Free Enigma by Robert Harris Page A

on the bulbs, would be the
original plaintext.
    The genius lay in the vast number of different permutations the
Enigma could generate. Electric current on a standard Enigma flowed
from keyboard to lamps via a set of three wired rotors (at least
one of which turned a notch every time a key was struck) and a
plugboard with twenty-six jacks. The circuits changed constantly;
their potential number was astronomical, but calculable. There were
five different rotors to choose from (two were kept spare) which
meant they could be arranged in any one of sixty possible orders.
Each rotor was slotted on to a spindle and had twenty-six possible
starting positions. Twenty-six to the power of three was 17,576.
Multiply that by the sixty potential rotor-orders and you got
1,054,560. Multiply that by the possible number of plugboard
connections—about 150 million million—and you were looking at a
machine that had around 150 million million million different
starting positions. It didn’t matter how many Enigma machines you
captured or how long you played with them. They were useless unless
you knew the rotor order, the rotor starting positions and the
plugboard connections. And the Germans changed these daily,
sometimes twice a day.
    The machine had only one tiny—but, as it turned out,
crucial—flaw. It could never encipher a letter as itself: an A
would never emerge from it as an A, or a B as a B, or a C as a
C…Nothing is ever itself: that was the great guiding principle in
the breaking of Enigma, the infinitesimal weakness that the bombes
exploited.
    Suppose one had a cryptogram that began:
IGWH BSTU XNTX EYLK PEAZ ZNSK UFJR CADV _
    And suppose one knew that this message originated from the
Kriegsmarine’s weather station in the Bay of Biscay, a particular
friend of the Hut 8 cribsters, which always began its reports in
the same way:
WEUBYYNULLSEQSNULLNULL
    (“Weather survey 0600”, WEUB being an abbreviation for
WETTERUBERSICHT and SEQS for SECHS; YY and NULL being inserted to
baffle eavesdroppers).
    The cryptanalyst would lay out the ciphertext and slide the crib
beneath it and on the principle that nothing is ever itself he
would keep sliding it until he found a position in which there were
no matching letters between the top and bottom lines. The result in
this case would be:
BSTUXNTXEYLKPEAZZNSKUF
WEUBYYNULLSEQSNULLNULL
    And at this point it became theoretically possible to calculate
the original Enigma settings that alone could have produced this
precise sequence of letter pairings. It was still an immense
calculation, one which would have taken a team of human beings
several weeks. The Germans assumed, rightly, that whatever
intelligence might be gained would be too old to be of use. But
Bletchley—and this was what the Germans had never reckoned
on—Bletchley didn’t use human beings. It used bombes. For the first
time in history, a cipher mass-manufactured by machine was being
broken by machine.
    Who needed spies now? What need now of secret inks and
dead-letter drops and midnight assignations in curtained
wagons-lits? Now you needed mathematicians and engineers with
oilcans and fifteen hundred filing clerks to process five thousand
secret messages a day. They had taken espionage into the machine
age.
    But none of this was of much help to Jericho in breaking
Shark.
    Shark defied every tool he could bring to bear on it. For a
start, there were almost no cribs. In the case of a surface Enigma
key, if Hut 8 ran out of cribs, they had tricks to get round
it—“gardening”, for example. “Gardening” was arranging for the RAF
to lay mines in a particular naval grid square outside a German
harbour. An hour later, you could guarantee, the harbour master,
with Teutonic efficiency, would send a message using that day’s
Enigma settings, warning ships to beware of mines in naval grid
square such-and-such. The signal would be intercepted, flashed to
Hut 8, and give them their missing crib.
    But you couldn’t do that with