7. The Bedrock of Cybersecurity: Cryptography 147 with the letter three places forward in the alphabet. In the English language, the Caesar cipher, as it became known, would transform the plaintext message attack at dawn into the ciphertext DWWDFNDWDGZQ.1 A cipher is an encryption scheme that operates on the level of letters, creating mappings from a plaintext alphabet to one or more ciphertext alphabets. Table 7.1 shows the full alphabet mapping for the Caesar cipher. Obviously, Caesar was concerned that his correspondence might be intercepted, and if that were to occur, he did not want the enemy to be able to read his military orders. The axiom “knowledge is power” is never more true than in wartime, so there is a long tradition of militaries employing cryptography in an effort to conceal as much information as possible from the enemy. One of the advantages of the Caesar cipher is that it is easy and inexpensive both in time and resources to use. The scheme is easy to remember and messages can be both encrypted and decrypted quickly. On the other hand, its effectiveness is dubious. One wonders if Caesar’s adversaries would have been able to reverse engineer his scheme to recover the plaintext. This is an example of cryptanalysis—the art and science of decrypting ciphertext when not in possession of the secret key. It is also called code cracking—an attack against ciphertext to reveal the encrypted message. Table 7.1 Caesar cipher plaintext-to-ciphertext alphabet mapping. The Caesar cipher is an example of an alphabetic shift cipher. The Caesar cipher uses a shift of three, but in the English alphabet, shifts from [0-25] all produce valid mappings. Because the letters wrap around the end (see how x is mapped to A in Table 7.1), shifts beyond twenty-five are equivalent to ones from zero to twenty-five (e.g., twenty-six is the same as zero, twenty-seven is the same as one, etc.). In these ciphers, the shift is the key—the secret knowledge that the corresponding parties share. The convention is that encryption shifts forward in the alphabet and decrypting shifts backwards to recover the original text. The cipher known as ROT-13 uses a shift of thirteen. It has the advantage of not having to remember whether encryption shifts forwards or backwards because the result is the same. ROT-13 is sometimes used in Internet forums to conceal spoiler alerts. Readers will not accidentally be exposed to spoilers but those that want to know the information can easily decode the ciphertext. 1 This text follows the convention of using lowercase letters for plaintext and uppercase letters for ciphertext. Also, spaces are removed from ciphertext to conceal word patterns—this makes cryptanalysis more difficult for the adversary, but poses only a minor inconvenience for the decrypting correspondent. These are conventions only and not constraints for real-world cryptography.
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