AES Encryption & Wireless Protocol Integrity

TL;DR

AES encryption alone won’t prevent bit modification in a wireless protocol. While AES protects the confidentiality of your data, it doesn’t guarantee its integrity. You need to add a Message Authentication Code (MAC) like HMAC-SHA256 or use authenticated encryption modes like GCM or CCM.

Understanding the Problem

Imagine sending a secret message in a locked box (AES encryption). Anyone with the key can open the box and read the message. But, they could also potentially change the message inside before re-locking the box. AES doesn’t tell you if someone has tampered with the contents.

Why AES Isn’t Enough

1. Confidentiality vs. Integrity: AES focuses on keeping data secret (confidentiality). It scrambles the data so only authorized parties can read it. It doesn’t inherently protect against changes to the data (integrity).
2. Bit Flips & Attacks: An attacker could intercept your wireless transmission, modify bits in the encrypted payload, and re-encrypt it. Without a way to detect these changes, the receiver will happily decrypt the altered message as if it were legitimate. This is particularly dangerous in protocols where even small changes can have big consequences (e.g., financial transactions).
3. Wireless Channel Issues: Wireless channels are noisy. Errors can occur during transmission that look like intentional modifications. AES doesn’t distinguish between accidental errors and malicious tampering.

How to Protect Against Bit Modification

You need to add a mechanism to verify the data hasn’t been altered. Here are your options:

1. Message Authentication Code (MAC)

1. What it is: A MAC is like adding a digital fingerprint to your message. The sender calculates the MAC based on the message and a secret key, then sends both the message and the MAC.
2. How it works: The receiver recalculates the MAC using the received message and the same secret key. If the calculated MAC matches the received MAC, the message is authentic (hasn’t been tampered with). If they don’t match, the message has been altered or corrupted.
3. Common Algorithms: HMAC-SHA256 is a popular choice. It uses SHA-256 as the underlying hash function.
4. Example (Python):

   import hmac
   import hashlib
   
   message = b'This is my secret message.'
   key = b'MySecretKey'
   
   hash_object = hmac.new(key, message, hashlib.sha256)
   digest = hash_object.hexdigest()
   print(f"MAC: {digest}")

2. Authenticated Encryption (AE)

AE modes combine encryption and authentication in a single step, providing both confidentiality and integrity.

1. GCM (Galois/Counter Mode): A widely used AE mode known for its performance and security.
2. CCM (Counter with CBC-MAC): Another popular option, often used in Wi-Fi protocols.
3. Example (Python using cryptography library):

   from cryptography.fernet import Fernet
   
   key = Fernet.generate_key()
   f = Fernet(key)
   token = f.encrypt(b'My secret message')
   decrypted_message = f.decrypt(token)
   print(f"Encrypted: {token}")
   print(f"Decrypted: {decrypted_message}")

   Note: Fernet is a simplified AE implementation, but GCM and CCM offer more control and flexibility.

Implementing in Your Wireless Protocol

1. Choose an Algorithm: Select either a MAC algorithm (like HMAC-SHA256) or an authenticated encryption mode (like GCM or CCM). GCM is generally preferred for new designs.
2. Key Management: Securely manage the secret key used for both encryption and authentication. This is crucial! A compromised key defeats the entire system.
3. Integrate into Protocol: Add the MAC calculation/verification steps to your protocol’s message structure. For AE modes, ensure you are using a library that correctly handles initialization vectors (IVs) and nonces.
4. Testing: Thoroughly test your implementation to verify it detects bit modifications under various attack scenarios.

cyber security Considerations

• Key Length: Use sufficiently long keys for AES (128-bit, 192-bit, or 256-bit) and your chosen MAC algorithm (e.g., SHA-256).
• IV/Nonce Management: For AE modes, use unique initialization vectors (IVs) or nonces for each message to prevent replay attacks.
• Side-Channel Attacks: Be aware of potential side-channel attacks that could leak information about your key during encryption/decryption.