(e) The effect of X2oneback limitation on key-breaking
Turing's original method had already been modified in two respects (see
above (c)). With the introduction of X2oneback limitation a certain amount of use
was at once made of this new feature in key-breaking, though it was realised at
the time that it should be possible to make very much greater use of it. The
powerful methods for using X2oneback which were finally perfected in early 1945
are described in 26. At the time, however, use was only made of the limitation to
obtain X2 after one other chi had been obtained and the psi repeat recognised.
This was done by examining the TM deduced from the PSI' already obtained, when
written on a width of 31. All TM dots imply X2oneback = x and columns where no
dots appear are extremely likely to correspond to a X2oneback dot. This allows
us to infer most of X2oneback which is then slid one to the left so that it
becomes X2 and is then compared with the Delta-X2 values obtained from the last
Turingery count for Delta-X2 to have been made. The combination of the two should
give a complete X2 which is then added to K2 to give PSI'2 . This PSI'2 combined
with the use of the known X2oneback should place most if not all, of the TM dots
whose position is ambiguous.
The disadvantage of the new modification was that recognising the psi repeat
was made much more difficult because the old rule disallowing more than 2
consecutive TM dots no longer held.
(f) A new feature
In the early months of the QEP era a new feature appeared. Messages no
longer invariably began and ended with the beginning and ending of transmissions,
nor did transmissions beginning in the middle of messages start with "Zwoter (etc)
teil . . . ". No serious difficulties were caused, apart from the greater
difficulty of breaking depths, and later de-chis, because we could no longer rely
on the starts of transmissions containing stereotyped message beginnings,
though a fair proportion still did.
(g) The Herring link and the first appearance of X2oneback P5twoback limitation.
The Rome-Tunis link known as Herring operated between Decmber, 1942 and the
final collapse of the German forces in Tunisia in May, 1943. It was on this link
that both the X2oneback and X2oneback + P5twoback limitations first appeared. The
method by which the X2oneback + P5twoback limitation was analysed and its method
of working understood is described in Ch.44. The X2oneback + P5twoback limitation
effectively prevents messages being in depth even when the initial settings are
the same owing to the divergence of the two PSI's under the influence of the
different P5's. Thus work on Herring was made impossible, until the operational
difficulties or passing a great quantity of traffic under pressure using the new
P5 attachment proved too great ( a single fifth impulse corruption in reception
would cause a breakdown, necessitating a complete retransmission) and the
attachment was abandoned. - (to reappear on nearly all links in December, 1943).
From then on the Germans sent an enormous quantity of traffic; the majority was
sent in depth (often multiple depth), presumably because they could hardly
spare the time to reset their machines. The effort and production of the Testery
reached an unprecedented peak, at a time when the messages broken were of great
operational importance. In May the section decoded over 1,400,000 letters,
a figure which was not equalled until March, 1944, when the Newmanry was in full