1. Introduction to AES
The Advanced Encryption Standard (AES) is a symmetric-key block cipher that is widely used for secure data encryption. It was established as the replacement for the Data Encryption Standard (DES) due to DES’s vulnerability to brute-force attacks. AES was adopted by the National Institute of Standards and Technology (NIST) in 2001 after a public competition to select a new encryption standard.
Key Features of AES
✅ Symmetric-key encryption (same key for encryption and decryption).
✅ Block cipher that encrypts data in 128-bit blocks.
✅ Supports three key lengths:
- AES-128 (128-bit key)
- AES-192 (192-bit key)
- AES-256 (256-bit key)
✅ Stronger security than DES and 3DES.
✅ Faster and more efficient than older encryption methods.
2. How AES Works
AES operates using the Substitution-Permutation Network (SPN), unlike DES, which uses the Feistel network. The encryption process involves multiple rounds of transformations.
AES Encryption Steps
The number of rounds depends on the key length:
- AES-128 → 10 rounds
- AES-192 → 12 rounds
- AES-256 → 14 rounds
Each round consists of the following steps:
Step 1: Key Expansion
- The original encryption key is expanded into multiple round keys using a key schedule algorithm.
Step 2: Initial Round
- AddRoundKey: The first round key is XORed with the plaintext block.
Step 3: Main Rounds (Repeated for Each Round)
Each round consists of four transformations:
- SubBytes (Substitution Layer)
- Each byte is replaced using a predefined S-Box (Substitution Box), introducing non-linearity for security.
- ShiftRows (Permutation Layer)
- Rows of the 4×4 state matrix are shifted to the left by different offsets to introduce diffusion.
- MixColumns (Mixing Layer, except in the last round)
- Each column is transformed using a mathematical function to further scramble the data.
- AddRoundKey
- The round key is XORed with the state matrix.
Step 4: Final Round
- The final round omits the MixColumns step but includes the other three transformations.
Step 5: Ciphertext Output
- The transformed state matrix is converted back into a 128-bit encrypted block (ciphertext).
3. AES Decryption Process
The decryption process follows the same steps in reverse order, using the inverse S-Box, ShiftRows, and MixColumns operations.
4. AES Key Lengths and Security
| AES Version | Key Size | Number of Rounds | Security Level |
| AES-128 | 128-bit | 10 rounds | Strong |
| AES-192 | 192-bit | 12 rounds | Stronger |
| AES-256 | 256-bit | 14 rounds | Most Secure |
AES-256 is the most secure version and is widely used for military and government encryption.
5. Why is AES Secure?
Strong Encryption Properties
🔒 Large Key Size: Prevents brute-force attacks.
🔒 Confusion & Diffusion: The S-Box and MixColumns operations create strong security.
🔒 No Known Practical Attacks: As of today, AES is resistant to all known cryptographic attacks.
Attacks on AES
- Brute-Force Attack: Theoretically possible but impractical due to key size.
- Side-Channel Attacks: Exploit weaknesses in implementation (not the algorithm itself).
6. Applications of AES
AES is used in a variety of applications, including:
✅ Wireless Security – Wi-Fi encryption (WPA2, WPA3).
✅ Data Storage – Encrypting hard drives, SSDs, and USB devices.
✅ Web Security – SSL/TLS for secure HTTPS connections.
✅ Banking & Financial Systems – Securing transactions and credit card data.
✅ Cloud Storage & Communications – End-to-end encryption for data security.
✅ Blockchain & Cryptocurrencies – Protecting transaction data.
7. AES vs. DES vs. 3DES
| Feature | DES (56-bit) | 3DES (168-bit) | AES (128/192/256-bit) |
| Security | Weak | Moderate | Strong |
| Key Length | 56-bit | 168-bit (3×56) | 128, 192, 256-bit |
| Number of Rounds | 16 | 48 (3×16) | 10, 12, 14 |
| Speed | Fast | Slower | Fastest |
| Vulnerability | Easily cracked | Still vulnerable | No practical attacks |
8. Conclusion
AES is the gold standard for modern encryption, offering strong security, efficiency, and versatility. It is used in nearly every industry that requires secure communication and data protection.