**1.4 Add round key**

The 16 bytes of the resultant matrix generated from mix column stage are then considered as 128 bits. In add round key stage, 128 bits of state are bitwise EX-ORed with 128 bits of round key. If this result belongs to the last round, then the output is cipher text else the resulting 128 bits is considered as 16 bytes, and another round is started with new byte substitution process. This is a column-wise operation between four bytes of state column and one word of round key. In the last round, there is no mix column step. **Figure 6** shows add round key stage in AES algorithm.

Decryption of cipher text, generated from AES encryption, contains all the stages in encryption but in reverse order. AES decryption starts with inverse initial round. The remaining nine rounds in decryption consist of processes like add round key, inverse shift rows, inverse byte substitution, and inverse mix columns.

Add round key: Add round key has its own inverse function since XOR functions its own inverse and the round keys should be selected in reverse order.

Inverse shift rows: Inverse shift rows functions exactly in the same way as shift row stage but in opposite direction. The first row is kept as it is, the second row is shifted by one-byte position to the right, the third row is shifted by two-byte position to the right, and the fourth row is shifted by three-byte position to the right. The resultant matrix consists of same 16 bytes but at different position. **Figure 7** shows inverse shift row stage in AES algorithm.

Inverse byte substitution: Inverse byte substitution is done using predefined substitution table known as inverse s-box. **Figure 8** shows inverse s-box in AES algorithm.

Inverse mix column: Transformation in inverse mix column is done using polynomials of degree less than 4 over Galois field (GF) 28 in which coefficients are the elements from the column of the state.

The rest of the chapter is organized as follows:

Section 2 presents the survey based on the various kinds of implementation of AES algorithm on reconfigurable platform. In Section 3, implementation of AES algorithm using the proposed approach is discussed. In Section 4, experimental results achieved using the proposed method along with the comparative analysis with existing methods are discussed.



**123**

**Figure 8.**

*Inverse S-box of AES algorithm.*

**Figure 6.**

**Figure 7.** *Inverse shift row.*

*Add round key stage.*

*High-Speed Area-Efficient Implementation of AES Algorithm on Reconfigurable Platform*

*DOI: http://dx.doi.org/10.5772/intechopen.82434*

**Figure 5.** *Mix column stage.* *High-Speed Area-Efficient Implementation of AES Algorithm on Reconfigurable Platform DOI: http://dx.doi.org/10.5772/intechopen.82434*

**Figure 6.** *Add round key stage.*

*Computer and Network Security*

**1.4 Add round key**

mix column stage in AES algorithm.

outputs completely new four bytes that replaces the original four bytes. **Figure 5** shows

The 16 bytes of the resultant matrix generated from mix column stage are then considered as 128 bits. In add round key stage, 128 bits of state are bitwise EX-ORed with 128 bits of round key. If this result belongs to the last round, then the output is cipher text else the resulting 128 bits is considered as 16 bytes, and another round is started with new byte substitution process. This is a column-wise operation between four bytes of state column and one word of round key. In the last round, there is no mix column step. **Figure 6** shows add round key stage in AES algorithm. Decryption of cipher text, generated from AES encryption, contains all the stages in encryption but in reverse order. AES decryption starts with inverse initial round. The remaining nine rounds in decryption consist of processes like add round

key, inverse shift rows, inverse byte substitution, and inverse mix columns.

its own inverse and the round keys should be selected in reverse order.

shows inverse shift row stage in AES algorithm.

the elements from the column of the state.

with existing methods are discussed.

The rest of the chapter is organized as follows:

Add round key: Add round key has its own inverse function since XOR functions

Inverse shift rows: Inverse shift rows functions exactly in the same way as shift row stage but in opposite direction. The first row is kept as it is, the second row is shifted by one-byte position to the right, the third row is shifted by two-byte position to the right, and the fourth row is shifted by three-byte position to the right. The resultant matrix consists of same 16 bytes but at different position. **Figure 7**

Inverse byte substitution: Inverse byte substitution is done using predefined substitu-

Section 2 presents the survey based on the various kinds of implementation of AES algorithm on reconfigurable platform. In Section 3, implementation of AES algorithm using the proposed approach is discussed. In Section 4, experimental results achieved using the proposed method along with the comparative analysis

tion table known as inverse s-box. **Figure 8** shows inverse s-box in AES algorithm. Inverse mix column: Transformation in inverse mix column is done using polynomials of degree less than 4 over Galois field (GF) 28 in which coefficients are

**122**

**Figure 5.** *Mix column stage.*

**Figure 7.** *Inverse shift row.*


**Figure 8.** *Inverse S-box of AES algorithm.*
