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In-Depth Information
b 1
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AddRoundKey
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C(x)
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Figure 4.12
AddRoundKey step.
The subkey is added by combining each byte of the state with the
corresponding byte of the subkey using a bitwise XOR [36,37].
The AddRoundKey step is shown in Figure 4.12. We will apply the
AES with a fixed block size of 128 bits and a key size of 128 bits.
4.6 Modes of Operation
Block ciphers can be run in different modes of operation, allowing users
to choose appropriate modes to meet the requirements of their appli-
cations. Using a certain mode in the encryption process restricts the
decryption process to using the same mode. In this section, we discuss
different possible ways in which block codes can be utilized to imple-
ment a cryptosystem. The possible block cipher modes of operation that
we treat are identified by the acronyms ECB, CBC, CFB, and OFB.
In each case, we assume that we have a block cipher of block length n,
with enciphering map, E K , and deciphering map, D K , for each key K.
4.6.1 he ECB Mode
ECB is the simplest mode of operation for encryption algorithms
where the data sequence is divided into blocks of equal sizes and
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