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CRYPTOGRAPHY
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Each round of DES is a complex round cipher, as shown in Figure 3014 Note that the structure of the encryption round ciphers is different from that of the decryption one
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Figure 3014 One round in DES ciphers
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Kj (48 bits)
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(48 bits)
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a Encryption round
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b Decryption round
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DES Function The heart of DES is the DES function The DES function applies a 48-bit key to the rightmost 32 bits R i to produce a 32-bit output This function is made up of four operations: an XOR, an expansion permutation, a group of S-boxes, and a straight permutation, as shown in Figure 3015 Figure 3015 DES function
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(32 bits)
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Function
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S-boxes
~---- -
K; (48 bits)
32 bits
SECTION 302
SYMMETRIC-KEY CRYPTOGRAPHY
Triple DES
Critics of DES contend that the key is too short To lengthen the key, Triple DES or 3DES has been proposed and implemented This uses three DES blocks, as shown in Figure 3016 Note that the encrypting block uses an encryption-decryption-encryption combination of DESs, while the decryption block uses a decryption-encryption-decryption combination Two different versions of 3DES are in use: 3DES with two keys and 3DES with three keys To make the key size 112 bits and at the same time protect DES from attacks such as the man-in-the-middle attack, 3DES with two keys was designed In this version, the first and the third keys are the same (KeYl = KeY3)' This has the advantage in that a text encrypted by a single DES block can be decrypted by the new 3DES We just set all keys equal to KeYl' Many algorithms use a 3DES cipher with three keys This increases the size of the key to 168 bits
Figure 3016 Triple DES
64-bit plaintext 64-bit plaintext
Encrypt DES
KeYl
Decrypt DES
KeYl
fil Q ,
Decrypt DES
KeY2
'I::
"S
gs ,
Encrypt DES
KeY2
Encrypt DES
KeY3
Decrypt DES
KeY3
64-bit ciphertext
64-bit ciphertext
a Encryption Triple DES
b Decryption Triple DES
Advanced Encryption Standard (AES)
The Advanced Encryption Standard (AES) was designed because DES's key was too small Although Triple DES ODES) increased the key size, the process was too slow The National Institute of Standards and Technology (NIST) chose the Rijndael algorithm, named after its two Belgian inventors, Vincent Rijmen and Joan Daemen, as the basis of AES AES is a very complex round cipher AES is designed with three key sizes: 128, 192, or 256 bits Table 301 shows the relationship between the data block, number of rounds, and key size
Table 301 AES configuration
Size ofData Block Number ofRounds
Key Size
128 bits 192 bits 256 bits
10 128 bits 12 14
CRYPTOGRAPHY
AES has three different configurations with respect to the number of rounds and key size
In this text, we discuss just the lO-round, 12S-bit key configuration The structure and operation of the other configurations are similar The difference lies in the key generation The general structure is shown in Figure 3017 There is an initial XOR operation followed by 10 round ciphers The last round is slightly different from the preceding rounds; it is missing one operation Although the 10 iteration blocks are almost identical, each uses a different key derived from the original key
Figure 3017 AES
12S-bit plaintext
+"" /
r---------
-----------,
Ronnd 1
I I I I
Round key generator
I I I I I I I I I
Round 2
12S-bit key
I I I I I I I I
Round 10 (slightly different)
'-----------,r
- - - ______ 1
l2S-bit ciphertext
Structure of Each Round Each round of AES, except for the last, is a cipher with four operations that are invertible The last round has only three operations Figure 3018 is a flowchart that shows the operations in each round Each of the four operations used in each round uses a complex cipher; this topic is beyond the scope of this book
Other Ciphers
During the last two decades, a few other symmetric block ciphers have been designed and used Most of these ciphers have similar characteristics to the two ciphers we discussed in this chapter (DES and AES) The difference is usually in the size of the block or key, the number of rounds, and the functions used The principles are the same In order not to burden the user with the details of these ciphers, we give a brief description of each
SECTION 302
SYMMETRIC-KEY CRYPTOGRAPHY
Figure 3018 Structure of each round
Round i SubByte Byte substitution ShiftRow Byte permutation MixCo1umn 128-bit data
1_ _ _ _ _ _ _ _ _ _ _ _ _
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