The Special Fish Report
Albert W. Small (December 1944)
Codes and Ciphers
TOP SECRET Special Fish Report Page 64
is examined for double letters, and those that would be double or triple
or more except for the doubted impulse, are considered as being really
doubled or tripled. Then the Z text from the "red form" is written out,
the P crib added to it, and a believed-to-be-correct-Key is obtained.
(This can also be obtained by adding the Newmanry X to the phoney? PSI':)
This is written down, the PSI is written under it, and the Key patterns
for the doubted impulse are written below that in the positions where
there are double or triple letters in the PSI' text only.
Thus, it X3 is doubted we might have:
The possible combinations of patterns are now matched to
the X wheel, finding new possible settings. The deX is modified by
the X wheel's new settings, to obtain new PSI's, to re attempt to set PSI.
"Alphabet rods" are also found useful by some PSI setters,
though others shun them. In a case like the following:
it is possible (given simple X2 limitation) for there to be an extension of
PSI at the position WT in the Z text. If so, W and T will both be
enciphered by the same letter. By juxtaposing the W and T rods, and
looking down them, all possible digraphs from such encipherment can be
seen at a glance, end if none is likely then there was probably a motor