AES Encryption Algorithm

Cryptocurrency

AES

The Advanced Encryption Standard (AES) is a symmetric encryption algorithm widely used to secure sensitive data. It was established by the U.S. National Institute of Standards and Technology (NIST) in 2001 and is based on the Rijndael cipher developed by Vincent Rijmen and Joan Daemen. AES operates on fixed-size blocks of data and can use key sizes of 128, 192, or 256 bits. Below is a detailed explanation of how the AES algorithm works:

Key Features of AES

  1. Block Size: AES operates on 128-bit blocks of data (16 bytes).
  2. Key Length: AES supports three key lengths: 128, 192, and 256 bits.
  3. Symmetrical: The same key is used for both encryption and decryption.

Structure of AES

AES consists of several key steps that can be broken down into the following phases:

  1. Key Expansion: The encryption key is expanded into multiple key schedules. Each round of the AES algorithm uses a different round key derived from the original key.

  2. Initial Round:

    • AddRoundKey: The input data block (plaintext) is combined with the round key using the bitwise XOR operation.

  3. Main Rounds: For each of the following rounds (10 rounds for 128-bit key, 12 for 192-bit key, and 14 for 256-bit key), the following operations are performed:

    • SubBytes: Each byte of the data block is replaced with its corresponding byte from a fixed substitution table called the S-box. This step introduces non-linearity.
    • ShiftRows: The rows of the data block are shifted cyclically. The first row is unchanged, the second row is shifted one byte to the left, the third row two bytes, and the fourth row three bytes.
    • MixColumns: Each column of the data block is treated as a polynomial and is mixed with other columns to diffuse the data. This step increases resistance to cryptanalysis.
    • AddRoundKey: The round key is XORed with the data block again.

  4. Final Round: This round is slightly different because it omits the MixColumns step:

    • SubBytes
    • ShiftRows
    • AddRoundKey

Pseudocode for AES Encryption

Here’s a high-level overview of the process in pseudocode:

      
      Function AES_Encrypt(plaintext, key)
    // Key Expansion
    roundKeys = KeyExpansion(key)

    // Initial Round
    state = AddRoundKey(plaintext, roundKeys[0])

    // Main Rounds
    for i from 1 to numberOfRounds do
        state = SubBytes(state)
        state = ShiftRows(state)
        state = MixColumns(state)
        state = AddRoundKey(state, roundKeys[i])

    // Final Round (no MixColumns)
    state = SubBytes(state)
    state = ShiftRows(state)
    state = AddRoundKey(state, roundKeys[numberOfRounds])

    return state

AES Decryption

Decryption in AES is essentially the reverse process of encryption. It uses a corresponding key schedule derived from the original key, but the operations are applied in reverse order:

  1. AddRoundKey
  2. Inverse ShiftRows
  3. Inverse SubBytes
  4. AddRoundKey
  5. Inverse MixColumns (for the main rounds)
  6. Repeat for the number of rounds
  7. Final Round (no Inverse MixColumns)

The decryption process can be summarized in a similar way to encryption, but it involves using the inverse operations to retrieve the original plaintext.

Security

AES is considered secure due to its strength against various cryptographic attacks. It has undergone extensive analysis and has not been successfully attacked with any practical method using current technology. However, it is important to use it properly, including secure key management practices, to ensure data security.

Conclusion

AES is a foundational encryption algorithm that provides robust security for data in transit and at rest. Its well-defined structure of rounds, key expansions, and transformation steps contributes to its strength and efficiency in various applications, making it the standard choice for many secure communications and data encryption requirements.