|Anoraks Corner by Tony Sale|
A menu is a set of instructions for setting up a Turing Bombe to search
for an Enigma configuration which satisfies all the constraints specified
by the menu.
The menu is worked out by writing the crib letter by letter, underneath the intercepted cipher text.
A weakness of the Enigma machine, that no letter can encipher to itself, can be exploited to check whether the crib is in a possible place with respect to the cipher text. No letter should appear at the same place in crib and cipher. If it does there is a "clash" and the crib is in the wrong place.
A strong menu requires that loops of letters be found such that at different places along the cipher/crib pair letters encipher to one another in a circle. i.e. A -> E, E -> B, B -> A all at different places.
An ever present problem is wheel turnover. On the Enigma machine a carry notch transfers the movement of one wheel to the next, "a carry", like on a car mileometer adding one to the tens from the units.
Because the code breaker has no knowledge as to the positions of the wheels a carry might occur within the length of the crib. If it does occur then the Bombe run is invalidated.
In order to reduce the risk of turnover, cribs are kept as short as possible consistent with getting a strong menu.
Another technique which can be used if a long crib is available, is the modulo 26 method.
This involves writing out the cipher/crib pair in rows of 26 letters.
...zabcd efghi jklmn opqrs tuvwx y
zz.RUHCY VCYVV ALACY OMKPM XFWAO W
...FLUBI BESTA NDXEI KSXEL FXGOT H
za.VRPOA RKYJL BURVD FTIMA KWJXC Y
...AXANN AXDRE PXEIN SNULL NDLLV I
zb.BADKP HBIKU XQEQK LLKGK TDUGG P
...ARXLI TERXJ AEGER XBERT AXVIE R
zc.DRYGJ NYQGV JUTID NDDPS ONJTN T
...XZWON ULLFU ENQNU LLXLI TERXH E
This works because if a wheel doesn't turn over within a five character length, then it will also not turnover in a five character length 26 characters further on. Thus all the elements in the third column can contribute to a menu.
The lower case letters are the offsets, row and column.
This was a very strong menu and was sent to Eastcote, the Bombe outstation in linear form:
NAZJ AXZL XABJ XUAK UNCK NDAN DLZK DUCN NICM
IVAM IYZN QEBM QEBK QTCL EGBL ERAL RKBN ECZM
In this form in each group of four letters, the first two letters are the cipher/crib pair, the second two letters are the offset along the crib.
This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum