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AES encryption algorithm: How does it work?
2022-02-30

Overview of AES Encryption Algorithm

AES (Advanced Encryption Standard) encryption algorithm is a widely used symmetric encryption algorithm, and its working principle can be briefly summarized as follows:

I. Key Selection

AES supports three key lengths: 128-bit, 192-bit, and 256-bit. The longer the key length, the higher the encryption strength, but the consumption of computational resources will also increase accordingly. In practical applications, the appropriate key length is selected based on the importance of the data and security requirements.

II. Key Expansion

The AES algorithm expands the input key into multiple round keys. These round keys are used in subsequent encryption rounds to ensure that a different key is used for each round, thereby enhancing the security of the algorithm.

III. Encryption Process

The encryption process of AES consists of multiple rounds, with each round performing the following four steps:

  • SubBytes (Byte Substitution): Replaces each byte of the input data using an S-box (Substitution box), increasing the confusion of the data.
  • ShiftRows: Circularly shifts each row of the data block (usually a 4x4 matrix), with different rows shifted by different distances, to further diffuse the data.
  • MixColumns (except for the last round): Performs matrix multiplication on each column of the data block using a fixed matrix, further mixing the data. However, this step is omitted in the last encryption round.
  • AddRoundKey: Performs an XOR operation on the current round's round key and the data block, incorporating key information into the encryption process.

IV. Decryption Process

The decryption process of AES is the reverse of the encryption process. Starting from the last round, the following steps are performed in reverse order:

  • AddRoundKey: Performs an XOR operation on the round key and the data block.
  • InvMixColumns (except for the first round): Performs inverse matrix multiplication on each column of the data block to restore the column data.
  • InvShiftRows: Circularly shifts each row of the data block to the right in reverse to restore the row data.
  • InvSubBytes: Replaces each byte of the data block using the inverse table of the S-box to restore the original data.

V. Characteristics and Advantages

High Security: Considered a very secure encryption algorithm after rigorous examination and practical verification.
Fast Speed: Employs efficient mathematical operations and hardware optimizations to quickly complete encryption and decryption operations.
Flexibility: Supports multiple key lengths and encryption modes to meet security requirements in different scenarios.