Weak Key Attacks: How to Prevent Them

TL;DR

Keys generated with low entropy (not enough randomness) are easily guessed by attackers. This guide shows you how to check for weak keys and improve your key generation process.

Checking for Weak Keys

1. Understand Entropy: Entropy measures the unpredictability of a random source. Higher entropy means more secure keys. A good rule of thumb is at least 128 bits of entropy for modern cryptography.
2. Check Your Random Number Generator (RNG): The quality of your RNG is crucial. Many systems have built-in RNGs, but they may not be suitable for security purposes.
   • Linux: Use /dev/urandom instead of /dev/random unless you absolutely need cryptographically strong blocking behaviour (which is rare).
   • Windows: Use the CryptGenRandom API.
   • macOS: Use the SecRandomCopyBytes function.
3. Test Key Randomness: Several tools can help you assess key randomness.
   • ent: A command-line tool for entropy estimation.

     ent -s /path/to/keyfile

     Look for a high value of ‘Entropy’.

   • Dieharder: A more comprehensive statistical test suite. Requires installation and is more complex to use, but provides detailed results.
4. Look for Patterns: Examine generated keys for repeating sequences or predictable patterns. Attackers exploit these weaknesses.
   • Hex Editors: Use a hex editor (like HxD on Windows or Hex Fiend on macOS) to visually inspect the key file.
   • Scripts: Write simple scripts to check for common patterns. For example, checking if all bytes are zero.

Improving Key Generation

1. Use a Cryptographically Secure RNG (CSRNG): This is the most important step.
   • OpenSSL: A widely used cryptography library.

     openssl rand -base64 32

     generates a 32-byte random key in base64 format.

   • Python (secrets module):

     import secrets
     key = secrets.token_bytes(32)

2. Increase Key Length: Longer keys are harder to crack.
   • AES: Use 128-bit, 192-bit or 256-bit keys.
   • RSA: Use at least 2048-bit keys; 3072 or 4096 bits are recommended for higher security.
3. Salt Your Keys (where appropriate): Salting adds randomness to key derivation, making precomputed tables less effective.
   • PBKDF2: A key derivation function that uses a salt.
   • bcrypt/scrypt: Password hashing algorithms that include salting.
4. Avoid Predictable Seeds: Never use timestamps, process IDs, or other easily guessable values as seeds for your RNG.
5. Hardware Security Modules (HSMs): For the highest security, consider using an HSM to generate and store keys. These devices are designed specifically for cryptographic operations.

Example: Generating a Secure Key in Python

import secrets
import hashlib

def generate_secure_key(length=32):
    # Generate random bytes using secrets module
    random_bytes = secrets.token_bytes(length)

    # Hash the random bytes to create a more uniform key distribution
    hashed_key = hashlib.sha256(random_bytes).digest()

    return hashed_key

secure_key = generate_secure_key()
print(f"Secure Key: {secure_key.hex()}")