Cryptography Reference
In-Depth Information
communications. Field cipher systems such as the U.S.
Signal Corps's cipher disk mentioned previously, lacked
sophistication (and security), however. Nevertheless, by
the end of the war some complicated cipher systems were
used for high-level communications, the most famous of
which was the German ADFGVX fractionation cipher,
described in the section "Product Ciphers."
The communications needs of telegraphy and radio
and the maturing of mechanical and electromechanical
technology came together in the 1920s to bring about a
major advance in cryptodevices: the development of rotor
cipher machines. Although the concept of a rotor had
been anticipated in the older mechanical cipher disks,
American Edward H. Hebern recognized in about 1917
(and made the first patent claim) that by hardwiring a
monoalphabetic substitution in the connections from
contacts on one side of an electrical disk (rotor) to con-
tacts on the other side and then cascading a collection of
such rotors, polyalphabetic substitutions of almost arbi-
trary complexity could be realized. A set of these rotors
is usually arranged in a stack called a basket; the rotation
of each of the rotors in the stack causes the next one to
rotate, much as the wheels in an odometer advance
1
/
10
of
a revolution for every full revolution of its driving wheel.
In operation, the rotors in the stack provide an electrical
path from contact to contact through all of the rotors. In a
straight-through rotor system, closing the key contact on
a typewriter-like keyboard sends a current to one of the
contacts on the end rotor. The current then passes through
the maze of interconnections defined by the remaining
rotors in the stack and their relative rotational positions
to a point on the output end plate, where it is connected
to either a printer or an indicator, thereby outputting the
ciphertext letter equivalent to the input plaintext letter.
