Mark Stamp - Information Security

Modern block ciphers use complex algorithms to generate ciphertext from plaintext (and vice versa), but at a higher level, a block cipher can be viewed as a codebook, where each distinct key determines a distinct codebook. That is, a modern block cipher consists of an enormous number of codebook ciphers, with the codebooks indexed by the key. The concept of an additive also lives on, in the form of an initialization vector, or IV, which is often used with block ciphers (and sometimes with stream ciphers as well). Modern block ciphers are discussed in detail in the next chapter.

The trouble with quotes on the Internet is that it's difficult to determine whether or not they're real.

—Abraham Lincoln

In this section, we take a brief look at three instances where classic ciphers played a role in historical events. First, we look at a weak cipher that was used during the controversial U.S. presidential election of 1876. Then we consider the Zimmermann Telegram, which played a pivotal role in World War I. The Zimmermann Telegram was encrypted with a classic codebook cipher. Finally, we discuss the VENONA messages, where Soviet spies in the United States used one‐time pad encryption. This system was used over a long period of time, but most notably for atomic espionage in the 1940s.

The U.S. presidential election of 1876 was a virtual dead heat. At the time, the Civil War was still fresh in people's minds, Radical Reconstruction was ongoing in the former Confederacy, and the nation was still bitterly divided.

The contestants in the election were Republican Rutherford B. Hayes and Democrat Samuel J. Tilden. Tilden had obtained a slight plurality of the popular vote, but it is the Electoral College that determines the winner of the presidency. In the Electoral College, each state selects a delegation and for almost every state, the entire delegation is supposed to vote for the candidate who received the largest number of votes in that particular state. 5

In 1876, the Electoral College delegations of four states 6 were in dispute, and these held the balance. A commission of 15 members was appointed to determine which state delegations were legitimate, and thus determine the presidency. The commission decided that all four states should go to Hayes and he became president of the United States. Tilden's supporters immediately charged that Hayes’ people had bribed officials to turn the vote in his favor, but no evidence was forthcoming.

Some months after the election, reporters discovered a large number of encrypted messages that had been sent from Tilden's supporters to officials in the disputed states. One of the ciphers used was a partial codebook together with a transposition on the words. The codebook was only applied to important words and the transposition was a fixed permutation for all messages of a given 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 telegramfor Warsawto 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.

As discussed above, a classic codebook cipher is a book containing (plaintext) words and their corresponding (ciphertext) codewords. Table 2.3contains 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 2.3to 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