The Special Fish Report

Albert W. Small (December 1944)

Page 99

Tony Sale's
Codes and Ciphers

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CX/MSS
TOP SECRET Special Fish Report Page 99

Positions of tapes relative to each other are shown on the

run as the first four digits dawn the columns, and the scores at such

positions are shown as the last four digits. The P tape was 2,510 long.

The Z tape was 2,512 long, of which only 1,222 characters were due to the

Z text and 1,290 were blank. (That is, the crib as 1,288 longer than

the cipher.) Start and stop impulses were placed on the Z tape.

The Z tape was made two positions longer than the P tape, so that as the

two revolved side by side in the Robinson "bedstead" (tape rack), the first

position of the Z tape changed relative to the P tape by two notches each

revolution. For this reason the setting "1052" shown on the accompanying

Robinson run really means position 2 x 1052 or 2104. (This was an even

run. On odd runs the position is one less than twice the recorded position.)

In this particular crib run it should be noted that

if the first position of the Z tape was set along 2,104 positions on the P

tape, then the first 406 characters of the Z tape matched the last

406 of the P tape. The remaining 816 characters of the text (non-blank) part

of the Z tape have been matched against the beginning of the P tape

(and are therefore random) This is considered better procedure however than

to let the Z tape "run off the end" of the P tape, in which case average

random scores would have to be re-figured at each of such positions.

The random score in the sample is obviously 1/16 times 1,222 = 76.4.

Sigma is (1,222*(1/16)*(15/16))^1/2 = 8.46. A score of 114 is therefore 4.5 sigma.

This was attained in spite of the fact that only 406 characters counted

toward the causal score, whereas 816 were random!

These 406 characters actually represent 406+341 or 747 letters of

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