Offline Device Authentication

TL;DR

This guide explains how to securely authenticate users to a device that isn’t always connected to the internet. We’ll focus on using local accounts with strong passwords, and consider methods like token-based authentication for improved security when intermittent connectivity is available.

1. Understanding the Challenge

Authenticating offline devices differs from typical cloud-based systems because you can’t constantly verify credentials against a central server. This means storing user data (or hashes of passwords) directly on the device, which introduces security risks if the device is compromised.

2. Local Accounts with Strong Passwords

1. Create User Accounts: Each user needs a unique account on the device. Avoid default usernames and passwords.
2. Password Complexity: Enforce strong password policies:
   • Minimum length (e.g., 12 characters)
   • Mix of uppercase, lowercase letters, numbers, and symbols
   • Regular password changes (optional, but recommended)
3. Password Hashing: Never store passwords in plain text! Use a strong hashing algorithm like bcrypt or Argon2.

   # Example using Python's passlib library
   import passlib.hash
   hash = passlib.hash.bcrypt.hash('mysecretpassword')
   print(hash)
   

4. Salt: Always use a unique salt for each password hash to prevent rainbow table attacks. Modern hashing libraries handle this automatically.

3. Token-Based Authentication (for intermittent connectivity)

If the device connects to the internet occasionally, you can implement token-based authentication:

1. Initial Login: When a user first logs in *while connected*, authenticate them against your server using their credentials.
2. Generate Token: Upon successful authentication, the server generates a secure token (e.g., JWT – JSON Web Token).
3. Store Token Locally: The device stores this token securely.
4. Subsequent Access: For subsequent access *while offline*, the device uses the stored token instead of requiring a password.

   # Example JWT generation (using Python's PyJWT library)
   import jwt
   import datetime
   payload = {'username': 'user123', 'role': 'admin'}
   token = jwt.encode(payload, 'your_secret_key', algorithm='HS256')
   print(token)
   

5. Token Expiration: Tokens should have a limited lifespan (e.g., 1 hour, 1 day).
6. Refresh Token: Implement a refresh token mechanism to obtain new tokens when the current one expires and the device is online.
   • The server issues both an access token *and* a refresh token.
   • When the access token expires, the device uses the refresh token to request a new access token (without requiring the user’s password).

4. Secure Storage

Protecting sensitive data on the device is crucial:

• Encryption: Encrypt the entire storage volume or at least the directory containing user credentials and tokens. Use strong encryption algorithms (e.g., AES).
• Key Management: Securely store the encryption key. Avoid hardcoding it in the application code. Consider using hardware security modules (HSMs) if available.
• Tamper Detection: Implement mechanisms to detect unauthorized modifications to the storage volume or critical files.

5. Two-Factor Authentication (2FA)

Adding 2FA significantly enhances security, even offline:

• Time-Based One-Time Passwords (TOTP): Generate TOTP codes locally using an algorithm like HOTP/TOTP. The user needs a shared secret to generate the codes.
  • The initial setup requires online connectivity to share the secret securely.
  • Offline access relies on the user’s ability to generate TOTP codes independently (e.g., using an authenticator app).
• Biometrics: Use device biometrics (fingerprint, face recognition) as a second factor.
  • Ensure biometric data is stored securely by the device’s operating system.

6. Regular Security Audits

Periodically review your implementation for vulnerabilities and ensure it aligns with best security practices. Consider penetration testing to identify potential weaknesses.