『The Code Book』要約

In The Code Book, Simon Singh traces the 2,000-year duel between codemakers and codebreakers, framing cryptography as a recurring cycle in which every new cipher provokes an eventual break. The narrative opens with ancient techniques such as the Spartan scytale and Caesar's shift cipher, then shows how frequency analysis, pioneered by the ninth-century Arab scholar al-Kindi, fatally weakened monoalphabetic substitution. Singh dramatizes the Elizabethan plot against Mary, Queen of Scots, where intercepted and deciphered letters helped seal her fate, and explains how the Vigenère cipher earned the nickname "le chiffre indéchiffrable" until Charles Babbage and Friedrich Kasiski independently broke it. The book's centerpiece is the twentieth-century battle over Enigma. Singh describes how Polish mathematicians, notably Marian Rejewski, first reconstructed the machine's wiring, and how Alan Turing and the codebreakers at Bletchley Park later exploited cribs and the Bombe to read German traffic during World War II. He then moves to the postwar era: the one-time pad as the only provably unbreakable system, the rise of public-key cryptography through Diffie, Hellman, and RSA, and the resulting clash between governments seeking surveillance access and citizens demanding privacy. Later chapters examine the Data Encryption Standard, quantum computing's threat to RSA, and quantum cryptography's promise of eavesdropping detection. A cryptographic contest Singh ran with readers, in which prizes were offered for breaking successive cipher challenges, illustrates the book's central argument: security depends on computational difficulty, not secrecy of method, and that difficulty is always provisional.