# The Special Fish Report

## Albert W. Small (December 1944)

### Page 103

Tony Sale's
Codes and Ciphers

| previous page | next page | index page |

CX/MSS
TOP SECRET Special Fish Report Page 103

Key Breaking on Colossus

Key-breaking runs are identical to wheel-breaking runs, already

described. Decibanning is done the same way. Wheels come out much

quicker, however.

In general, a DeltaK45 rectangle is made from the Delta Key.

This rectangle is usually flagged (cross product) and the flag converged

by crude convergence. A pattern is thus obtained for DeltaX5 and this may

be pushed through the rectangle (crude scoring) to obtain DeltaX4. If

necessary the DeltaKl5 DeltaK25 DeltaK35 rectangles are also made as described

on Page 80, but this is not usually necessary on key obtained from cribs

since such key is sufficiently long to give good results with less work.

As soon as patterns X4 and X5 have been obtained, a DeltaK tape is placed

on Colossus and the following runs made (wheel-breaking runs):

4p/5 as a test of score.

3=/5 (or if it is known that the 5
pattern is correct as to direction of
signs, and not inversed, then

2=/5, (or 2./5. if there is no possibility
that 5 pattern signs have been inversed)

1=/5 (or 1./5. if as before.)

Note that an attempt is made to build up a large number of
stronger
partial wheels, rather than to make a \ run such as 2=/4=5 which would have

less text for study, and might therefore not have resulted in any stronger

pattern anyway at this stage.

The next wheel-breaking run is often 4=/3=5 which should "finish off"

wheel X4. Then comes 3=/4=5 which usually finishes X3. Then 2=/3=4=5. Finally

comes l=/2=3=4=5 and in half-an-hour after starting on Colossus, "the wheels

are out". (On short messages with low values of b, it is or course not as

easy as appears here.)

| previous page | next page | index page |