Communication in the Presence of Adversaries - Burdens of Proof - page 20

The resulting ciphertext would read “ftjfdgaqbn.” Note that each of the
two p s, h s, and o s of the plaintext encrypt to distinct symbols in the
ciphertext.
Polyalphabetic substitution improves over a simple substitution cipher
because the key space is much larger: there are 26 m keywords of length m
(26 choices for the first letter times 26 choices for the second, and so on),
so even for small values of m, an exhaustive key search would be quite
long, at least by hand (26 5 is greater than 11 million combinations, 26 6
greater than 300 million). In addition, because each alphabetic character
of the plaintext can be mapped to one of m possible alphabetic characters
(if the key contains all distinct letters), cryptanalysis based on letter fre-
quency is more difficult. Still, such a cipher can also be broken through
more complex forms of statistical analysis. 7
Codebooks
Both of the ciphers mentioned in the previous section ignore the semantic
units of the messages and view the plaintext as strings of letters, encrypting
them either individually or in blocks. Codebooks encrypt whole words at
a time by replacing each plaintext word with a numerical or other symbolic
equivalent as listed in the codebook, such as [England, 34]. Deciphering
proceeds by a corresponding inverted codebook, such as [34, England].
Although such a system might seem rather simplistic, it actually domi-
nated the cryptographic world longer than any other enciphering method:
for about 500 years, from the fourteenth century to the nineteenth century,
much of the world's diplomatic correspondence was secured through the
use of nomenclators , a mixed cipher-code system that relied on codebooks
to replace certain specific words or sentences together with a simple mono-
alphabetic cipher to replace the rest of the text. 8
A major advantage of nomenclators over ciphers was easier (and thus
more accurate) encryption and decryption: semantic units such as words
and sentences are more readily processed by humans than meaningless
blocks of letters of fixed length. Nomenclators also offered substantial secu-
rity, including a large key space, and good resistance to frequency analysis. 9
However, because specific words are always encoded with the same code
words, codebooks are susceptible to cryptanalysis by known-plaintext attack ,
where the cryptanalyst has access to the encrypted version of a message
of her own creation. The attack immediately reveals code words for all