A History of Cryptography - Introduction to Cryptography with Java Applets - page 15

Each row is a permutation of the ordinary alphabet; the leftmost letter of each row is
referenced by the keyword. The first row in the table represents the plaintext letter. To enci-
pher the plaintext letter T using the key letter D, for example, we find the letter in the cell
referenced by row D, column T. This yields the ciphertext letter J.

E XAMPLE .

Encipher the message

HARKONNEN RULZ

using the keyphrase

SPICE.

By locating each ciphertext letter in the manner described previously, we get

OZTJY JTZGD KPX.

Decryption should be simple to figure out. What makes the full Vigenere cipher slightly
superior to the simple shift Vigenere is that the full relative frequency distribution of the lan-
guage may be necessary to break the former, whereas only the most common letter is needed
to break the latter.

1.10

THE AUTO-KEY VIGENERE CIPHER
Vigenere ciphers are our earliest examples of stream ciphers. Stream ciphers are those that
encipher letters based on their position in the plaintext. Ideally, the key being used should
never repeat, as this aids the cryptanalyst. Some stream ciphers make the plaintext and/or
the ciphertext part of the encryption process; such is the case with the auto-key Vigenere.
This type of cipher begins with a priming key of length n , say k 0 , k 1 , . . . , k n 1 . Encryp-
tion for the first n characters is done the same way (using the key) as for the simple shift
Vigenere, but after that, to encipher the i th character of the plaintext, we add to it (with
wrap-around) the ( i n )th letter of the plaintext. This is easily seen with an example.

E XAMPLE . For this example, it is convenient to see the letter-number associations of the
ordinary alphabet. (See Table 1.14.)
Suppose we wish to encipher the message

LIGHT SPEED CHEWIE NOW

ABCDE F GHI J KL MNOP QRS T UVWXYZ
0123456789 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

TABLE 1.14 Letter-Number Associations of the Ordinary Alphabet