Hash Chaining: Strengthening Security

TL;DR

Yes, repeatedly hashing a resource (like a password or file) can make attacks harder, but it’s not a magic bullet. It increases the computational cost for attackers and makes certain types of attacks more difficult, especially pre-computation attacks like rainbow table lookups. However, it doesn’t solve all security problems – strong, unique salts are still essential.

How Hash Chaining Works

Hash chaining involves taking the hash of a resource and then hashing that result again (and potentially multiple times). Let’s break down why this can be useful:

Step-by-step Guide to Implementing Hash Chaining

1. Choose a Strong Hashing Algorithm: Start with a secure hash function like SHA-256 or Argon2. Avoid older algorithms like MD5 and SHA-1, which are known to have weaknesses.
   • Example using Python’s hashlib library:

   import hashlib
   
   resource = "mysecretpassword"
   encoded_resource = resource.encode('utf-8') # Important for consistent hashing!
   hash1 = hashlib.sha256(encoded_resource).hexdigest()
   print(f"First Hash: {hash1}")

2. Salt the Resource: Before any hashing, add a unique, randomly generated salt to your resource. This prevents rainbow table attacks.
   • Example of generating a random salt:

   import os
   salt = os.urandom(16).hex()
   print(f"Generated Salt: {salt}")

3. Then, combine the salt and resource before hashing:

salted_resource = salt + resource
encoded_salted_resource = salted_resource.encode('utf-8')
hash1 = hashlib.sha256(encoded_salted_resource).hexdigest()
print(f"First Hash (with Salt): {hash1}")

4. Repeat the Hashing Process: Take the output of the first hash and hash it again. Repeat this process several times.
   • Example of chaining two hashes:

   hash2 = hashlib.sha256(hash1.encode('utf-8')).hexdigest()
   hash3 = hashlib.sha256(hash2.encode('utf-8')).hexdigest()
   print(f"Second Hash: {hash2}")
   print(f"Third Hash: {hash3}")

5. Store the Final Hash and Salt: Store the final hash result *and* the original salt. You’ll need both to verify the resource later.
   • Important: Never store the plain text resource!
6. Verification Process: When a user tries to authenticate (or you need to check a file’s integrity), repeat the hashing process using the stored salt and compare the resulting hash with the stored hash.
   • Example of verification:

   input_resource = "mysecretpassword"
   stored_salt = "your_stored_salt"
   
   # Re-hash the input resource with the stored salt
   salted_input = stored_salt + input_resource
   encoded_salted_input = salted_input.encode('utf-8')
   hash1 = hashlib.sha256(encoded_salted_input).hexdigest()
   hash2 = hashlib.sha256(hash1.encode('utf-8')).hexdigest()
   hash3 = hashlib.sha256(hash2.encode('utf-8')).hexdigest()
   
   # Compare the final hash with the stored hash
   if hash3 == "your_stored_final_hash":
       print("Authentication successful!")
   else:
       print("Authentication failed.")

Benefits of Hash Chaining

• Increased Computational Cost: Each additional hash layer increases the time and resources an attacker needs to crack the resource.
• Mitigation of Pre-computation Attacks: Rainbow tables become less effective because each hash iteration changes the input for the table lookup.
• Defense in Depth: Adds another layer of security on top of salting, making it harder for attackers even if one layer is compromised.

Limitations and Considerations

• Not a Replacement for Strong Salts: Hash chaining *requires* strong, unique salts to be effective. Without salts, it provides limited security.
• Increased Processing Time: Repeated hashing adds overhead, which might impact performance in high-volume applications.
• Key Stretching is Better: For passwords specifically, key stretching algorithms like Argon2, bcrypt, and scrypt are generally preferred over simple hash chaining because they are designed to be slow and memory-hard, making them more resistant to brute-force attacks. Hash chaining can be *part* of a larger strategy but shouldn’t be the sole security measure.
• Cyber security best practice: Always use well-vetted libraries for hashing and encryption. Avoid implementing your own cryptographic functions unless you are an expert in the field.