Cryptography Reference
In-Depth Information
a frequency of 0.0169, and T occurs only half as often.
The cryptanalyst would, of course, need a much larger
segment of ciphertext to solve a running-key Vigenère
cipher, but the basic principle is essentially the same as
before—i.e., the recurrence of like events yields identical
effects in the ciphertext. The second method of solving
running-key ciphers is commonly known as the probable-
word method. In this approach, words that are thought
most likely to occur in the text are subtracted from the
cipher. For example, suppose that an encrypted message
to President Jefferson Davis of the Confederate States of
America was intercepted. Based on a statistical analysis of
the letter frequencies in the ciphertext, and the South's
encryption habits, it appears to employ a running-key
Vigenère cipher. A reasonable choice for a probable word
in the plaintext might be “PRESIDENT.” For simplicity a
space will be encoded as a “0.” PRESIDENT would then
be encoded—not encrypted—as “16, 18, 5, 19, 9, 4, 5, 14,
20” using the rule A = 1, B = 2, and so forth. Now these
nine numbers are added modulo 27 (for the 26 letters plus
a space symbol) to each successive block of nine symbols
of ciphertext—shifting one letter each time to form a new
block. Almost all such additions will produce random-like
groups of nine symbols as a result, but some may pro-
duce a block that contains meaningful English fragments.
These fragments can then be extended with either of the
two techniques described above. If provided with enough
ciphertext, the cryptanalyst can ultimately decrypt the
cipher. What is important to bear in mind here is that
the redundancy of the English language is high enough
that the amount of information conveyed by every
ciphertext component is greater than the rate at which
equivocation (i.e., the uncertainty about the plaintext that
the cryptanalyst must resolve to cryptanalyze the cipher)
is introduced by the running key. In principle, when the
