The Special Fish Report

Albert W. Small (December 1944)

Page 95

Tony Sale's
Codes and Ciphers

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CX/MSS
TOP SECRET Special Fish Report Page 95

"Cribbing"

Solution of parametric equation (3) K = P + Z, can be done by

Robinson "crib runs," and key obtained used to solve equation (4), PSI' + X' = K.

With X2oneback limitation, or X2oneback + P5twoback limitation, cribs

used should be at least 500 letters in length. Without limitation, or with X2oneback

+ PSI1oneback or with X2oneback + PSI1oneback + P5twoback limitation, cribs should

be over 1,000 letters. A usable crib arises when

the Germans use the same message tape in the transmission of the

(same) message on different keys. Messages deciphered by Testery and

suspected to be circular, are checked by "Sixta" (traffic analysis people)

against log receipts on other circuits. Sixta also looks for messages,

the last two digits of whose serial numbers are the same. (Serial numbers' last

two digits are sent in clear by the German radio operators in receipting procedure.

These clicks of course may be accidental.

An average of two recoveries a week results from "cribbing."

It is shown in Fried #F46 P4 that for X2oneback + P5twoback limitation

transmission:

and the crib run for X2oneback + P5twoback limitation consists therefore of running

the first function in the square brackets against the second function, on Robinson
counting dots.

The value of "k" is usually 3,4, 9, and 11. (See P.2 of #F 75 report by

Fried.)

A Delta-P2 + P5twoback tape is therefore constructed for Robinson as follows:

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