Cryptography Reference
In-Depth Information
internal functions and a shortening of the key size from
112 bits to 56 bits, the full details of the algorithm that
was to become the Data Encryption Standard (DES) were
published in the
Federal Register
in 1975. Following almost
two years of public evaluation and comment, the standard
itself was adopted at the end of 1976 and published at the
beginning of 1977. As a consequence of certification of the
standard by the NBS and its commitment to evaluate and
certify implementations, it was mandated that the DES be
used in unclassified U.S. government applications for the
protection of binary-coded data during transmission and
storage in computer systems and networks and on a case-
by-case basis for the protection of classified information.
The use of the DES algorithm was made manda-
tory for all financial transactions of the U.S. government
involving electronic fund transfer, including those con-
ducted by member banks of the Federal Reserve System.
Subsequent adoption of the DES by standards organiza-
tions worldwide caused the DES to become a de facto
international standard for business and commercial data
security as well.
The DES is a product block cipher in which 16 iter-
ations, or rounds, of substitution and transposition
(permutation) process are cascaded. The block size is 64
bits. The key, which controls the transformation, also con-
sists of 64 bits; however, only 56 of these can be chosen by
the user and are actually key bits. The remaining eight are
parity check bits and hence totally redundant. The figure
is a functional schematic of the sequence of events that
occurs in one round of the DES encryption (or decryp-
tion) transformation. At each intermediate stage of the
transformation process, the cipher output from the pre-
ceding stage is partitioned into the 32 left-most bits,
L
i
,
and the 32 right-most bits,
R
i
.
R
i
is transposed to become
the left-hand part of the next higher intermediate cipher,
