Understanding JWT Token Transformation: Bridging OAuth2 and PostgREST

TLDR: Here is the github repo to pull and try (you will need your own oauth2 provider like google etc) https://github.com/jcianci12/Client-Server-postgrest-jwt-demo

🍒 In modern web applications, authentication and authorization are critical components that often involve multiple systems working together. One common challenge is integrating OAuth2 authentication with PostgREST, which requires a specific JWT format. This is where JWT token transformation comes into play.

The Authentication Landscape

When building secure applications, we often need to:

Authenticate users through OAuth2 providers (like Google, GitHub, etc.)
Access protected resources through PostgREST
Implement Row-Level Security (RLS) in PostgreSQL

Each of these components has its own requirements for token format and claims, creating a need for transformation.

Why Transformation is Necessary

1. Different Systems, Different Requirements

🍒 OAuth2 providers issue tokens in their own format, with specific claim names and structures. For example:

{
  "iss": "https://accounts.google.com",
  "sub": "1234567890",
  "email": "user@example.com",
  "scope": "openid email profile"
}

PostgREST, on the other hand, expects a different format:

{
  "role": "authenticated",
  "sub": "1234567890",
  "email": "user@example.com"
}

2. Security Context Mapping

🍒 The transformation process isn’t just about changing field names. It’s about:

Mapping OAuth2 scopes to PostgREST roles
Ensuring proper permission inheritance
Maintaining security context across systems

3. Claims Standardization

🍒 Different OAuth2 providers use different claim names for the same information. For example:

Google uses email
GitHub might use user_email
Microsoft might use upn

The transformation middleware standardizes these into a consistent format that PostgREST can understand.

The Transformation Process

🍒 Here’s what happens during token transformation:

Validation
- Verify token signature
- Check expiration
- Validate required claims
Mapping
- Convert provider-specific claims to standard format
- Map scopes to roles
- Add any required PostgREST-specific claims
Security Checks
- Verify token hasn’t been revoked
- Check for required permissions
- Validate audience and issuer

Best Practices

🍒 When implementing token transformation:

Keep it Simple
- Transform only what’s necessary
- Maintain clear mapping rules
- Document all transformations
Security First
- Validate all incoming tokens
- Use secure signing keys
- Implement proper error handling
Performance
- Cache transformed tokens when possible
- Optimize transformation logic
- Monitor transformation latency

Real-World Example

🍒 Consider a scenario where a user authenticates with Google and needs to access a PostgREST API:

User logs in with Google OAuth2
Google issues an ID token
JWT middleware transforms the token
PostgREST receives the transformed token
RLS policies are applied based on the transformed claims

Conclusion

🍒 JWT token transformation is a crucial component in modern authentication architectures. It bridges the gap between different authentication systems while maintaining security and proper access control. By understanding and implementing proper token transformation, developers can create secure, flexible, and maintainable authentication systems.