A History of Cryptography - Introduction to Cryptography with Java Applets - page 10

A

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Y B

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N W

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T L G

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J A

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FIGURE 1.2 A Sample Code Wheel

Figure 1.2. To decipher, one would position the keyword letter under “A,” but would go from
the inner ciphertext wheel to the outer plaintext wheel.

1.7 BREAKING SIMPLE VIGENERE CIPHERS

If enough ciphertext is received, and if the analyst makes a good guess for the key length,
say , frequency analysis also breaks these types of polyalphabetic substitution ciphers. An

analyst can separate the ciphertext into

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categories, and then do a separate frequency analy-
sis on each category. In this way, one could derive all of the

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shift values. The problem with
using a keyword in this way is that it would eventually repeat, and this fact could be
exploited.

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