The Special Fish Report
Albert W. Small (December 1944)
Codes and Ciphers
TOP SECRET Special Fish Report Page 77
difference row for familiar sequences, dragging probable words against this
difference row to see where good text #2 results. They believe hand methods
to be quicker than machine, and are unimpressed by our I.B.M. methods. In
some ways I think them right. Facility gained at 32-letter arithmetic
and visual recognition of the various forms of P text, stands them in
good stead later when it comes to breaking psi wheels. And it is not too great
a mental burden for them, because they are reading messages that mean something
to the war effort. Our failure at Arlington to work operationally destroys
the huge incentive that makes the British daily mental gymnastics not only
bearable but also pleasurable.
When bits of plain texts in scattered stretches are read, there
is a difficulty in the association of these plain texts with their correct
messages. P1 + Z1 = K1 it is true, but Pi + Z2 = ?? Aids in the
correct association of plain and cipher texts are:
1. The log book gives the type of plain texts to be
2. Hand transmissions tie in stretches, and also result in
characteristic plain text.
3. Go-backs are self-identifying.
4. When a X2oneback limitation exists, the Delta Key can
be distinguished from the Delta "rubbish."
The last process is done as follows:
Delta "believed-to-be-key" is written on a width of 31. The
of Delta-K2 so written
excess of dots (or crosses) in any column \ should have a proportional bulge of
Beta, because of the X2oneback limitation. Let the modular sum of these 31 columns
excesses be called X. (i.e., X = Sum|X1|.) Then a quick significance test
to determine if the text is Delta-K2 with a X2oneback limitation is that shown on
page 29, i.e., X >= .8(RW)^1/2 + 1.2 (R)^1/2