The Special Fish Report
Albert W. Small (December 1944)
Page 63
Tony Sale's
Codes and Ciphers
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CX/MSS
TOP SECRET Special Fish Report Page 63
by the "dechi registrar" (a Testery man) with "Hand," "Auto," and "Pause," and all
other useful operators' information that can be gleaned from the "red form"
(message intercept from Knockholt.) The Testery knows from experience that after
each pause or hand transmission, the resumed automatic sending is likely
to be a repeat of 30 to 150 letters of plain text (or on rare occasions
is often a new message and
when not, then it \ states address, call signs, etc.). Such a P
text repeat is called a "go-back." If a stretch of P text has been fitted
in near the beginning or end of one Auto transmission, then an attempt
is made to find a possible go-back area at the end or beginning of
another such auto-transmission. To do this, Delta-D texts are written out
by hand for about 150 letters from the D texts involved, and slid against
each other, counting "clicks." For if both stretches are correctly superimposed,
every place there is a motor dot in both texts simultaneously, the resultant
Delta-D texts will be the same. These positions can be further checked by any
limitation if it exists; since if the Delta-Ds are correctly set, then
the P5twoback is the same in each Delta-D, and therefore the X2oneback
signs must agree or else one out of the supposed pair of motor dots wasn't
a dot.
If accurate scoring is desired, tables exist to show what characteris-
tics the sum of Delta-D(1st transmission) plus Delta-D(2nd transmission) should take
on, since this sum is simply the sum of the first PSI'stream and the second PSI'
stream, and the PSI's always have definite characteristics depending on
motor dottage. I shall give these tables a few pages later on. Go-backs are
also an excellent way to detect whether the Xs are off in one impulse; the clicks
from superimposing Delta-D texts will be good on only 4 impulses if a X wheel is
off,
(unless the distance between the go-backs is a multiple of the length of the wheel)
and this method determine very easily which X wheel is misset, as well as permitting
it to be corrected without obtaining plain text first.
In setting PSIs, quite often some PSI refuses to set, so that the
corresponding X wheel setting is suspect. In such a case the PSI' text being worked
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