Seizing the Enigma by David Kahn Read Free Book Online

collars and a dark blue bow tie with white polka dots, Ewing was a distinguished engineer. He had dealt with cables in Uruguay and had, a year or so before, described a cipher mechanism to Oliver. Oliver regarded him as of “very great brain power, in fact a man who stood out among clever men.” Everyone thought naval education would not be much needed during the few months until victory was won. Oliver told Ewing he had no one to deal with the intercepts; would Ewing see if he could make anything of them? Grasping “at even the most unpromising chance of being useful,” Ewing accepted at once.
    To assist him, he called on some people whose abilities would be useful and who were discreet and available: faculty members, particularly instructors in German, at the Royal Naval Colleges at Dartmouth and Osborne, which were on vacation in August. One of the first volunteers was Alastair Denniston, thirty-three, a German master at Osborne. A short, quiet Scot, he had studied at Paris and Bonn and had helped win a bronze for Great Britain in field hockeyas a member of the Scottish team in the 1908 Olympics in London. Like the others, Denniston was, in his own words, “singularly ignorant of cryptography.”
    He and his colleagues worked in Ewing’s cramped office. They did little more than sort and file intercepts, learn to distinguish German naval messages from military ones, and discover that call signs such as POZ and KAY, the “names” of radio stations, were not the same as the coded texts of messages. But they made not a dent on the German naval messages.
    A month later, the
Signalbuch
from the
Magdeburg arrived in Ewing’s office.
But what seemed to be the answer to a cryptanalyst’s prayer did not at first turn many of the coded messages into plain German. The book consisted essentially of hundreds of pages of columns of five-digit and three-letter groups standing opposite German words:
63940 OAT
Ohnmacht -ig
41 OAU
Ohr, Ohren-
42 OAÜ
Okkupation, Okkupations, -ieren
43 OAV
Ökonomie -isch
44 OAW
Oktant
45 OAX
Oktober
    This meant that
Oktober
would be encoded as OAX or 63945, and OAÜ (the Germans usually used the letters, not the numbers), would mean
Okkupation
or its derivatives. But attempts to reduce the intercepts to German by this straightforward method produced gibberish in most cases; the only messages that were solved were weather reports and messages to auxiliary vessels. To resolve the mystery, Ewing brought in the head of the Intelligence Division’s German Section, Fleet Paymaster Charles J. E. Rotter, who had spent many leaves in Germany. He was installed in Ewing’s secretary’s office.
    A break came when the
Handelsschiffsverkehrsbuch
, or
HVB
, another maritime code, seized from a merchantman off Melbourne, Australia, arrived at Ewing’s office, along with a method for disguisingthe code’s four-letter codewords. The letters of the codewords were replaced with other letters given in a list, or key. For example, the codeword for
Fregattenkapitän
(commander) was RABL; the key specified that the substitute for R was T, for A, L, and so forth, so that RABL would actually be transmitted as TLIN. This procedure is called superencipherment.
    Rotter seems to have reasoned that the Germans were using the same system to encipher the codewords of the
Signalbuch
, so that OAO might become, for example, JVJ. Working with a succession of messages whose serial numbers the Germans had enciphered—“Their folly was greater than our stupidity,” Alastair Denniston commented—Rotter by early November had discovered the key to the superencipherment, thus exposing the main messages of the High Seas Fleet.
    This breakthrough suddenly gave the handful of cryptanalysts plenty to do. More cryptanalysts were taken on, and a new, larger workplace was found, Room 40 of the Old Building of the Admiralty. “Room 40” became the unofficial name for the codebreaking agency.
    Early in December this thriving and fortunate