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As discussed above, a classic codebook cipher is a book containing (plaintext) words and their corresponding (ciphertext) codewords. Table 2.3 contains an excerpt from a famous World War I codebook cipher. This particular codebook was used to encrypt the infamous Zimmermann Telegram, which we discuss in some detail in this section.

Table 2.3 Excerpt from a German codebook

Plaintext	Ciphertext
Februar	13605
fest	13732
finanzielle	13850
folgender	13918
Frieden	17142
Friedenschluss	17149

For example, to use the codebook in Table 2.3 to encrypt the German word Februar , the entire word would be replaced with the five‐digit codeword 13605. This codebook was used for encryption, while the corresponding inverse codebook, arranged with the five‐digit codewords in numerical order, would be used for decryption. A codebook is a form of a substitution cipher, but the substitutions are far from simple, since we substitute for entire words, or in some cases, entire phrases.

Figure 2.4 Reproduction of the Zimmermann Telegram

At the height of World War I in 1917, the German Foreign Minister, Arthur Zimmermann, sent an encrypted telegram to the German ambassador in Mexico City. The ciphertext message, a reproduction of which appears in Figure 2.4 [95], was intercepted by the British. At the time, the British, French, and Russians were at war with Germany, while the United States was striving to remain neutral.

The Russians had recovered a damaged version of the German codebook, and the partial codebook had been passed on to the British. Through painstaking analysis, the British were able to fill in the gaps in the codebook, so that by the time they obtained the Zimmermann Telegram, they could decrypt it. The telegram stated that the German government was planning to begin unrestricted submarine warfare and had concluded that this would likely lead to war with the United States. As a result, Zimmermann told his ambassador to Mexico that Germany should try to recruit Mexico as an ally to fight against the United States. Among other incentives, Mexico was to “reconquer the lost territory in Texas, New Mexico and Arizona.″ When the Zimmermann Telegram was released in the U.S., public opinion turned sharply against Germany and, after the sinking of the Lusitania, the United States declared war.

The British were initially hesitant to release the Zimmermann Telegram since they feared that the Germans would realize that their cipher was broken and, presumably, stop using it. After decrypting the Zimmermann Telegram, the British took a closer look at other intercepted messages that had been sent at about the same time. To their amazement, they found that a variant of the incendiary telegram had been sent unencrypted.⁷ The British subsequently released a version of the Zimmermann Telegram that closely matched this unencrypted version. As the British hoped, the Germans concluded that their codebook had not been compromised and continued to use it for sensitive messages throughout the war.