March and April, but changed between these two months.
42 D APRIL 1942
(a) Breaking the wheels
One or two depths were found in April, but no attempt was made to analyse
the keys obtained. The break into April was made on a near depth of April 22nd.
The indicators of the two messages concerned were
M H S L P E I S V O I U
M H S L P E I . . O I O
Two of the indicator letters in the second message could not be determined,
owing to corruptions. By a curious coincidence both were found, after the near
depth was broken, to represent different wheel settings from those used in the
first message. The fifth X indicator differed between the two messages.