## Captain Walter Fried's Fish Notes March 1944 to January 1945## Page 18 |
Tony Sale's Codes and Ciphers |

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- TOP SECRET CX/MSS Page 3

resolution into its components. The method is apparently not applicable

to relays because the relaying operator makes up a new tape and this will

vary in minor details, notably punctuation. But if Berlin sends the same

message to both Rome and Paris the same tape will be used on automatic

transmission and the two texts will be letter perfect. An occasional

garble, even if on the 5th impulse, will not invalidate the procedure

although an omitted letter of course will. The real risk is interruption

of the tape with stretches of hand transmission or "go-backs", that is,

repetitions of portions of what has already been transmitted. "Go-backs"

in the crib can be found and corrected for but if the crib happens to be

in a portion of the unknown cipher text which includes "hand" or "go-backs"

it will not be possible to lodate it. A possible aid to setting cribs is

provided by the fact that where a transmission includes several messages

the receipts, which are sent in clear, state the exact time at which each

message was concluded. Correlation of these times with time of interception

and with each other should make it possible to deteraine approximately the

portion or the message which contains the crib. However, this information

has not been used because the Germans make mistakes and it is therefore

not too reliable and because once the necessary preliminary work has been

done it is almost as easy to try all possible settings.

- 8. In the description which follows I will assume that the X2 + P5

but is fully applicable if only the X2 limitation is used. To develop the

theory of this procedure we use the usual notation - a = proportion of

crosses in MT; b proportion of crosses in Delta Psi and assume that ab = 1/2

Let a' = proportion of crosses in MB. Then, with X2 + P5 limitation, which

is assumed to produce equal numbers of dots and crosses

- 1 - a = 1/2(1 - a') or a' = 2a - 1

chance that MB =. , namely 1 - a'. Therefore the chance that DeltaPsi'2 = . is

- 1 - a' + a'(l - b) = 1 - a'b = 1 - 2ab + b = b

DeltaPsi'2 = x is b. Consequently the chance is b that

- DeltaPsi'2 + P5two back + X2one back = x

- DeltaPsi'2.+Delta X2 + P5two back = X2one back + DeltaX2 + x

- DeltaZ2 + DeltaP2 +P5two back = X2one back + DeltaX2 + x

texts are properly juxtaposed. X2one back+DeltaX2 (which is called X2hat) is

unknown but it must be periodic. Therefore, DeltaZ2 + DeltaP2 +P5two back

combined with itself at an interval of 31 should yield a high proportion of

coincidences (or