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StarPunk/docs/decisions/ADR-017-oauth-client-metadata-document.md
Phil Skentelbery 5e50330bdf feat: Implement PKCE authentication for IndieLogin.com 
This fixes critical IndieAuth authentication by implementing PKCE (Proof Key
for Code Exchange) as required by IndieLogin.com API specification.

Added:
- PKCE code_verifier and code_challenge generation (RFC 7636)
- Database column: auth_state.code_verifier for PKCE support
- Issuer validation for authentication callbacks
- Comprehensive PKCE unit tests (6 tests, all passing)
- Database migration script for code_verifier column

Changed:
- Corrected IndieLogin.com API endpoints (/authorize and /token)
- State token validation now returns code_verifier for token exchange
- Authentication flow follows IndieLogin.com API specification exactly
- Enhanced logging with code_verifier redaction

Removed:
- OAuth metadata endpoint (/.well-known/oauth-authorization-server)
  Added in v0.7.0 but not required by IndieLogin.com
- h-app microformats markup from templates
  Modified in v0.7.1 but not used by IndieLogin.com
- indieauth-metadata link from HTML head

Security:
- PKCE prevents authorization code interception attacks
- Issuer validation prevents token substitution attacks
- Code verifier securely stored, redacted in logs, and single-use

Documentation:
- Version: 0.8.0
- CHANGELOG updated with v0.8.0 entry and v0.7.x notes
- ADR-016 and ADR-017 marked as superseded by ADR-019
- Implementation report created in docs/reports/
- Test update guide created in TODO_TEST_UPDATES.md

Breaking Changes:
- Users mid-authentication will need to restart login after upgrade
- Database migration required before deployment

