The Special Fish Report
Albert W. Small (December 1944)
Page 67
Tony Sale's
Codes and Ciphers
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CX/MSS
TOP SECRET Special Fish Report Page 67
Let us define "Li" as a letter having "i" crosses in it. Then:
If((DeltaPSI')A +
(DeltaPSI')B)= The sum most likely come from:
L0
L0 L0 70%
L4 L4 12%
L5 L5 11%
L1
L4 L5 42%
L3 L4 35%
L2 L3 11%
L1 L2 10%
L2
L0 L2 24%
L3 L5 21%
L4 L4 21%
L3 L3 14%
L2 L4 14%
L3
L0 L3 48%
L3 L4 27%
L2 L3 13%
L2 L5 9%
L4
L0 L4 76%
L2 L4 12%
L3 L3 9%
L5
L0 L5 89%
L2 L3 7%
L1 L4 4%
Another useful table assumes that only 4 impulses are satisfactory,
the remaining one having been wrongly set on the chi
wheel; it also gives the probability of crosses in the sum of the delta psi
primes.
It is known that:
Given only 4 impulses:
P(0 crosses in delta psi prime) = 1-a +a(1-b)^4 = .277 = A0
1/4 * P1 = ab(l-b)^3 = .016 = A1
A2 = .034
A3 = .074
A4 = .161
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