INVITATION TO CYBERSECURITY 154 there are practical issues with implementing the two special properties of the one-time pad scheme. The one-time pad’s name comes from a notepad of paper filled with random characters. This is the keystream used to encrypt and decrypt messages. A keystream is a sequence of characters used to encrypt and decrypt messages on a per character basis. A source of randomness must be used to produce the keystream so that no pattern of any kind can emerge—producing the high number of random characters needed is not trivial. Two identical copies of the notepad must be made—one for the encrypting party and one for the decrypting party—and both must be kept secure at all times. The keystream is consumed quickly because every plaintext letter requires a keystream character, and once a keystream has been used, it must be discarded—this is why it is called a one-time pad. So bottomline, one-time pads are difficult to implement. As a real-world example, the one-time pad encryption scheme was used by the Soviet Union for nuclear espionage purposes in the 1940s. Unfortunately for them, when they ran out of keystream, the Soviet spies reused portions of their one-time pads. This is called using a one-time pad in-depth, and it forfeits the provably secure property. It turns the one-time pad into a type of Vigenère cipher, albeit one with a really long key. The Venona project was an United States signals counterintelligence operation that collected and analyzed the Soviet’s ciphertext correspondence. Due to the misuse of the one-time pad scheme, the team was able to decode many of the Soviet’s messages, exposing important details of the spying operation. For even such a vital, sophisticated, and well-financed operation, fully implementing the one-time pad scheme proved too onerous. 7.1.2 Word Substitution: Codebooks “The United States…[made] use of a resource that virtually no other combatant had: pools of tongues so recondite that almost no one else in the world understood them. These were the American Indian languages, which are isolated both geographically and linguistically.” - on the United States use of code talkers during both World Wars in The Codebreakers by David Kahn All of the above cryptographic schemes are ciphers that employ substitution at the level of letters. Another cryptographic scheme called codebooks also uses substitution, but it does so at the level of words. In these schemes, two codebooks are created containing plaintext-to-codeword mappings. One book is ordered by the plaintext words and is used for encrypting, and the other is ordered by codewords and is used for decrypting. Unlike with ciphers, for codebook schemes the vocabulary is limited to the words for which mappings exist, so forethought is needed to know which plaintext words to include. Table 7.5 shows a partial codebook that uses random three digit numbers as codewords. In this scheme, attack at dawn becomes 915301717.
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