Unformatted text preview: s permitted for subject £ on object O in the current protection
state of the system. If yes, the access is permitted; otherwise the access is denied.
Cryptography
Cryptography is a means of protecting private information against unauthorized
access in those situations where it is difficult to ensure access control. The basic
idea behind this security technique is that if it is not possible to ensure access
control, it is better to prevent comprehension of information.
Basic Concepts and Terminologies
Two primitive operations employed by cryptography are encryption and
decryption. Encryption (also called enciphering) is the process of transforming an
intelligible information (called plaintext or cleartext) into an unintelligible form
(called ciphertext). Decryption (also called deciphering) is the process of
transforming thi information back from ciphertext to plaintext.
Encryption is basically a mathematical function (encryption algorithm) having the
following form:
C = E(P, Ke)
where P is the plaintext to be encrypted, K e is an encryption key, and C is the
resulting ciphertext. Decryption o C is performed by a matching function
(decryption algorithm) that has the following form:
P=D(C,Kli)
where Kd is the decryption key. Note that the decryption function D is the inverse
of the encryption function L Therefore we have
D (E (P, Ke),
=P
To prevent the plaintext from being easily revealed, it must be possible to
transform a given plaintext into a large variety of possible ciphertexts selected by
a specific parameter. The keys K e and Kd serve as this parameter. Tha is, the
function parts remain the same but the keys are changed as often as necessary.
The above described general structure of a cryptosystem is illustrated in Figure
14.25.
Symmetric and Asymmetric Cryptosystems There are two broad classes of cryptosystems  symmetric and asymmetric. In a
symmetric cryptosystem, either both the encryption key (Ke) and decryption key
(Kd) are the same or one is easily derivable from...
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This document was uploaded on 04/07/2014.
 Spring '14

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