23G Page 90
to have a very high sigma-age, especially if the slide between the two parts is found to be small.
(e) Doctoring of tapes.
If it is difficult to set the other wheels, it may be worth
while to do a slide run on 1+2., for when the settings of the various
parts have been found it is possible to “doctor” the tape, i.e. to put the
parts in their correct relative positions by inserting or removing letters:
this is in fact rarely done except during wheel-breaking [illegible].
(f) Break-ins with spanning.
If it is thought likely that a message contains a slide, especially when it is to be “flogged” [23J(b)], the break-in runs may be done whilst spanning a suitable part of the text. Suitable spans are first two-thirds and last two-thirds.
Alternatively, since slides are more probable when interception fails to identify letters, which are then represented by 9’s, count99’s with spanning, and select a stretch with as few as possible.
Raw tapes are spanned over the first 2500 letters, and, if long enough (> 4000) over the last 2500.
(g) Message slides and wheel slides.
It is important not to confuse message slides with wheel slides [23G] whose only common feature is that they give rise to rival settings.
(h) M Procedure.
Occasionally two messages are sent without resetting wheels:
switching the Tunny machine out and in, automatically inserts exactly two
letters of key between them; the two messages are therefore punched on the
same tape with [illegible] two letters inserted, and become a single message
so far as Χ-setting is concerned. [of 11B(k), 11D(d)]
Note: The interruption disturbs the stepping of the Ψ‘s so that when setting Ψ‘s the two cannot always be treated as a single message.
23G WHEEL SLIDES
(a) Definition.
Consider a wheel which has a large number of agreements with
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