15-12-2012, 01:58 PM
Data Encryption Standard
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INTRODUCTION
In 1972, the NBS Institute for Computer Sciences and
Technology (ICST) initiated a project in computer
security, a subject then in its infancy. One of the first
goals of the project was to develop a cryptographic
algorithm standard that could be used to protect sensitive
and valuable data during transmission and in
storage. Prior to this NBS initiative, encryption had been
largely the concern of military and intelligence organizations.
The encryption algorithms, i.e., the formulas or
rules used to encipher information, that were being used
by national military organizations were closely held
secrets. There was little commercial or academic
expertise in encryption. One of the criteria for an
acceptable encryption algorithm standard was that the
security provided by the algorithm must depend only on
the secrecy of the key, since all the technical specifications
of the algorithm itself would be made public. NBS
was the first to embark on developing a standard encryption
algorithm that could satisfy a broad range of
commercial and unclassified government requirements
in information security.
DES uses a sequence of operations, including several
substitution and permutation primitives, to encrypt a
data block. These primitives are subsequently used to
reverse the encryption operation. Horst Feistel defined a
variety of substitution and permutation primitives which
are iteratively applied to data blocks for a specified
number of times [3,4]. Each set of primitive operations
is called a “round,” and the DES algorithm uses 16
rounds to ensure that the data are adequately scrambled
to meet the security goals. The secret key is used to
control the operation of the DES algorithm. Each key
contains 56 bits of information, selected by each user to
make the results of the encryption operations secret to
that user. Any of approximately 1016 keys could be used
by the DES, and an attacker trying to “crack” a DES
encrypted message by “key exhaustion” (trying every
key) must, on average, try half of the total possible keys
before succeeding.