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abbreviated to FORT): since messages were often sent in several parts, FORT was a very handy
crib. The common group of letters EINS (meaning ‘one’) was an ever-reliable crib—there was
about a 90% chance that any given message would contain EINS somewhere within it.^56 Bombes
needed longer cribs than human codebreakers; for example ‘FORT EINS EINS VIER NEUN’
(‘Continuation 1149’).^57 Cribs as short as fifteen letters were workable on the bombes, but cribs
of thirty to forty letters or more were desirable.^58
More exotic cribs resulted from the sheer naivety of many of Enigma’s users. ‘German opera-
tors were simple souls with childish habits’, Denniston said drily.^59 The weather stations regu-
larly sent messages containing routine phrases like ‘WETTER FUER DIE NACHT’ (‘weather
for the night’) and ‘ZUSTAND OST WAERTIGER KANAL’ (‘situation eastern Channel’). One
naval station even transmitted the confirmation ‘FEUER BRANNTEN WIE BEFOHLEN’
(‘Beacons lit as ordered’) every single evening. This was ‘a very excellent crib’, said Mahon,
himself a gifted cribster.^60 German Air Force and Army cribs included ‘RAF plane over airport’,
‘Quiet night. Nothing special to report’, and ‘Wine barrels on hand’.^61
The cribs used to achieve Victory’s first breaks were captured by the Royal Navy from an
armed German trawler, Schiff 26, which was intercepted and boarded while carrying a cargo
of munitions to the Norwegian port of Narvik.^62 Subsequent attacks on German vessels were a
rich source of cribs, as well as other Enigma-related materials.
Figure 12.8 shows a crib of 25 letters. The codebreakers could pinpoint the location of a
cribbed phrase within a message by using the basic fact that the Enigma machine never encoded
a letter as itself. This was the effect of a component called the ‘reflector’. Because of the presence
of the reflector, the letter that came out of the wheels was bound to be different from the one
that went in (see Fig. 12.4). So the cribster would slide a suspected fragment of plaintext (such
as ZUSTANDOSTWAERTIGERKANAL) along the ciphertext, looking for a location at which
no letters matched up. Positions where a letter in the crib stood alongside the very same letter in
the ciphertext were called ‘crashes’: a crash showed that the crib was in the wrong place. When a
crib was more than about thirty letters long, it was more likely than not that it would ‘crash out’
everywhere except at its correct location in the message.
Figure 12.9 summarizes the action of the bombe. The machine searched at high speed to find
• which wheels were in the Enigma
• the plugboard wiring pattern
• the wheels’ starting position at the beginning of the message (such as XYZ).
The correct combination of these elements would produce the right cipher letters from the
crib. In an ideal world the bombe could do this by enciphering the crib at every possible
combination of these elements, looking for the combination that produced (for example)
1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 2122 23 24 25
D A E D A Q O Z S I Q M M K B I L G M P W H A I V
K E I N E Z U S A E T Z E Z U M V O R B E R I Q Tfigure 12.8 A crib of twenty-five letters. The crib (bottom line) is positioned against the corresponding ciphertext
(middle line).^63 ‘Keine Zusaetze zum Vorberiqt’ (the crib) means ‘No additions to preliminary report’. (‘Vorberiqt’ is a
shortening of ‘Vorbericht’: Enigma operators regularly replaced CH by Q.) The numbers along the top indicate the
successive steps of the encryption process.