Asymmetric Encryption vs Modern Authentication

TL;DR

While asymmetric encryption can be used for authentication, it’s generally not a direct replacement for modern strategies like OAuth 2.0 and OpenID Connect. It’s more complex to implement securely, less flexible, and harder to scale. Modern authentication offers better user experience, security features (like multi-factor), and integration with existing systems.

Understanding the Problem

You’re asking if you can replace things like passwords and session cookies with purely asymmetric encryption for logging users in. Let’s break down why this is tricky.

1. How Asymmetric Encryption Works (Briefly)

Asymmetric encryption uses a key pair: a public key and a private key.

• Public Key: Everyone can have this. You use it to encrypt data.
• Private Key: Only you should have this. You use it to decrypt data encrypted with your public key.

If someone encrypts a message with your public key, only you can read it using your private key.

2. How Asymmetric Encryption Could Be Used for Authentication

Here’s one way it could work:

1. User Generates Key Pair: When a user signs up, they create their own public/private key pair on their device (e.g., in the browser).
2. Public Key Registration: The user sends their public key to your server and you store it associated with their account.
3. Authentication Process:
   1. Your server generates a random ‘challenge’ message (a unique string of data).
   2. The server encrypts this challenge using the user’s stored public key.
   3. The encrypted challenge is sent to the user’s browser.
   4. The user’s browser decrypts the challenge with their private key.
   5. The decrypted challenge is signed (hashed) using the private key, and this signature is sent back to the server.
   6. The server verifies the signature against the original challenge message and the user’s public key. If it matches, authentication succeeds.

This proves the user possesses the private key associated with their account.

3. Why It’s Not a Simple Replacement

There are several significant drawbacks:

• Key Management: The biggest problem! Users need to securely store and manage their private keys. Losing the key means losing access. Backing up keys is complex and prone to security issues. Browser-based storage isn’t always secure enough.
• Revocation: If a user’s private key is compromised, you need a way to revoke it. This is difficult without a central authority (like a Certificate Authority).
• Complexity: Implementing this correctly requires deep cryptographic expertise. It’s easy to make mistakes that create security vulnerabilities.
• Performance: Asymmetric encryption is slower than symmetric encryption and hashing used in modern authentication. This can impact user experience, especially on mobile devices.
• Lack of Features: Modern authentication provides features like:
  • Multi-factor authentication (MFA)
  • Account recovery
  • Session management
  • Delegated authorization (OAuth 2.0)

  Asymmetric encryption doesn’t inherently provide these.

4. Modern Authentication Strategies: A Better Approach

Strategies like OAuth 2.0 and OpenID Connect are built on top of secure protocols (like TLS/SSL) and address the problems above.

• OAuth 2.0: Allows users to grant limited access to their data without sharing passwords.
• OpenID Connect: An identity layer on top of OAuth 2.0, providing user authentication information.

These protocols use tokens (short-lived credentials) and rely on established security practices.

5. When Asymmetric Encryption *Is* Useful in Authentication

Asymmetric encryption has a place in modern authentication, but not as the sole method.

• Key Exchange: Used to securely establish a shared secret key for symmetric encryption (e.g., during TLS/SSL handshake).
• Digital Signatures: Used to verify the integrity of data and authenticate messages.
• Passwordless Authentication (WebAuthn): WebAuthn uses asymmetric cryptography with hardware security keys (like YubiKeys) for strong, phishing-resistant authentication. This is a good example of where it’s used effectively.

Example: Generating a Challenge in Python

import secrets
import hashlib

def generate_challenge():
    # Generate a random challenge string
    random_string = secrets.token_hex(32)
    return random_string

def hash_challenge(challenge):
  hashed_challenge = hashlib.sha256(challenge.encode('utf-8')).hexdigest()
  return hashed_challenge

challenge = generate_challenge()
hashed_challenge = hash_challenge(challenge)
print("Challenge:", challenge)
print("Hashed Challenge:", hashed_challenge)

Conclusion

While theoretically possible, using asymmetric encryption as a complete replacement for modern authentication is complex and generally not recommended. Modern strategies offer better security, usability, and flexibility. Asymmetric encryption plays important roles within these systems but isn’t a suitable standalone solution.