Cryptography Reference
In-Depth Information
QAV SW VRGUWFARSTW RA HCW
URDCWFHWLH.
W occurs 21 times in the cipher, H occurs 18, and so
on. Even the rankest amateur, using the frequency data in
the table, should have no difficulty in recovering the plain-
text and all but four symbols of the key in this case.
It is possible to conceal information about raw frequency
of occurrence by providing multiple cipher symbols for each
plaintext letter in proportion to the relative frequency of
occurrence of the letter—i.e., twice as many symbols for E
as for S, and so on. The collection of cipher symbols repre-
senting a given plaintext letter are called homophones. If the
homophones are chosen randomly and with uniform proba-
bility when used, the cipher symbols will all occur (on average)
equally often in the ciphertext. The great German math-
ematician Carl Friedrich Gauss (1777-1855) believed that he
had devised an unbreakable cipher by introducing homo-
phones. Unfortunately for Gauss and other cryptographers,
such is not the case, since there are many other persistent
patterns in the plaintext that may partially or wholly survive
encryption. Digraphs, for example, show a strong frequency
distribution: TH occurring most often, about 20 times as fre-
quently as HT, and so forth. With the use of tables of digraph
frequencies that partially survive even homophonic sub-
stitution, it is still an easy matter to cryptanalyze a random
substitution cipher, though the amount of ciphertext needed
grows to a few hundred instead of a few tens of letters.
Types of crypTanalysis
There are three generic types of cryptanalysis, charac-
terized by what the cryptanalyst knows: (1) ciphertext
only, (2) known ciphertext/plaintext pairs, and (3) chosen
 
Search WWH ::




Custom Search