Mastering Algorithms

AES (Advanced Encryption Standard)

Overview

AES (Advanced Encryption Standard) is a symmetric block cipher algorithm that has been the standard for encrypting data since 2001. It was selected by the U.S. National Institute of Standards and Technology (NIST) to replace DES and is now used worldwide for securing sensitive data.

AES operates on fixed-size blocks of data (128 bits) and supports key sizes of 128, 192, or 256 bits. It's fast, secure, and efficient, making it suitable for both software and hardware implementations.

How It Works

AES uses a substitution-permutation network (SPN) structure:

  1. Key Expansion: Derives round keys from the original key
  2. Initial Round: AddRoundKey operation
  3. Main Rounds: Repeated application of:
    • SubBytes - Substitution using S-box
    • ShiftRows - Permutation of rows
    • MixColumns - Mixing of columns
    • AddRoundKey - XOR with round key
  4. Final Round: Same as main rounds but without MixColumns

AES Algorithm


AES_Encrypt(plaintext, key):
    state = plaintext
    expanded_key = KeyExpansion(key)
    
    AddRoundKey(state, expanded_key[0])
    
    for round = 1 to Nr - 1:
        SubBytes(state)
        ShiftRows(state)
        MixColumns(state)
        AddRoundKey(state, expanded_key[round])
    
    SubBytes(state)
    ShiftRows(state)
    AddRoundKey(state, expanded_key[Nr])
    
    return state

Implementation

Note: Full AES implementation is complex. Here's a simplified example using Python's cryptography library: 


from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
import os

def aes_encrypt(plaintext, key):
    """AES encryption using CBC mode"""
    # Generate random IV
    iv = os.urandom(16)
    
    # Create cipher
    cipher = Cipher(
        algorithms.AES(key),
        modes.CBC(iv),
        backend=default_backend()
    )
    
    encryptor = cipher.encryptor()
    ciphertext = encryptor.update(plaintext) + encryptor.finalize()
    
    return iv + ciphertext

def aes_decrypt(ciphertext, key):
    """AES decryption using CBC mode"""
    iv = ciphertext[:16]
    encrypted_data = ciphertext[16:]
    
    cipher = Cipher(
        algorithms.AES(key),
        modes.CBC(iv),
        backend=default_backend()
    )
    
    decryptor = cipher.decryptor()
    plaintext = decryptor.update(encrypted_data) + decryptor.finalize()
    
    return plaintext

# Example usage
key = os.urandom(32)  # 256-bit key
plaintext = b"Hello, AES Encryption!"
ciphertext = aes_encrypt(plaintext, key)
decrypted = aes_decrypt(ciphertext, key)

Specifications

Key Size Block Size Number of Rounds Security Level
128 bits 128 bits 10 High
192 bits 128 bits 12 Very High
256 bits 128 bits 14 Very High

Security

AES is considered highly secure when properly implemented:

  • No known practical attacks against full AES
  • Resistant to known cryptanalytic techniques
  • Widely analyzed and tested
  • Approved for use with classified information (AES-256)

Best practices:

  • Use AES-256 for maximum security
  • Always use a secure mode (CBC, GCM, etc.)
  • Generate keys using cryptographically secure random number generators
  • Use proper key management practices

Applications

  • Data Encryption: File encryption, database encryption
  • Network Security: SSL/TLS, VPN protocols
  • Disk Encryption: Full disk encryption systems
  • Wireless Security: WPA2, WPA3
  • Government Use: Classified information protection

Related Algorithms

Explore other encryption algorithms: