General Report on Tunny

25A Page 152

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25 CHI-BREAKING FROM CIPHER
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25A The short wheel-breaking run
25B Weighing the evidence
25C General plan of wheel-breaking
25D







 Particular methods
          (a) Doubts
          (b) Setting other messages
          (c) Spanning of message slides
          (d) Spanning for changes in ΔP
          (e) Wheel characteristics
          (f) Inside out
          (g) Flogging
25E Special methods for Χ2 limitation.
25F Special method for ab≠ ½
25G Exhibits
25W Derivation of formulae for the weighing of evidence.
25X The number of legal wheels
25Y Proportional bulges relating to

This chapter describes all aspects of chi-breaking from cipher except the details of rectangles and flags (24). The special case of chi-breaking from key is treated separately (26).


25A     THE SHORT WHEEL-BREAKING RUN

(a) General description.

The basic method is the short (i.e. one-wheel) wheel-breaking run which consists essentially of choosing each character of a wheel to make the ΔD letter count, against that character, as good as possible.

Suppose for example that Χ1, Χ2, Χ3, Χ4, are known: then ΔD1( = ΔZ 1 + ΔΧ1). ΔD2, ΔD3, ΔD4 can be found.

ΔD1, ΔD2, ΔD3, ΔD4, ΔZ5 (a partial de-chi) represented at each place by a single letter, may be written out in widths of 23 so that all entries in a column are against the same character of ΔΧ5.

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