Cryptography Reference
In-Depth Information
teletypewriter to decrypt the cipher. Vernam initially
believed that a short random key could safely be reused
many times, thus justifying the effort to deliver such a large
key, but reuse of the key turned out to be vulnerable to
attack by methods of the type devised by Kasiski. Vernam
offered an alternative solution: a key generated by combin-
ing two shorter key tapes of
m
and
n
binary digits, or bits,
where
m
and
n
share no common factor other than 1 (they
are relatively prime). A bit stream so computed does not
repeat until
mn
bits of key have been produced.
This version of the Vernam cipher system was adopted
and employed by the U.S. Army until Major Joseph O.
Mauborgne of the Army Signal Corps demonstrated dur-
ing World War I that a cipher constructed from a key
produced by linearly combining two or more short tapes
could be decrypted by methods of the sort employed to
cryptanalyze running-key ciphers. Mauborgne's work
led to the realization that neither the repeating single-
key nor the two-tape Vernam-Vigenère cipher system
was cryptosecure. Of far greater consequence to modern
cryptology—in fact, an idea that remains its corner-
stone—was the conclusion drawn by Mauborgne and
William F. Friedman that the only type of cryptosystem
that is unconditionally secure uses a random onetime key.
The proof of this, however, was provided almost 30 years
later by another AT&T researcher, Claude Shannon, the
father of modern information theory.
In a streaming cipher the key is incoherent—i.e., the
uncertainty that the cryptanalyst has about each successive
key symbol must be no less than the average information
content of a message symbol. In a long “message” such as
this topic, the raw frequency of occurrence pattern is lost
when the text is encrypted with a random onetime key, as
indicated by the dotted curve in the following figure.
