Cryptography and Network Security:
Principles and Practice
Eighth Edition
Chapter 7
Block Cipher Operation
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Copyright © 2020 Pearson Education, Inc. All Rights Reserved.
Figure 7.1 Multiple Encryption (1 of 2)
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Meet-in-the-Middle Attack
• The use of double D E S results in a mapping that is not
equivalent to a single D E S encryption
• The meet-in-the-middle attack algorithm will attack this
scheme and does not depend on any particular property of
D E S but will work against any block encryption cipher
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Figure 7.1 Multiple Encryption (2 of 2)
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Figure 7.2 Known-Plaintext Attack on
Triple D E S
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Triple D E S with Three Keys
• Many researchers now feel that three-key 3D E S is the preferred
alternative
• Three-key 3D E S has an effective key length of 168 bits and is
defined as:
– C = E( K3, D( K2, E( K1, P)))
• Backward compatibility with DES is provided by putting:
– K3 = K2 or K1 = K2
• A number of Internet-based applications have adopted three-key
3D E S including P G P and S/M I M E
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Modes of Operation
• A technique for enhancing the effect of a cryptographic
algorithm or adapting the algorithm for an application
• To apply a block cipher in a variety of applications, five modes of
operation have been defined by N I S T
– The five modes are intended to cover a wide variety of
applications of encryption for which a block cipher could be
used
– These modes are intended for use with any symmetric block
cipher, including triple D E S and A E S
Copyright © 2020 Pearson Education, Inc. All Rights Reserved.
Table 7.1 Block Cipher Modes of Operation
Mode Description Typical Application
Electronic
Codebook (E C B)
Each block of plaintext bits is encoded
independently using the same key.
• Secure transmission of single
values (e.g., an encryption
key)
Cipher Block
Chaining (C B C)
The input to the encryption algorithm is the
X O R of the next block of plaintext and the
preceding block of ciphertext.
• General-purpose block-
oriented transmission
• Authentication
Cipher Feedback
(C F B)
Input is processed s bits at a time.
Preceding ciphertext is used as input to the
encryption algorithm to produce
pseudorandom output, which is X O Red
with plaintext to produce next unit of
ciphertext.
• General-purpose stream-
oriented transmission
• Authentication
Output Feedback
(O F B)
Similar to C F B, except that the input to the
encryption algorithm is the preceding
encryption output, and full blocks are used.
• Stream-oriented transmission
over noisy channel (e.g.,
satellite communica
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