Cryptography Reference
In-Depth Information
been to use the number found in selected locations of one
maximal-length feedback shift register, in which the key
is used as the initial register fill, to control the number of
steps from the plaintext
n
-tuple to the ciphertext
n
-tuple
in the cycle of another linear feedback shift register. In
schemes of this sort the key register is generally stepped
forward to hide the key itself before any encryption of
plaintext is carried out and then advanced sufficiently
many steps between encryptions to ensure diffusion of
the keying variables. To encrypt an
n
-bit block of plain-
text, the text is loaded into the main shift register and the
machine stepped through a specified number of steps,
normally a multiple of the number of bits in the key, suf-
ficient to diffuse the information in the plaintext and in
the key over all positions in the resulting ciphertext. To
decrypt the resulting ciphertext it is necessary to have
an inverse combiner function or for the original encryp-
tion function to be involutory—i.e., the encryption and
decryption functions are identical, so that encrypting
the ciphertext restores the plaintext. It is not difficult to
design the feedback logic to make an involutory machine.
Pictorially, the machine has simply retraced its steps in the
cycle(s). Linearity in the logic, though, is a powerful aid to
the cryptanalyst, especially if a matched plaintext/cipher-
text attack is possible.
With a slight modification, this approach constitutes
the basis of several commercially available cryptographic
devices that function in a manner quite similar to the
pin-and-lug cipher machines previously described. One
such cryptomachine has six maximal-length linear feed-
back shift registers in which the stepping is controlled by
another shift register; the contents of the latter are used
to address a (nonlinear) lookup table defined by keys sup-
plied by the user.
