**3. Advanced encryption standard (AES)**

The AES encryption algorithm is symmetric in the group, and there are three different key lengths: 128 bits, 196 bits, and 256 bits, with the packet size being 128 bits. The algorithm is reasonably flexible in its application. The AES algorithm is widely used in software and hardware. In the three key lengths, the 128bit key length is commonly used. The internal algorithm performs a ten-time iterative process when the key length is under. The five sections of the final round are joined by the Sub Bytes, S-box, Shift Rows, Mix Columns, and Add Round Key. AES has five different units of measurement: bits, bytes, characters, groups, states. A round of AES is composed of byte replacement (Sub Bytes), line displacement (Shift Rows), mixed column displacement (Mix Columns), key replacement (Add Round Key), and so on. AES algorithm design should meet three criteria during all phases of the data packet transformation, in the beginning, and ending stages of encryption:


**Figure 2** shows the process of AES Encryption and Decryption. It relies on the packet size and the length of the key, and it is controlled by the key. The iteration round of the number is controlled by the key and the length of the block.

As a **Figure 2**, a cryptographic algorithm is shown on the left and a cryptographic algorithm is shown on the right of the figure. A key expansion algorithm is shown in the middle of the figure. It consists of N iterations having four different steps: byte replacements (Sub Bytes), line displacements (Shift Rows), mixed column shifts (Mix

#### **Figure 2.**

*Process of AES encryption and decryption.*

Columns), and key shifts (Add Round Key). There are no mixed column transformations in the final round. The decryption algorithm is the opposite of encryption (inverse byte substitution, inverse shift rows, and inverse mix columns).

For full encryption, the data is passed through Nr rounds (Nr = 10, 12, 14). These rounds are governed by the following transformations:


*A Survey of Lightweight Image Encryption for IoT DOI: http://dx.doi.org/10.5772/intechopen.104431*

• *key*: Even if an eavesdropper knows the plaintext and ciphciphertexte AES algorithm cannot be determined, because the secret key is known to both the sender and the receiver. According to its specifications, AES uses one of three key sizes (Nk). AES-126, AES-196, and AES-256 respectively use 128 bit (16 bytes, 4 words) and 196 bit (24 bytes, 6 words) key sizes. Key values have no weak point, unlike DES. All key values are equally secure, therefore no key-value renders encryption more vulnerable than the other. Key values are expanded via key expansion routines before being used in the AES algorithm. In addition to performing "on the fly" word expansion, this routine can be performed at any time.
