Cryptography Reference
In-Depth Information
To avoid the problems associated with linearity, cryp-
tographers have devised a number of nonlinear feedback
logics that possess such desirable properties as diffusion
of information (to spread the effects of small changes in
the text) and large-cycle structure (to prevent exhaustive
search) but which are computationally infeasible to invert
working backward from the output sequence to the initial
state(s), even with very many pairs of matched plaintext/
ciphertext. The nonlinear feedback logic, used to deter-
mine the next bit in the sequence, can be employed in
much the same way as linear feedback logic. The com-
plicating effect of the key on the ciphertext in nonlinear
logic, however, greatly contributes to the difficulty faced
by the cryptanalyst. Electronic cipher machines of this
general type were widely used, both commercially and by
national cryptologic services.
The significance of the above historical remarks is
that they lead in a natural way to the most widely adopted
and used cipher in the history of cryptography—the Data
Encryption Standard (DES).
des and aes
In 1973 the U.S. National Bureau of Standards (NBS; now
the National Institute of Standards and Technology) issued
a public request for proposals for a cryptoalgorithm to be
considered for a new cryptographic standard. No viable
submissions were received. A second request was issued in
1974, and International Business Machines (IBM) submit-
ted the patented Lucifer algorithm that had been devised
by one of the company's researchers, Horst Feistel, a
few years earlier. The Lucifer algorithm was evaluated in
secret consultations between the NBS and the National
Security Agency (NSA). After some modifications to the
