Cryptography Reference
In-Depth Information
message is a sequence of such marks and spaces, which can be represented
by the presence or absence of an electrical current. Vernam's idea was to
print out a key consisting of a random sequence of such marks and spaces
and to add it to the teleprinter message. At the receiving end, the key
would be subtracted from the ciphertext. The arithmetic was simple:
mark
+ mark = space
,
mark + space = mark
,
space + mark = mark
, and
space + space
= space
. In binary notation, where mark = 1 and space = 0, this gives the
exclusive-or (XOR) operator of binary arithmetic:
XOR
mark/1
space/0
mark/1
space/0
mark/1
space/0
mark/1
space/0
The beauty was that the process necessitated little modification to an
ordinary teleprinter—only the addition and subtraction logic, and some
tape-feeding mechanism to be coordinated at either end: “Messages were
enciphered, transmitted, received, and deciphered in a single operation—
exactly as fast as a message in plain English. . . . The advantage was the
assimilation of encipherment into the overall communication process.
Vernam created what came to be called 'on-line encipherment.' He freed
a fundamental process in cryptography from the shackles of time and error.
He eliminated a human being—the cipher clerk—from the chain of
communication.”
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It suffered however from two important limitations: in order to ensure
secrecy, the key tape had to be made extremely long. In fact, even though
no formal proof existed, it was believed the tape had to be of a length
comparable to the message itself. Furthermore, it appeared the key had to
be absolutely random, and any attempt to circumvent these requirements
by reusing key material resulted in rapid break of the system. These limita-
tions severely limited the practical use of the cipher, and the device failed
commercially: “Cable companies passed it over in favor of the old-fash-
ioned commercial codes, which substantially shortened messages, thereby
cutting cable tolls, and which gave a modicum of secrecy as well.”
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The practical impact of Vernam's cipher was thus limited to certain
diplomatic channels, in particular those of Germany that instituted the
system between 1921 and 1923. The Germans based it on simple pads of
