INVITATION TO CYBERSECURITY 148 In ancient times the Hebrews employed a similar scheme known as the Atbash cipher. The Atbash cipher maps letters to the opposite end of the alphabet (see Table 7.2). Twice in the Hebrew Bible, the word Babylon is encrypted as the word Sheshak (see Jeremiah 25:26 and 51:41). Babylon in Hebrew is BBL (bet bet lamed), and the letters at the opposite end of the Hebrew alphabet are SHSHK (shin shin kaph), or Sheshak. Reflecting its Hebrew origin, the Atbash cipher got its name from the first two pairs of letter mappings in Hebrew—A (aleph) to T (taw) and B (bet) to SH (shin). Using the English alphabet, it would be called the “Azby cipher.” In the Atbash cipher, attack at dawn maps to ZGGZXPZGWZDM. To decrypt the message, the correspondent finds the letter in the ciphertext alphabet and maps it back to the plaintext letter. Going in the reverse direction undoes the encryption and returns the original plaintext message. Table 7.2 Atbash cipher plaintext-to-ciphertext alphabet mapping. Ciphers are not limited to formulaic mappings like alphabetic shifts or patterns. Any mapping from a plaintext alphabet to a ciphertext alphabet is valid, and there are a mind-boggling number of such mappings. The total number of mappings is the number of permutations of the English alphabet. There are 26! permutations, or over 4 × 1026 unique mappings. Table 7.3 shows one such random permutation. In this cipher, attack at dawn would turn into the ciphertext WOOWJUWOMWDX. Because it is not based on a simple pattern, the key for this cipher is more difficult to memorize. It is the twenty-six ciphertext alphabet letters in order: WRJMPVBCTKUFNXAZEQSOHYDLIG. Table 7.3 A randomly generated plaintext-to-ciphertext alphabet mapping. All of these ciphers are examples of monoalphabetic substitution ciphers. A monoalphabetic substitution cipher is a cipher that uses one plaintext-to-ciphertext alphabet mapping (monos means single in Greek). The ciphertext characters are not limited to English letters like in the above examples. Numerical digits or any other symbol, including groups of symbols, can be used. The pigpen cipher is an ancient cipher that uses geo-
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