Cryptography Reference
In-Depth Information
TABLE 1.6
Plaintext
DE F CON F OUR
Shift value
5
13
2
7
5
13
2
7
5
13
Ciphertext
I
R HJ
T A HVZ E
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 1.7
The message to send is
IRHJT
AHVZE.
Note that the way we group the letters has nothing to do with how many shift values are
being used; in fact, we don't want to give the analyst any clues by grouping the letters in
blocks the same size as the number of shift values!
It was difficult for classical cryptographers to remember shift values when using a large
number of them. They certainly didn't want to write them down, because the shift values
were the secret key. So instead they used letters to represent the shifts in the form of a key-
word, or a long keyphrase. Each letter in the alphabet was associated with its position, as
shown in Table 1.7.
From now on, when our alphabet consists of only capital English characters we will call
this the “ordinary” alphabet. These keywords and keyphrases were easily remembered. For
example, the keyphrase
BLAST OFF
represents the shift values
1
12
0
18
19
14
5
5.
These are the 8 shift values that would be used on a message, repeating the sequence every
eighth letter.
1.6
THE VIGENERE CIPHER AND CODE WHEELS
One convenient tool used for the previous type of cipher (called a simple shift Vigenere
cipher) was a code wheel. The outer ring of the wheel represented plaintext letters, and the
inner wheel represented ciphertext letters. Using a letter from a keyword or keyphrase, say
“S,” one would rotate the inner wheel and position the keyword letter under the letter “A.”
To encipher, one would go to the plaintext letter in the outer wheel, say “G,” and find its cor-
responding ciphertext letter, in this case “Y.” This is the position of the wheel illustrated in
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