ENCYCLOPEDIA OF Espionage, Intelligence, and Security Volume ...

Enigmaletter A on the right-hand (ciphertext) disk. The other twosubstitutions are produced by the other two wires: B → C,C → B. When the scrambler is rotated so that its input 1moves from A to C on the plaintext disk, its output 1 movesfrom A to C on the ciphertext disk. Now, instead of producingA → A, wire 1 produces C → C. The other two wiresnow produce the substitutions A → B, B → A. Thus, eachtime the scrambler is rotated by one letter position, a newdifferent substitution code is produced. This continuesuntil the scrambler returns to its starting position, whereuponthe substitution codes produced by the device beginto repeat. In this example, repetition begins with the thirdshift of the scrambler.Rotation of the scrambler can be used to make acipher that is more formidable than a straightforwardsubstitution. Consider a three-letter plaintext message isto be sent: ABA. First, A is enciphered with the scramblerin the first position described above: A → A. Before thesecond letter is encrypted, the scrambler disk is rotated byone letter-position. The second plaintext letter is thenenciphered: B → A. The disk is rotated, and A is encipheredagain: A → C. Although in this case one would startrepeating substitutions after only three letters, the resultingcipher is significantly more complex, and thus harderto crack, than a static substitution cipher.Decryption in this system is simple as long as thereceiving party possesses an identical machine; the wiresin the scrambler disk work equally well in either direction,so decryption is simply encryption run backwards. Thereceiver must, however, begin decrypting with their scramblerset to the same position as the sender’s at the startof transmission, otherwise the substitution codes usedby the receiver to decipher the message will be out ofstep with those used by the sender to encipher it, anddecipherment will fail.The Enigma system was based upon the scramblerdiskprinciple described above. Enigma used not a 3-letter,but a 26-character alphabet and not one, but four scramblerdisks. The first scrambler scrambled plaintext orciphertext, the second scrambler scrambled the outputs ofthe first scrambler, the third scrambled the outputs of thesecond, and the fourth fed back, or “reflected,” the outputsof the third so that messages passed through theother three scramblers before the encrypted ciphertext (ordecrypted plaintext) was read. Each letter was thus scrambleda total of seven times during its passage through themachine. Three of the scrambler disks could be rotatedfreely, but the fourth, the “reflector,” was stationary.In order to use an Enigma unit, its operator typedplaintext or ciphertext into a keyboard. For each keystroketyped, Enigma automatically shifted one or more of itsscramblers and lit up a letter on a display board. The letteron the display board showed the output text for the typedinput letter: ciphertext if plaintext was input, plaintext ifciphertext was input. To produce further scrambling betweenciphertext and plaintext, each Enigma also had aEncyclopedia of Espionage, Intelligence, and Securitybuilt-in commutator or “plugboard” that enabled the operatorto crisscross paired letters of the alphabet beforetheir signals fed into the first scrambler disk. The resultwas that Enigma had over 10 20 different “keys” or distinctsettings of scramblers and plugboard. Simply guessingthe correct key for a given message was, therefore, essentiallyimpossible. Every day at midnight, all operators of agiven Enigma system would switch to a new key; theseinitial daily keys were printed in a codebook that wasdistributed to the operators. For added security, the scrambler-diskspart of the key was changed for every singlemessage sent; this message-key information was transmittedtwice at the beginning of every message. Thistechnique was intended to prevent message loss due totransmission errors, but in fact reduced Enigma’s effectivenessby introducing an element of predictability.The defeat of Enigma. Enigma was long considered impossibleto crack. However, in 1931, a disgruntled German exofficergave drawings for the machine to the French secretservice. The French, who considered Enigma too tough tocrack even with this information in their possession, gaveit to the Polish government. Polish mathematician MarianRejewski (1905–1980) used it to devise automatic devices(specialized electromechanical calculators) for re-crackingthe ever-changing Enigma cipher on a daily basis. Justbefore the fall of Poland in 1939, Rejewski’s findings weretransferred to the British government, which continued toimprove them.During World War II, the German military modifiedthe Enigma system at intervals, requiring the British tocontinue re-cracking the cipher throughout the war. Withthe help of a motley team of crossword-puzzle experts,bridge devotees, chess champions, mathematicians, andlinguists led by British mathematician and computingpioneer Alan Turing (1912–1954), the group succeeded.Tragically, however, Turing was persecuted after the warfor his homosexuality. His security clearance was revoked,he was forced to undergo debilitating hormone treatments,and he was banned from the development of thedigital computer. Turing committed suicide in 1954, some20 years before his crucial contribution to the cracking ofEnigma, and thus, to the Allied victory, was declassified.❚ FURTHER READING:BOOKS:Churchouse, Robert. Codes and Ciphers. Cambridge, England:Cambridge University Press, 2002.Singh, Simon. The Code Book. New York: Doubleday,1999.SEE ALSOCipher MachinesCodes and Ciphers407