The Special Fish Report Albert W. Small (December 1944) Page 102

Tony Sale's Codes and Ciphers

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CX/MSS
TOP SECRET Special Fish Report Page 102



If the limitation in the sample run given had been X2oneback only, and


not X2oneback + P5twoback, tapes would have been made the same as those described on

page 96 except that the P5twoback impulse would have been omitted entirely. Thus the

P tape would have been simply:


Level 1 (DeltaP2) differenced at 93


(etc.)


If there had been no limitation whatsoever, or a X2oneback plus psi-one-


prime-bar limitation (and therefore unpredictable at this stage)

the original tape would have had to have been different. The mathematics

leading to the differencing of P and Z at small multiples of the X2

wheel only, would have been of no avail. X wheels would have to have been

of P and Z

considered in pairs, and differencing \ done at products of their cycles. The

P tape then would have looked like:


Level 1 (DeltaP1&5) differenced at 943

Level 2 (DeltaP4&5) differenced at 598

Level 3 (DeltaP3&5) differenced at 667

Level 4 (DeltaP2&5) differenced at 713

Level 5 Control . x . x .x . x


and the Z tape would have been the same with "Z" substituted for "P."

The same type of runs would have been made, and the same tests employed,

since probabilities of getting dots are the same. The above tape however

requires much longer texts because the differencing interval is so much

greater.


Key obtained from Robinson cribbing may now be solved as


described under "Key Breaking" (Page 79) or by key breaking runs on Colossus,

next to be described.





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