Seizing the Enigma (51 page)

Two other reasons, of a far less fundamental nature, also conduced to Allied superiority. First, since Germany used a single cryptosystem very extensively, the Allies could concentrate more manpower on it, had more intercepts in it to work on, and looked forward to greater rewards from solving it than if they had to work on many systems of several nations, as the Germans did. Second, the Allies ruled the sea. They thus could seize documents from enemy ships. The Germans captured only a few Royal Navy cryptographic documents.

One day during the war, a can of Spam appeared on the table of Leonard Forster, a translator in Hut 4. “Look,” said his wife, “here’s this new thing that’s come from America.” When he saw the can, Forster felt a great swelling of pride. For he had had a hand in getting that food to Britain.

ULTRA
had helped bring food to his table and to millions of others in Britain. It was one of the great intellectual achievements of the century, no less remarkable because it was achieved against a secret produced by men rather than one of nature. The unraveling of the Enigma was the equivalent of those endeavors that are awarded Nobel prizes. And, like those, it benefited humankind.

By bringing peace closer,
ULTRA
shortened the time that fathers were separated from their children, husbands from wives. And it spared an untold number of people—men in the cargo ships and their escorts; men at the fighting fronts; men, women, and children under the bombs in the cities of the home fronts. That was
ULTRA
’s greatest gift: it saved lives. Not only British and American lives, but German lives as well. That is the debt the world owes the Bletchley codebreakers; that is the crowning human value of their triumphs.

P
REPARING TO ENCIPHER A MESSAGE IN NAVAL
E
NIGMA WAS A
complicated and multistep procedure. It required an indicators book, which listed groups of three letters to specify a particular key net, that month’s machine-setting list, which gave each day’s settings, tables to encipher pairs of letters into other pairs, and other papers.

The first steps, preparing the so-called “inner settings,” could be done only by an officer. He would do the following:

1.
Select from the eight rotors available the three that the machine-setting list specified for that day.

2.
On each rotor, turn the alphabet ring to the position prescribed in the machine-setting list and lock it in place with the pin.

3.
Assemble the rotors on their shaft in the order prescribed by the machine-setting list and insert them into the machine.

The radioman would then prepare the outer settings. He would:

1.
Turn the rotors until the three letters specified in the machine-setting list appeared in the windows of the machine’s closed lid.

2.
Insert the plugs at both ends of the plugboard cables into the proper sockets of the plugboard to connect the pairs of letters prescribed by the machine-setting list.

The radioman next readied the message key. He would:

1.
Determine the key net on which the message is to be set.

2.
In the distribution list in force, find the numbers of the columns in the indicators book assigned to that key net.

3.
From one of those columns, pick out at random a three-letter indicator.

4.
Write this key-net indicator in the last three cells of the first line of the encipherment form’s book-group column (perhaps called that because for some years the German plaintext was encoded in the
Allgemeines Funkspruchbuch
before being enciphered in Enigma).

5.
Make up a dummy letter at random (a null) and write it in the first cell.

6.
In the indicators book, pick out at random any three-letter indicator.

7.
Write it in the first three cells of the second line of the encipherment form’s book-group column.

8.
Make up a null at random and write it in the last cell of that line.

9.
Determine the bigram table in force from the key list.

10.
Combine the letters of the first cells in the first two lines into a vertical pair.

11.
Look up the vertical pair in the bigram table and replace it with its cipher pair.

12.
Write the two letters of this cipher pair horizontally into the first two cells of the first line of the radio-group column of the encipherment form.

13.
Repeat this process with the three remaining vertical pairs in the book-group column, writing them horizontally into the first two lines of the radio-group column.

14.
Press, on the Enigma keyboard, the three letters of the original, unenciphered key-net indicator and write down at the top of the message form the letters lit up on the illuminable panel (this becomes the message key).

15.
Turn the rotors until the letters of the message key show in the lid windows.

A naval Enigma encipherment worksheet. The cipher clerk would write the plaintext in the right-hand column under the heading
Bedeutung
(meaning). The first word, Wespe, is a priority indicator. The message begins, “
Leipzig an Flotte. Köln Standort Norderney Leuchtturm …” (Leipzig
to the fleet:
Cologne
location Norderney lighthouse …). The clerk would then follow the instructions for choosing and enciphering the indicators and finally for enciphering the plaintext. Translations, beginning at the left-hand column:
Anfangskenngruppen
, beginning indicator groups;
Verschlüsselt mil Schlüssel M
, enciphered by Enigma;
Endkenngruppen
, final indicators groups;
Uhrzeitgruppe
, time group;
Gruppenzahl
, number of groups;
Funkgruppen
, radio groups;
Buchgruppen
, book groups;
Spruchschlüssel
, message key;
gültig für 3.8.
, valid for August 3;
Schlüsselkenngruppe
, key-net indicator;
Verfahrenkenngruppe
, message-grade indicator (but actually a random group).

Part of a page of a
Kennbuch
, or indicators book, showing the random three-letter groups that form part of the Enigma key. A deciphering portion has the indicators in alphabetical order with their column numbers next to them.

A portion of a bigram table, which serves to encipher part of the Enigma key.

A portion of a key list for the commercial Enigma used by German forces in the Spanish civil war. For each day it shows, under
Innere Einstellung
(inner setting), the positions for the three removable rotors in Roman numerals and the alphabet ring settings for the three removable rotors and the reversing rotor. Under
Ausseneinstellung
(outer setting), it gives the rotor positions for the start of encipherment of each message sent during two periods of the day.

A distribution list for the indicators book. It shows, for example, that the three-letter indicators in columns 81 to 140 of the book are to serve as key-net indicators for the
TRITON
key net, used by Atlantic U-boats. Indicators from other columns specify other key nets.

The cipher clerk would then write the plaintext into the book-group columns of the cipher form without word breaks but with
x
or
y
to separate sentences. He replaced the common letter-pairs
ch
and
ck
with
q.
Priority indications, such as SSD (for
sehr sehr dringend
, very very urgent), would be replaced by a variety of words, such as
Wespe.
Ready at last for the actual encipherment, he would summon a colleague. As he pressed the successive letters of the plaintext on the typewriter keyboard, his co-worker would write down in the radio-group columns of the form the letters that lit up on the illuminable panel—the letters of the cryptogram, the secret message that was to be sent. The cipher clerk would cross out the book-group column to avoid its being transmitted by mistake. He would transmit the enciphered indicators before the enciphered message. At the other end, the recipient would decipher the indicators, recover the message key, and translate the message.