In today’s digital age, securing information is paramount. Encryption algorithms are essential tools in safeguarding data from unauthorized access. One of the most robust and widely used encryption standards is the Advanced Encryption Standard (AES).

Developed by the National Institute of Standards and Technology (NIST) in 2001, AES has become the benchmark for encryption, replacing the older Data Encryption Standard (DES). This article delves into the basics of AES, its structure, differences from DES, and its operational parameters.

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## Explanation of Advanced Encryption Standard Structure

Advanced Encryption Standard is a symmetric block cipher, meaning it uses the same key for both encryption and decryption and processes fixed-size blocks of data. Specifically, AES takes 128-bit blocks of plaintext and converts them into 128-bit blocks of ciphertext through a series of transformations. The primary components of the AES structure include the following:

**Input and Output**: Both are 128-bit blocks.**State Array**: A 4×4 matrix where each element is a byte, totaling 128 bits.**Initial Transformation**: The plaintext is loaded into the state array and an initial transformation is applied.**Rounds**: Depending on the key size, AES performs 10, 12, or 14 rounds of transformations to produce the ciphertext.**Final Transformation**: After the last round, the state array contains the ciphertext.

## Differences between DES and Advanced Encryption Standard

While DES (Data Encryption Standard) was the go-to encryption standard for many years, its vulnerabilities necessitated the development of AES. Here are the key differences between DES and Advanced Encryption Standard:

**Operation**:

**DES**: Uses a 56-bit key and operates on 64-bit blocks of data.**AES**: Uses 128, 192, or 256-bit keys and operates on 128-bit blocks of data.

**Rounds**:

**DES**: Consists of 16 rounds of a Feistel network structure.**AES**: Depending on the key size, it has 10 (for 128-bit keys), 12 (for 192-bit keys), or 14 (for 256-bit keys) rounds of transformations.

**Key Scheduling**:

**DES**: The key schedule generates 16 subkeys, each 48 bits in length, derived from the original 56-bit key.**AES**: The key schedule produces a number of 128-bit round keys. The total number of round keys depends on the key size, but all round keys are 128 bits in length.

## Introduction to Transformations in Advanced Encryption Standard

AES encryption involves a series of transformations applied to the data during each round. These transformations are designed to provide security by ensuring confusion and diffusion of the plaintext. The main transformations in AES include:

**Substitute Bytes (SubBytes)**:

- A non-linear substitution step where each byte in the state array is replaced with another byte using a fixed S-box.

**Shift Rows**:

- A transposition step where each row of the state array is shifted cyclically a certain number of steps.

**Mix Columns**:

- A mixing operation that operates on the columns of the state array, combining the four bytes in each column.

**Add Round Key**:

- Each byte of the state array is combined with a byte of the round key using bitwise XOR.

## Advanced Encryption Standard Parameters and the Relationship between Key Size and Number of Rounds

AES can operate with three different key sizes: 128 bits, 192 bits, and 256 bits. The key size directly impacts the number of rounds performed during the encryption process:

**AES-128**:

**Key Size**: 128 bits.**Rounds**: 10.**Transformations**: 9 rounds of four transformations, final round of three transformations.

**AES-192**:

**Key Size**: 192 bits.**Rounds**: 12.**Transformations**: 11 rounds of four transformations, final round of three transformations.

**AES-256**:

**Key Size**: 256 bits.**Rounds**: 14.**Transformations**: 13 rounds of four transformations, final round of three transformations.

## Key Schedule in Advanced Encryption Standard

The key schedule in AES expands the original key into an array of key material. Each round of the encryption process requires a round key, and the key schedule generates these round keys from the original key. The number of generated round keys equals the number of rounds plus one.

## Conclusion

AES is a powerful encryption standard that has become the backbone of modern data security. Its robust structure, variable key sizes, and multiple rounds of transformations ensure a high level of security, making it a superior alternative to DES. Understanding the basics of Advanced Encryption Standard, its structure, and operational parameters is crucial for anyone involved in data security. With its widespread adoption and proven effectiveness, AES continues to be a vital tool in protecting sensitive information across the globe.