A History of Cryptography - Introduction to Cryptography with Java Applets - page 16

TABLE 1.15

Plaintext LI GHTSPEEDCHEWI ENO W

Key ARG HLI GHT S P E ED CHE WI

Ciphertext L ZMOEAVL XVRL I Z KL RK E

using the keyword

ARGH

and an auto-key Vigenere. First, we write the plaintext, and underneath it we write the prim-
ing key, followed by as much of the plaintext as necessary to fill out the line. Underneath
this, we do a simple shift to generate the ciphertext shown in Table 1.15.
How does one recover the plaintext when the plaintext is part of the key? It should be
easy to see that only knowledge of the priming key is necessary. Once we use the key to
decrypt the first n characters of the ciphertext, we derive the first n characters of the plain-

text, and hence can use it to decrypt more ciphertext.
One must be particularly careful with ciphers like these that no errors are made in the
encryption phase, for a single miscalculated character affects an entire series of characters
following it. Care must also be taken to ensure that no errors occur during transmission.

1.11

THE RUNNING KEY VIGENERE CIPHER
Another alternative to the auto-key Vigenere is called a running key Vigenere. It makes use
of a very long key in the form of meaningful text, as in a topic, of which both the sender
and intended receiver have a copy.

E XAMPLE . Suppose we are working with the ordinary alphabet. Again, we show the ordinary
letter/number associations, in Table 1.16, for quick reference.
To encrypt the message

TORA TORA TORA

we use a passage from a topic, such as a particular edition of the Bible, as the key:

AND GOD SAID LET THERE BE LIGHT.

The encryption proceeds as a simple shift, as shown in Table 1.17.
To decrypt, one simply needs to know which passage from which topic to use, and the
plaintext is easily regained.

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 1.16 Letter-Number Associations of the Ordinary Alphabet