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 |
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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.
2.4.3 Project VENONA
The so‐called VENONA project [130] provides an interesting example of a real‐world use of the one‐time pad. In the 1930s and 1940s, spies from the Soviet Union who entered the United States brought with them one‐time pad keys. When it was time to report back to their handlers in Moscow, these spies used the one‐time pads to encrypt their messages, which were then sent. These spies were extremely successful, and their messages dealt with the most sensitive U.S. government secrets of the time. In particular, the development of the first atomic bomb was a focus of much of the espionage. The Rosenbergs, Alger Hiss, and many other well‐known traitors—and many who were never identified—figure prominently in VENONA messages.
The Soviet spies were well trained and never reused the key, yet many of the intercepted ciphertext messages were eventually decrypted by American cryptanalysts. How can that be, given that the one‐time pad is provably secure? In fact, there was a flaw in the method used to generate the pads, so that, in effect, long stretches of the keys were repeated. As a result, many messages were in depth, which allowed for successful cryptanalysis of about 3000 VENONA messages.
Part of one interesting VENONA decrypt is given in Table 2.4. This message refers to David Greenglass and his wife Ruth. LIBERAL is Julius Rosenberg who (along with his wife Ethyl) was eventually executed for his role in nuclear espionage.8 The Soviet codename for the atomic bomb was, appropriately, ENORMOUS. For any World War II‐era history buff, the VENONA decrypts at [130] make for fascinating reading.
Table 2.4 VENONA decrypt of message of 21 September 1944
[C% Ruth] learned that her husband [v] was called up by the army
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but he was not sent to the front. He is a mechanical engineer
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and is now working at the ENORMOUS [ENORMOZ] [vi] plant in
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SANTA FE, New Mexico.
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