Advanced Encryption Standard (AES)

AES also known by its original name Rijndael is a symmetric block cipher algorithm, which means the sender and receiver use the same key for encryption and decryption.

AES operates on a 4 × 4 column-major order array of 16 bytes b₀, b₁, …, b₁₅ termed the state.

The key size of AES specifies the number of transform rounds that convert the input (plain text) to the output (cipher text).

• 10 rounds for 128-bit keys.
• 12 rounds for 192-bit keys.
• 14 rounds for 256-bit keys.

Encryption

Key-Expansion – Each round uses a different key that is transformed from the secret key. The round keys are generated at the beginning of the encryption.

Each round comprises of 4 steps:

1. Sub-Bytes: Each byte is replaced with another according to a lookup table.
2. Shift-Rows:
   • The first row is not shifted.
   • The second row is shifted once to the right.
   • The third row is shifted twice to the right.
   • The fourth row is shifted thrice to the right.
3. Mix-Columns:
   • In this step, we multiply each column of the state array by a fixed matrix (called the Mix Columns matrix). The result is a new column for the subsequent state array.
   • The Mix Columns matrix is defined as:
     
   • To calculate the new column we do:
     
     where:
     • ⊕ represents the bitwise XOR operation.
     • ⋅ represents multiplication in the finite field (GF(2^8)).
   • Example
     
4. Add-Round-Key
   we perform an XOR operation the result of step 3 with the round key of the next round, then proceed back to Sub-Bytes step to begin the next round. This cycle is repeated until the final round is reached.

Note: In the final round, we skip the Mix-Columns step.

Decryption

First, we generate the keys similar with the Key-Expansion step in the encryption.

Next, we do the invert steps:

1. Add-Round-Key
   Do same operation with the Add-Round-Key of the encryption process.
2. Invert-Sub-Bytes
   Each byte is replaced with another according to a Inverse S-box.
3. The first row is not shifted.
   • The second row is shifted once to the left.
   • The third row is shifted twice to the left.
   • The fourth row is shifted thrice to the left.
4. Invert-Mix-Columns
   We also do the same actions with the Mix-Columns step in the encryption, but using the invert Mix Column Matrix.
   

Note: In the final round, we also skip the Inv-Mix-Columns step.

Here is a simple demo. Thank for reading!

References

Advanced Encryption Standard – Wikipedia

Advanced Encryption Standard (AES) – GeeksforGeeks

AES Encryption: Secure Data with Advanced Encryption Standard (simplilearn.com)

Online AES Encryption / Decryption | Anycript