What Is AES Encryption and How Does It Work?

The Advanced Encryption Standard (AES), also known as Rijndael, is a robust encryption algorithm widely employed to secure electronic data. Operating as a symmetric block cipher, AES employs a single key for both encrypting and decrypting data, emphasizing the importance of key confidentiality. In 2001, the U.S. National Institute of Standards and Technology (NIST) adopted AES to replace the outdated Data Encryption Standard (DES), recognizing its superior security and efficiency. With key lengths of 128, 192, or 256 bits, AES has become a cornerstone in safeguarding sensitive information, finding application in various domains such as online communications, file encryption, and network security.

Key Features:
  1. Block Size: 128 bits (16 bytes)
  2. Key Sizes: 128, 192, or 256 bits
  3. Rounds: 10 for 128-bit keys, 12 for 192-bit keys, 14 for 256-bit keys
  4. Structure: Substitution-Permutation Network (SPN)

How AES Works?

AES encrypts data through a series of well-defined and carefully orchestrated steps, including key expansion, initial and multiple rounds of transformations (SubBytes, ShiftRows, MixColumns, and AddRoundKey), and a final round. These operations collectively provide a high level of security and ensure that the encrypted data (ciphertext) is difficult to reverse without the correct key. Let's dive into a detailed explanation of how AES (Advanced Encryption Standard) works, step by step:

Key Expansion

Purpose: To generate a series of round keys from the initial key.

The process begins with the expansion of the initial key into a series of round keys. This is achieved through a key expansion algorithm, which generates a set of round keys that will be used in each round of the encryption process.

Initial Round

Purpose: To start obscuring the plaintext using a round key.

The plaintext is divided into blocks, and the initial round involves adding (XOR operation) the plaintext block with the first round key.


Purpose: To repeatedly scramble the data, making it increasingly difficult to decipher.

Each round consists of four main transformations: SubBytes, ShiftRows, MixColumns, and AddRoundKey. The number of rounds depends on the key length: 10 rounds for AES-128, 12 rounds for AES-192, and 14 rounds for AES-256.

  1. SubBytes:In this step, each byte of the state (the current block of data being processed) is substituted with a corresponding byte from a predefined substitution table called the S-box. This introduces non-linearity into the encryption process.
  2. ShiftRows:The state is then subjected to a row-shifting operation. Each row of the state is shifted by a varying number of bytes. This step provides diffusion and ensures that each byte influences multiple bytes in the subsequent rounds.
  3. MixColumns:This step involves mixing the columns of the state using a matrix multiplication operation. Each column is treated as a polynomial, and matrix multiplication is performed to introduce further diffusion and confusion.
  4. AddRoundKey:The current state is XORed with the round key for the current round. The round key is derived from the original key during the key expansion phase. This step introduces the key's influence into the state, adding another layer of security.

Final Round

Purpose: To finalize the encryption process.

The final round is similar to the previous rounds, with the exception that the MixColumns step is omitted. The final round ensures that the ciphertext is derived from the processed state.


After the last round, the final state represents the encrypted form of the original plaintext, known as the ciphertext. This ciphertext is the output of the AES encryption process and can be safely transmitted or stored, as long as the encryption key remains secure.

Result The final state after all rounds is the encrypted ciphertext.


Implementing AES in C#, Python, and Java involves utilizing the cryptographic libraries and frameworks provided by each language. Here are brief notes on how you can approach AES implementation in these languages:


In C#, AES encryption is implemented using the System.Security.Cryptography namespace. To initialize the process, create an instance of the AesManaged class and set the key and IV properties. Encryption and decryption are carried out using the CreateEncryptor and CreateDecryptor methods to obtain the encryptor and decryptor, respectively. Finally, the TransformFinalBlock method is used to execute the encryption or decryption on the data. This streamlined process ensures the secure implementation of AES encryption in C# applications.

Encrypt and Decrypt a message using AES Algorithm in C#


In Python, cryptographic operations, including AES encryption, are often performed using the cryptography library. To initiate the process, import modules such as Fernet for symmetric encryption or AES for low-level AES operations. Initialize the key and IV before proceeding with encryption or decryption. Create a cipher object using the specified key and IV, and employ the encrypt or decrypt methods to execute the desired cryptographic operation. This concise approach ensures the effective implementation of AES encryption in Python applications using the cryptography library.

Encrypt and Decrypt a message using AES Algorithm in Python


In Java, cryptographic operations, including AES encryption, are facilitated by the javax.crypto package. To start the process, initialize encryption/decryption objects using classes like Cipher and SecretKeySpec. Set the key and IV by employing the SecretKeySpec and IvParameterSpec classes. For encryption or decryption, obtain a cipher instance and initialize it with the desired transformation (e.g., "AES/CBC/PKCS5Padding"). Utilize the doFinal method to execute the encryption or decryption on the specified data. This succinct procedure ensures the secure implementation of AES encryption in Java applications through the javax.crypto package.

Encrypt and Decrypt a message using AES Algorithm in Java

Implementing AES in these languages involves understanding the specifics of each language's cryptographic libraries and incorporating them into the overall application logic. Always follow best practices for secure key management and error handling to ensure the robustness of the implementation.


AES is widely regarded as highly secure and has proven its resilience against extensive cryptographic analysis since its introduction. The algorithm's robust security is underpinned by the challenging nature of fundamental mathematical operations integral to its design, particularly the SubBytes substitution and the MixColumns mixing. These operations contribute to the algorithm's resistance against various cryptographic attacks, ensuring that the encrypted data remains well-protected. The consistent performance of AES in the face of rigorous scrutiny and testing has solidified its reputation as a trustworthy and formidable encryption standard, making it a preferred choice for securing sensitive information in diverse applications.

Key Strength

  1. AES-128 is generally considered strong enough for most purposes.
  2. AES-192 and AES-256 provide even higher security levels for sensitive data.

Modes of Operation

AES supports multiple modes of operation, providing flexibility for diverse cryptographic applications. Among these modes are Electronic Codebook (ECB), Cipher Block Chaining (CBC), Counter (CTR), and several others, each tailored to meet specific needs. ECB involves encrypting individual blocks of data independently, making it suitable for parallel processing but potentially vulnerable to patterns in the plaintext. CBC, on the other hand, introduces an element of feedback, where the ciphertext of one block influences the encryption of the next, enhancing security but sacrificing parallelism. CTR operates by encrypting a counter value, offering parallelism and efficiency, while other modes cater to different scenarios. The choice of mode depends on the desired balance between security considerations and operational requirements, allowing AES to be adaptable to a wide range of encryption scenarios.


AES is widely used for securing sensitive data in various applications, including online communication, file encryption, disk encryption, and securing network traffic.


AES is a symmetric encryption algorithm that provides a high level of security and efficiency, making it suitable for a broad range of cryptographic applications. Its adoption as a standard by organizations and governments worldwide has contributed to its widespread use in securing digital communication and data.