Information Security. Mark Stamp

length. The allowed message lengths were 10, 15, 20, 25, and 30 words, with all messages padded to one of these lengths. A snippet of the codebook appears in Table 2.2.

Table 2.2 Election of 1876 codebook

Plaintext	Ciphertext
Greenbacks	Copenhagen
Hayes	Greece
votes	Rochester
Tilden	Russia
telegram	Warsaw

      The permutation used for a message of 10 words was

      One actual ciphertext message was

      which was decrypted by undoing the permutation and substituting telegram for Warsaw to obtain

The cryptanalysis of this weak cipher was relatively easy to accomplish [45]. Since a permutation of a given length was used repeatedly, many messages were in depth—with respect to the permutation as well as the codebook. A cryptanalyst could therefore compare all messages of the same length, making it relatively easy to discover the fixed permutation, even without knowledge of the partial codebook. Of course, the analyst first had to be clever enough to consider the possibility that all messages of a given length were using the same permutation, but, with this insight, the permutations were easily recovered. The codebook was then deduced from context and also with the aid of some unencrypted messages that provided additional context for the ciphertext messages.

      And what did these decrypted messages reveal? The reporters who broke the messages were amused to discover that Tilden's supporters had tried to bribe officials in the disputed states. The irony here—or not, depending on your perspective—is that Tilden's people were guilty of precisely the same crime of which they had accused Hayes.

      By any measure, this cipher was poorly designed and weak. One lesson is that the overuse of a key can be an exploitable flaw. In this case, each time a permutation was reused, it gave the cryptanalyst more information that could be collated to recover the permutation. In modern cipher systems, we try to limit the use of a key so that we do not allow a cryptanalyst to accumulate too much information, and to limit the damage if a particular key is exposed.

      2.4.2 Zimmermann Telegram

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