Cryptography Reference
In-Depth Information
equivocation is reduced to zero, the cipher can be solved.
The number of symbols needed to reach this point is
called the unicity distance—and is only about 25 symbols,
on average, for simple substitution ciphers.
V
ernam
-V
igenère
C
iPhers
In 1918 Gilbert S. Vernam, an engineer for the American
Telephone & Telegraph Company (AT&T), introduced
the most important key variant to the Vigenère system.
At that time all messages transmitted over AT&T's tele-
printer system were encoded in the Baudot Code, a binary
code in which a combination of marks and spaces repre-
sents a letter, number, or other symbol. Vernam suggested
a means of introducing equivocation at the same rate at
which it was reduced by redundancy among symbols of the
message, thereby safeguarding communications against
cryptanalytic attack. He saw that periodicity (as well as
frequency information and intersymbol correlation), on
which earlier methods of decryption of different Vigenère
systems had relied, could be eliminated if a random series
of marks and spaces (a running key) were mingled with the
message during encryption to produce what is known as a
stream or streaming cipher.
There was one serious weakness in Vernam's system,
however. It required one key symbol for each message
symbol, which meant that communicants would have to
exchange an impractically large key in advance—i.e., they
had to securely exchange a key as large as the message
they would eventually send. The key itself consisted of a
punched paper tape that could be read automatically while
symbols were typed at the teletypewriter keyboard and
encrypted for transmission. This operation was performed
in reverse using a copy of the paper tape at the receiving
