The Special Fish Report
Albert W. Small (December 1944)
Codes and Ciphers
TOP SECRET Special Fish Report Page 73
On high-motor-dot traffic, "55M889" is a wonderful help since
plain text can usually be guessed between any close-together-"stops" whenever the
nature of the message is foretellable from the log. Go-backs are a help because they
(usually) combine psi prime streams at just the right interval to project
results successfully back and forth.
Perhaps the greatest occasional aid in breaking a set of psi's is the
discovery and utilization of "stretches of psi in the de-chi." These stretches
occur when the German message tape runs out of the tape reader in such a way
as to cause blanks to be enciphered until the operator comes over and stops the
machine. Since Testery is working with a de-chi, it discovers these stretches
as pure psi prime. At the end of such a stretch a pause in the radio-transmission
usually occurs (while the German operator fixes his machine.) Said pause helps the
Testery locate and believe said stretches. Also, if the German tape gets stuck,
some one letter may be enciphered repeatedly until the operator pulls the tape
through (usually with the same sort of pause). Thirty two possibilities for the
letter that was repeatedly enciphered at this point must be tried (in order of
preference) when obtaining the resultant psi patterns.
It is sometimes possible to utilize recovered stretches of broken
pattern which appear to be pattern repeats. If such repeats are visible on
two impulses, the exact psi distance may be calculated by solving a diophantine
equation (e.g., 43n + 17 = 47m + 12 where PSI'1 and PSI'2 contain repeats.)
Patterns can then be played back and forth as developed.
Little can be said about Dragon that is not already known by
Arlington. It is indeed proving itself to be quite worth while.
The British are not using it to break psi's; and they are not using it to set
psi's on days of high motor dottage, since this can be done quickly by hand.