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Communication in the Presence of Adversaries 27 Yet another problem involved authentication of encrypted messages. Encryption between parties relied on a shared secret key. If enciphering material and keys were seized by the enemy, one could be fed false information , in what the Germans called a funkspiel. To authenticate encrypted material, and thus prevent funkspiels, two distinct methods were used. The first relied on inserting within the message itself an authenticator, a security check agreed upon in advance—for example, an additional x after every tenth letter of plaintext. The absence of an authenticator was meant as a “silent alarm,” indicating to the receiving party that something was amiss.27 The second method took advantage of the fact that Morse operators could not help but identify themselves in the process of keying information . The radio signal itself embedded biometric information uniquely identifying its human operator—its “fist,” or “sending touch,” “as distinctive as handwriting.”28 Such biometrics provided another layer of authentication, one much more difficult to defeat: Kahn explains that “Nazi radio spies were trained in a school near Hamburg . . . each agent’s ‘fist’ was recorded to make radiotelegraph forgery by the Allies that much more difficult.”29 Electromechanical Devices In the aftermath of the Great War, technological advances made possible the development of ciphers both remarkably powerful and simple to operate —the Enigma machines used by Germany during World War II and the Vernam one-time pad, used in diplomatic communications. These would spur corresponding advances in both theory and practice of cryptanalysis and eventually pave the way for Alan Turing’s foundational work in digital computing and Claude Shannon’s mathematical theory of information and communication. Enigma and Hagelin Machines In 1918 and 1919, several inventors developed, refined, and commercialized a series of electromechanical devices that provided extremely powerful encryption capabilities: One of these inventors, Swede Boris Hagelin, was to become cryptography’s first millionaire; another one, German Arthur Scherbius, commercialized a portable, battery-powered machine called the 28 Chapter 2 Enigma that was to enter history as the Nazi regime’s cryptosystem of choice to secure its military communications. The essential component of these devices was the wired rotor wheel, a mechanism that hardwired a single monoalphabetic substitution of 26 inputs to 26 outputs—for example, a to m, p to z, and so on. Two techniques vastly multiplied the power of this simple mechanism: first, wheels were placed alongside and connected to two or more additional rotors wheels, each implementing distinct substitutions, so that the outputs of one rotor fed the inputs of its neighbor; second, after the encryption of each single letter, a stepping mechanism moved the first rotor forward one position, while the second wheel moved forward one position after a complete revolution of the first wheel, and so on with the third wheel—in the manner of a car odometer. This resulted in each letter of the plaintext being encoded with a completely different set of substitutions.30 Using a different ciphering alphabet for each plaintext letter was a wellestablished cryptographic technique, dating back as far as Trithemius’s 1516 Six Books of Polygraphy. The longer the period—the number of different cipher alphabets used before the same sequence of plaintext/ciphertext alphabets is reused—the stronger the cipher and the more involved the ciphering and deciphering process. The automation provided by rotor machines enabled the period to grow to astronomical length (for the Enigma, 263 or approximately 17,000) without loss of accuracy. Because most military messages were of a much shorter length, Enigma-encrypted ciphertexts thus rarely exhibited the kind of frequency patterns so crucial to cryptanalysis. Brute force attacks were similarly difficult, as the total key space exceeded 3 × 10114 : 263 choices for the initial position of the rotors, 6 further choices for their ordering, and an additional 26! choices from a plugboard that implemented a fixed but easily modified monoalphabetic substitution. The procedures for key agreement were sophisticated. Each organizational unit (groups of planes, U-boats, etc.) was issued a codebook listing the original position of the rotors and plugboard for each day. After setting up the machine with the codebook parameters, the operator would observe the following procedure: for each new message, randomly select new rotor positions, encrypt these using the codebook settings for the day, and transmit the result as the first part of the message (the indicator); continue encryption of the message using the random rotor positions. In order to...

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