# Software Design and Experimental Evaluation of a Reduced AES for IoT Applications
## 3 Materials and Methods
### Encryption Process in LAES  
The main transformations components of LAES encryption rounds are mainly inherited from the original AES; however, the AES encryption components operate on GF (2\({}^{8}\)), while the LAES encryption components operate on GF(2\({}^{4}\)). This means that the components of the LAES operate by following the same logical operations of the AES, but the entries of each component are completely different, and operate on a smaller scale. For example, the S-box in the AES is a lookup table of 256 entries, while the S-box of the LAES is a lookup table of 16 entries. The input to the encryption process is a block of 64 bits, and it is reshaped into 4 \(\times\) 4 matrix. This matrix is called a state matrix, which is modified based on the encryption rounds until the 10 th round of the encryption is completed. The first nine rounds of LAES consist of four transformation components: LSubNibble, LShiftRows, LMixColumns, and LAddRoundKey. The final round of LAES contains only three transformations components, similar to the AES, which are LSubNibble, LShiftRows, and LAddRoundKey, as described below. LSubNibble: This component is used to perform the simple transformation of the state matrix bits into another matrix based on the S-box lookup table (see Table 5). For example, the state matrix entry of 0 will be transferred into 6. Different bits of the state matrix are transformed into different bits from the S-box lookup table. The S-box is intended to provide an invertible transformation of the state matrix entries during this component, which is inverted by the LInvSubNibble in the decryption process by the entries of the Inv S-box (see Table 6). In AES, the S-box is a 16 \(\times\) 16 matrix covering a total of 256 bytes, \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Round** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\
**Number** & & & & & & & & & & \\ \hline Round & 1 & 2 & 4 & 8 & 3 & 6 & C & B & 5 & A \\ constant & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Round constant table. represented in hexadecimal, and indexed in a column and row pattern. The work in [45] indicated that the SubBytes component in AES has an egregious processing time and power consumption, and the use of smaller lookup tables with sizes ranging from 16 to 128 bytes has become more reliable for obtaining higher speeds to meet the needs of different lightweight applications, such as smartphone applications, sensor networks, smart cards, and RFID. In LAES, the S-box and the Inv S-box are reduced into 16 nibbles, indexed from 1 to 16, which is expected to improve the processing speed of LAES and provide the required confusion for lightweight applications. LShiftRows: This component operates similar to the ShiftRows in AES, by shifting the entries of each row of the state matrix cyclically. The LShiftRows operates on a state matrix of 64 bits. The shift is made by certain offsets of 0, 1, 2, and 3 for rows 1 to 4 of the state matrix, respectively. The objective of this component is to avoid the columns of the state matrix form being linearly independent, and to enhance the overall diffusion of the encryption process. For the decryption process, the LInvShiftRows component cyclically shifts the rows of the state matrix to the right by certain offsets of 0, 1, 2, and 3 for rows 1 to 4 of the state matrix, respectively. LMixColumns: In this component, the state matrix obtained from the LSubNibble is diffused to provide a further level of diffusion following the operation performed by LShiftRows component. The LMixColumns performs permutation at the column level of the state matrix, which provides another level of diffusion over the LShiftRows component. The LMixColumns transformation is computed as illustrated in Equation (5), where S' is the new state matrix, P is the polynomial coefficient, and S is the current state matrix.