Cryptography Reference
In-Depth Information
TABLE 1.2 A Sample
Decoding Codebook
Codeword
Word
...
...
Computer
Dawn
...
...
Explode
Enemy
...
...
Lion
At
...
...
Run
Attack
...
...
A decoding codebook would provide the reverse mappings, organized alphabetically by
codeword, as shown in Table 1.2.
In practice, both the encoding and decoding codebooks would probably be incorporated
into one topic.
So, using the previous codebook, the message
ATTACK ENEMY AT DAWN
would be encoded as
RUN EXPLODE LION COMPUTER.
Though there is some evidence that codes may be more secure than most ciphers, they
are not used widely today because of the high overhead involved in distributing, maintain-
ing, and protecting the codebooks.
1.2
MONOALPHABETIC SUBSTITUTION CIPHERS
The oldest cryptosystems were based on monoalphabetic substitution ciphers. These ciphers
mapped individual plaintext letters to individual ciphertext letters. They are considered inse-
cure because they are all vulnerable to a type of analysis called frequency analysis, which
breaks these ciphers.
The oldest cipher known is called the Caesar cipher. The enciphering and deciphering
transformations map an individual letter to another letter in the same alphabet. Specifically,
a plaintext letter is shifted down 3 letters, with letters near the end of the alphabet wrapping
around again to the front, as shown in Table 1.3.
Thus, using this cipher,
FIRE MISSILE
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