Indicators were replaced by the QEP system. This meant a serious reduction in the
amount of traffic decoded because we had to rely entirely on depths. Fortunately
the Germans sent frequent and sometimes multiple depths - sometimes as many as 10
messages on the same QEP number (or QSN number as it was at first called).
Keys were broken from depths as before, but the wheel settings had to be found for
each depth broken for a month for which the chi and psi patterns were known. The
method for doing this is described below. The motors constructed in the same way
as those used in Tunny.
(b) Setting depths with no-limitation motors.
The P obtained by anagramming the depth is added to Z to form K. Where it
is not possible to determine which P belongs to which Z the second possible K has
to be tried if the first fails.
K5 is written out and de-chied at all 23 possible settings of X5 to give 23
possible versions of PSI'5 . This process is called 'making a drag' . The
problem is to find the true PSI'5. The majority can be discarded immediately
because it can be seen at once that they cannot fit the known PSI's whatever
extensions are assumed. This process is greatly helped by the fact that the
Tunny type Mu61 and Mu37 only have singleton dots and therefore cannot give more
than two consecutive dots in TM. The remaining candidates are examined by
reference to another impulse in the following manner.
For each assumed PSI'5 pattern, all TM dots which have to be assumed for
the pattern to fit PSI5 are marked. At each of these places we know that Delta-K
= Delta-X . So at all X4 settings which satisfy this condition we de-chi K4 and
examine the resultant possible PSI'4's. Unless the key is very short (the length
normally used is from 14 to 30) the correct X4 setting based on the correct X5
setting will yield a PSI'4 pattern which when contracted by using the assumed TM
dots will fit on the known PSI'4. We then do the same for PSI3 and so on until all
psis and chis are set. It remains to anagram by using the known psis and chis
sufficient to break (or, if the motor patterns are known, to set) the motors.
(c) Advances in Key-breaking
Recognising the psi repeat and numbering , were devised in the winter of
1942 and were never discarded (See 26D).
(d) Setting depths on X2oneback limitation
The X2oneback limitation first appeared in February, 1943. The number of
dots in Mu37 was doubled to give the same proportion of dots in TM as before. The
X2oneback limitation necessitated changes in setting and motor working and caused
some changes in Key-breaking methods.
The method of setting depths on X2oneback limitation is essentially the same
as with the "No-limitation" motor. But the drag is made on K2 instead of K5 and
use is made or the fact that for each setting of X2 used, we know where the
compulsory crosses in TM fall and therefore we know where we are not permitted to
assume extensions when trying to fit possible PSI'2 on to PSI2 . On the other
hand we no longer have the useful feature of the old type motor, which precludes
more than two consecutive TM dots.