AES Encryption Algorithm The Advanced Encryption Standard (AES) is a widely used symmetric encryption algorithm that has become the global standard for securing sensitive data. Developed as a replacement for the older Data Encryption Standard (DES), AES was adopted by the U.S. National Institute of Standards and Technology (NIST) in 2001 after a competition to find a more secure and efficient encryption algorithm. The winning algorithm, called Rijndael, was designed by Belgian cryptographers Vincent Rijmen and Joan Daemen. AES is known for its strong security, flexibility, and efficiency, making it suitable for a wide range of applications, including data encryption, secure communications, and digital rights management. Key Features of AES 1.Symmetric-Key Algorithm: AES uses the same key for both encryption and decryption, meaning both the sender and receiver must share the key securely before communication can occur. 2.Block Cipher: AES operates on fixed-length blocks of data, specifically 128-bit blocks. 3.Key Sizes: AES supports three different key sizes: ● AES-128: Uses a 128-bit key. ● AES-192: Uses a 192-bit key. ● AES-256: Uses a 256-bit key. 4.Number of Rounds: The number of encryption rounds depends on the key size: ● AES-128: 10 rounds. ● AES-192: 12 rounds. ● AES-256: 14 rounds. Each round involves several processing steps, including substitution, permutation, and mixing of the input data and encryption key. AES Encryption Process The AES encryption process consists of multiple rounds of transformations, where each round applies a series of operations to the input data (plaintext) and a subkey derived from the encryption key. Here’s a detailed breakdown of the AES encryption process: 1. Initial Key Expansion Before encryption begins, the original encryption key is expanded into a series of subkeys, known as the key schedule. These subkeys are used in each round of the encryption process. The key expansion process is designed to ensure that each subkey is different and unrelated to the others, adding complexity to the encryption. 2. Initial Round The first round involves a simple XOR operation between the plaintext and the first subkey. This step is called AddRoundKey. 3. Main Rounds The main rounds of AES encryption consist of four primary steps that are repeated for each round: ● SubBytes: This is a substitution step where each byte in the block is replaced with another byte using a fixed S-box (substitution box). The S-box is a nonlinear transformation that introduces confusion into the encryption, making it resistant to attacks like linear and differential cryptanalysis. ● ShiftRows: In this step, the rows of the block matrix are shifted to the left by different offsets. Specifically, the first row remains unchanged, the second row is shifted by one position, the third row by two positions, and the fourth row by three positions. This step helps achieve diffusion, ensuring that small changes in the input affect the entire ciphertext. ● MixColumns: This operation mixes the columns of the block by treating each column as a polynomial and multiplying it with a fixed polynomial modulo an irreducible polynomial. This step further increases diffusion by spreading the influence of each byte across the entire block. ● AddRoundKey: In this step, the current block is XORed with the round subkey, combining the data with a portion of the expanded encryption key. 4. Final Round The final round of AES encryption is similar to the main rounds, but it omits the MixColumns step. The final steps are: ● SubBytes ● ShiftRows ● AddRoundKey After the final round, the resulting block is the ciphertext, which can then be transmitted or stored securely. AES Decryption Process AES decryption is essentially the reverse of the encryption process. It uses the same key schedule but applies the transformations in reverse order: 1.Inverse ShiftRows: The rows of the block are shifted to the right instead of the left. 2.Inverse SubBytes: Each byte is substituted using the inverse S-box. 3.Inverse MixColumns: The columns are mixed using the inverse of the polynomial used during encryption. 4.AddRoundKey: The block is XORed with the round subkey. The decryption process follows the same structure as encryption, with the transformations applied in reverse order. After the final round, the output is the original plaintext. Key Concepts in AES 1. SubBytes and S-Box The SubBytes step uses a fixed S-box that operates on each byte independently. The S-box is designed to provide nonlinearity and confusion, making it difficult for attackers to find a linear relationship between the plaintext and the ciphertext. The S-box is constructed using the multiplicative inverse over a finite field combined with an affine transformation. 2. ShiftRows ShiftRows ensures that bytes from different columns are mixed in subsequent rounds. This step increases the complexity of the encryption by making sure that the influence of each byte is spread across the entire block. 3. MixColumns The MixColumns step treats each column as a 4-byte word and mixes them by multiplying them with a fixed matrix over the finite field . This operation ensures that the output depends on all four bytes in the column, adding to the diffusion property of the cipher. 4. AddRoundKey This step is critical to AES’s security, as it combines the data with the subkey at each round. The subkey is generated from the original key through the key expansion process, ensuring that each round key is different and unpredictable. AES Key Schedule The key schedule in AES is responsible for generating the round keys from the original key. The key expansion process depends on the key size (128, 192, or 256 bits) and involves several rounds of transformations, including: ● RotWord: Rotating a 4-byte word. ● SubWord: Applying the S-box substitution to a word. ● Rcon: XORing the result with a round constant, which varies for each round. The key schedule ensures that the round keys are independent and unpredictable, making it harder for attackers to recover the original key. AES Modes of Operation AES is a block cipher, meaning it operates on fixed-size blocks of data. To encrypt data longer than the block size, AES can be used in various modes of operation: 1.Electronic Codebook (ECB): Each block is encrypted independently. While simple, this mode is insecure for most applications because identical plaintext blocks produce identical ciphertext blocks. 2.Cipher Block Chaining (CBC): Each plaintext block is XORed with the previous ciphertext block before encryption, ensuring that identical plaintext blocks produce different ciphertext blocks. This mode requires an initialization vector (IV) for the first block. 3.Counter (CTR): Each block is XORed with a counter value before encryption. The counter is incremented for each block, and no padding is required. CTR mode allows for parallel encryption and decryption. 4.Galois/Counter Mode (GCM): A mode that combines the security of CTR with integrity protection through an authentication tag. GCM is widely used for secure communication protocols like TLS and IPSec. Security of AES AES is considered highly secure and has been extensively analyzed by cryptographers. Some key reasons for its security include: ● Key Size: AES supports key sizes of 128, 192, and 256 bits. The larger key sizes provide greater security against brute-force attacks. ● Number of Rounds: The multiple rounds of transformations, especially the nonlinear S-box and the mixing operations, make it resistant to common cryptographic attacks such as differential and linear cryptanalysis. ● Extensive Testing: AES has been subjected to years of scrutiny by cryptographers and is considered secure for most applications, including government and military use. Conclusion AES is the global standard for encryption due to its efficiency, flexibility, and robust security. Its design, based on a series of well-defined transformations, ensures both confidentiality and integrity when used in conjunction with appropriate modes of operation. AES is widely used in many applications, including securing web traffic (SSL/TLS), encrypting files, and protecting wireless communications (WPA2).