Understanding and Implementing Aes: a Practical Guide with Example Calculations

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Advanced Encryption Standard (AES) is a widely used encryption algorithm that ensures data security. It is employed in various applications, from securing communications to protecting stored data. This article provides a practical overview of AES, including example calculations to help understand its operation.

Basics of AES

AES is a symmetric key encryption algorithm, meaning the same key is used for both encryption and decryption. It operates on fixed-size blocks of data, typically 128 bits, and supports key sizes of 128, 192, or 256 bits. The encryption process involves multiple rounds of substitution, permutation, and mixing of the data to produce a secure ciphertext.

Encryption Process

The AES encryption process consists of several steps:

  • SubBytes: Byte substitution using a predefined S-box.
  • ShiftRows: Cyclically shifts rows of the state array.
  • MixColumns: Mixes the columns to diffuse the data.
  • AddRoundKey: Combines the state with a round key derived from the main key.

This sequence repeats for a number of rounds depending on the key size, with the final round omitting the MixColumns step.

Example Calculation

Consider encrypting a simple 128-bit block with a 128-bit key. The initial plaintext is represented in hexadecimal as 00112233445566778899aabbccddeeff. The key is 000102030405060708090a0b0c0d0e0f. The first step involves adding the round key to the plaintext, resulting in a new state. Subsequent rounds apply the substitution, shifting, mixing, and key addition steps, transforming the data into ciphertext.

For example, after the first AddRoundKey operation, the state becomes:

Plaintext: 00112233445566778899aabbccddeeff

Round key: 000102030405060708090a0b0c0d0e0f

Resulting state (hex): 00102030405060708090a0b0c0d0e0f

This process continues through all rounds, ultimately producing the encrypted ciphertext.