Secure Key Exchange vs. Password Hashing

TL;DR

No, a secure key exchange algorithm cannot directly replace hashing for password authentication. They serve different purposes. Hashing stores a one-way representation of the password, while key exchange establishes a shared secret. You can combine them for improved security (e.g., using key exchange to protect a salt before hashing), but they aren’t interchangeable.

Understanding the Difference

1. Password Hashing: This process takes a password and transforms it into a fixed-size string of characters (the hash). It’s designed to be one-way – you can’t easily get the original password back from the hash. Common algorithms include bcrypt, Argon2, and scrypt.
   • Why it’s used: To securely store passwords without storing them in plain text. If a database is compromised, attackers only get hashes, not actual passwords.
   • Example (Python with bcrypt):

     import bcrypt
     password = b"mysecretpassword"
     hash = bcrypt.hashpw(password, bcrypt.gensalt())
     print(hash)

2. Secure Key Exchange: This process allows two parties to establish a shared secret key over an insecure channel without revealing the key itself. Common algorithms include Diffie-Hellman and Elliptic-Curve Diffie-Hellman (ECDH).
   • Why it’s used: To encrypt communication, verify identities, or securely share data.
   • Example (Conceptual): Alice and Bob exchange public keys; they then independently calculate the same shared secret key using their private keys and each other’s public keys.

Why Key Exchange Doesn’t Replace Hashing

1. Hashing is for Storage: Hashing creates a static representation of the password that can be stored indefinitely. Key exchange generates a secret key that’s typically used for a specific session or transaction.
2. Reversibility: Key exchange aims to create a shared secret, not a one-way transformation. If you could reverse a key exchange process, it would defeat its purpose of secure communication.
3. Computational Cost: Modern password hashing algorithms are deliberately slow (computationally expensive) to make brute-force attacks harder. Key exchange isn’t necessarily designed with this in mind.

How to Combine Hashing and Key Exchange for Better Security

1. Protecting the Salt: A salt is a random value added to passwords before hashing to prevent rainbow table attacks. You could use key exchange to securely transmit the salt from the server to the client before password hashing.
   • This prevents an attacker who intercepts network traffic from easily obtaining the salts used for hashing.
2. Authenticated Key Exchange: Use a strong authentication method (like passwords hashed with bcrypt) *before* establishing a key exchange connection. This verifies the user’s identity before any sensitive data is exchanged.
3. Forward Secrecy: Implement key exchange protocols that provide forward secrecy, meaning that if a private key is compromised, past communication sessions remain secure. (e.g., using Diffie-Hellman Ephemeral).

In Summary

Hashing and key exchange are different tools for different jobs. Hashing protects stored passwords, while key exchange secures communication channels. Using them together can significantly improve the overall security of your system.