At the end of October, 1942, there was a complete change in the nature of
the Tunny traffic. The Tunny link itself closed down, and it was for a time
supposed that the Germans had abandoned the "Tunny" cipher machine. Two other
teleprinter links (called Codfish and Octopus) came into operation at this time,
and it was shown, by the analysis of depths of three that both these links were
using the "Tunny" machine. These links did not transmit twelve letter
indicators, but only a "QSN" number (QSN was later replaced by QEP ). Messages
having the same QSN number on the same day and belonging to the same link were, it
was found, in depth.
Messages were soon being sent in greater numbers than ever, but now only
those messages which were in depth with others could be read. So during the first
half of the year 1943, the Tunny Section confined itself to the reading of
Fortunately the German operators began to send depths in great profusion,
and so on many links it was still possible to read a fairly large fraction of the
traffic. (From this time on, many new links were coming into operation, or were
Codfish was one of the links which gave a large proportion of depths.
Depths of more than a dozen messages were not unknown on this link. Octopus
depths were much rarer.
(c) The New Cryptographic Problem
It was found that each link had its own set of wheel patterns, that X and
PSI patterns were changed monthly, and that motor wheel patterns were still
changed daily. Here there was one difference from the old Tunny link, for which
it had been demonstrated that the PSI patterns were changed only quarterly.
The Germans could not be relied upon to continue to send much a proportion
of depths, and in any came the single messages presented an urgent problem. The
wheel patterns for a link could be obtained from the depths but there seemed
to be no way by which single messages could be set on these patterns.
It was clear that single messages had now to be considered in isolation, for
it was no longer possible to relate them to one another by means of their
indicators, as in the method of analysis described in 42E. Had there been
reliable cribs, the method of message-setting described in Section VI could have
been employed, but the Germans had now taken precautions against the use of
stereotyped beginnings, the chief precaution being the use of padding words.
Sometimes a fairly reliable crib for a link would be found, but positions of the
crib in the message was then so variable that the method was still not