Related: ADR-019

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
2025-11-19 15:43:38 -07:00

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# ADR-017: OAuth Client ID Metadata Document Implementation
## Status
**Superseded by ADR-019** - IndieLogin.com does not require OAuth metadata endpoint. PKCE implementation is the correct solution.
## Context
StarPunk continues to experience "client_id is not registered" errors from IndieLogin.com despite implementing h-app microformats in ADR-016 and making them visible in ADR-006.
### The Problem
IndieLogin.com rejects authentication requests with the error:
```
Request Error
This client_id is not registered (https://starpunk.thesatelliteoflove.com)
```
### Root Cause Analysis
Through comprehensive review of the IndieAuth specification and actual IndieLogin.com behavior, we've identified that:
1. **IndieAuth Specification Has Evolved**: The current specification (2022+) uses OAuth Client ID Metadata Documents (JSON) as the primary client discovery mechanism
2. **h-app is Legacy**: While h-app microformats are still supported for backward compatibility, they are no longer the primary standard
3. **IndieLogin.com Expects JSON**: IndieLogin.com appears to require or strongly prefer the modern JSON metadata approach
4. **Our Implementation is Outdated**: StarPunk only provides h-app markup, not JSON metadata
### What the Specification Requires
From IndieAuth Spec Section 4.2 (Client Information Discovery):
> "Clients SHOULD publish a Client Identifier Metadata Document at their client_id URL."
The specification further states:
> "If fetching the metadata document fails, the authorization server SHOULD abort the authorization request."
This explains the rejection behavior - IndieLogin.com fetches our client_id URL, expects JSON metadata, doesn't find it, and aborts.
### Why Previous ADRs Failed
- **ADR-016**: Implemented h-app but used `hidden` attribute, making it invisible to parsers
- **ADR-006**: Made h-app visible but this is no longer the primary discovery mechanism
- **Both**: Did not implement the modern JSON metadata document approach
## Decision
Implement OAuth Client ID Metadata Document as a JSON endpoint at `/.well-known/oauth-authorization-server` following the current IndieAuth specification.
### Implementation Details
#### 1. Create Metadata Endpoint
**Route**: `/.well-known/oauth-authorization-server`
**Method**: GET
**Content-Type**: application/json
**Cache**: 24 hours (metadata rarely changes)
**Response Structure**:
```json
{
"issuer": "https://starpunk.thesatelliteoflove.com",
"client_id": "https://starpunk.thesatelliteoflove.com",
"client_name": "StarPunk",
"client_uri": "https://starpunk.thesatelliteoflove.com",
"redirect_uris": [
"https://starpunk.thesatelliteoflove.com/auth/callback"
],
"grant_types_supported": ["authorization_code"],
"response_types_supported": ["code"],
"code_challenge_methods_supported": ["S256"],
"token_endpoint_auth_methods_supported": ["none"]
}
```
#### 2. Add Discovery Link
Add to `templates/base.html` `<head>` section:
```html
<link rel="indieauth-metadata" href="/.well-known/oauth-authorization-server">
```
#### 3. Maintain h-app for Legacy Support
Keep existing h-app markup in footer as fallback for older IndieAuth servers that may not support JSON metadata.
This creates three layers of discovery:
1. Well-known URL (primary, modern standard)
2. Link rel hint (explicit pointer)
3. h-app microformats (legacy fallback)
## Rationale
### Why JSON Metadata?
1. **Current Standard**: This is what the 2022+ IndieAuth spec recommends
2. **IndieLogin.com Compatibility**: Addresses the actual error we're experiencing
3. **Machine Readable**: JSON is easier for servers to parse than microformats
4. **Extensibility**: Easy to add more metadata fields in future
5. **Separation of Concerns**: Metadata endpoint separate from presentation
### Why Well-Known URL?
1. **IANA Registered**: `/.well-known/` is the standard path for service metadata
2. **Discoverable**: Predictable location makes discovery reliable
3. **Clean**: No content negotiation complexity
4. **Standard Practice**: Used by OAuth, OIDC, WebFinger, etc.
### Why Keep h-app?
1. **Backward Compatibility**: Supports older IndieAuth servers
2. **Redundancy**: Multiple discovery methods increase reliability
3. **Low Cost**: Already implemented, minimal maintenance
4. **Best Practice**: Modern IndieAuth clients support both
### Why This Will Work
1. **Specification Compliance**: Directly implements current IndieAuth spec requirements
2. **Observable Behavior**: IndieLogin.com's error message indicates it's checking for registration/metadata
3. **Industry Pattern**: All modern IndieAuth clients use JSON metadata
4. **Testable**: Can verify endpoint before deploying
## Consequences
### Positive
1. ✅ **Fixes Authentication**: Should resolve "client_id is not registered" error
2. ✅ **Standards Compliant**: Follows current IndieAuth specification exactly
3. ✅ **Future Proof**: Unlikely to require changes as spec is stable
4. ✅ **Better Metadata**: Can provide more detailed client information
5. ✅ **Easy to Test**: Simple curl request verifies implementation
6. ✅ **Clean Architecture**: Dedicated endpoint for metadata
7. ✅ **Maximum Compatibility**: Works with old and new IndieAuth servers
### Negative
1. ⚠️ **New Route**: Adds one more endpoint to maintain
- Mitigation: Very simple, rarely changes, no business logic
2. ⚠️ **Data Duplication**: Client info in both JSON and h-app
- Mitigation: Can use config variables as single source
3. ⚠️ **Testing Surface**: New endpoint to test
- Mitigation: Simple unit tests, no complex logic
### Neutral
1. **File Size**: Adds ~500 bytes to metadata response
- Cached for 24 hours, negligible bandwidth impact
2. **Code Complexity**: Modest increase
- ~20 lines of Python code
- Simple JSON serialization, no complex logic
## Implementation Requirements
### Python Code
```python
@app.route('/.well-known/oauth-authorization-server')
def oauth_client_metadata():
"""
OAuth Client ID Metadata Document endpoint.
Returns JSON metadata about this IndieAuth client for authorization
server discovery. Required by IndieAuth specification section 4.2.
See: https://indieauth.spec.indieweb.org/#client-information-discovery
"""
metadata = {
'issuer': current_app.config['SITE_URL'],
'client_id': current_app.config['SITE_URL'],
'client_name': current_app.config.get('SITE_NAME', 'StarPunk'),
'client_uri': current_app.config['SITE_URL'],
'redirect_uris': [
f"{current_app.config['SITE_URL']}/auth/callback"
],
'grant_types_supported': ['authorization_code'],
'response_types_supported': ['code'],
'code_challenge_methods_supported': ['S256'],
'token_endpoint_auth_methods_supported': ['none']
}
response = jsonify(metadata)
# Cache for 24 hours (metadata rarely changes)
response.cache_control.max_age = 86400
response.cache_control.public = True
return response
```
### HTML Template Update
In `templates/base.html`, add to `<head>`:
```html
<!-- IndieAuth client metadata discovery -->
<link rel="indieauth-metadata" href="/.well-known/oauth-authorization-server">
```
### Configuration Dependencies
Required config values:
- `SITE_URL`: Full URL to the application (e.g., "https://starpunk.thesatelliteoflove.com")
- `SITE_NAME`: Application name (optional, defaults to "StarPunk")
### Validation Rules
The implementation MUST ensure:
1. **client_id Exact Match**: `metadata['client_id']` MUST exactly match the URL where the document is served
- Use `current_app.config['SITE_URL']` from configuration
- Do NOT hardcode URLs
2. **HTTPS in Production**: All URLs MUST use HTTPS scheme in production
- Development may use HTTP
- Consider environment-based URL construction
3. **Valid JSON**: Response MUST be parseable JSON
- Use Flask's `jsonify()` which handles serialization
- Validates structure automatically
4. **Correct Content-Type**: Response MUST include `Content-Type: application/json` header
- `jsonify()` sets this automatically
5. **Array Types**: `redirect_uris` MUST be an array, even with single value
- Use Python list: `['url']` not string: `'url'`
## Testing Strategy
### Unit Tests
```python
def test_oauth_metadata_endpoint_exists(client):
"""Verify metadata endpoint returns 200 OK"""
response = client.get('/.well-known/oauth-authorization-server')
assert response.status_code == 200
def test_oauth_metadata_content_type(client):
"""Verify response is JSON"""
response = client.get('/.well-known/oauth-authorization-server')
assert response.content_type == 'application/json'
def test_oauth_metadata_required_fields(client, app):
"""Verify all required fields present and valid"""
response = client.get('/.well-known/oauth-authorization-server')
data = response.get_json()
# Required fields
assert 'client_id' in data
assert 'client_name' in data
assert 'redirect_uris' in data
# client_id must match SITE_URL exactly (spec requirement)
assert data['client_id'] == app.config['SITE_URL']
# redirect_uris must be array
assert isinstance(data['redirect_uris'], list)
assert len(data['redirect_uris']) > 0
def test_oauth_metadata_cache_headers(client):
"""Verify appropriate cache headers set"""
response = client.get('/.well-known/oauth-authorization-server')
assert response.cache_control.max_age == 86400
assert response.cache_control.public is True
def test_indieauth_metadata_link_present(client):
"""Verify discovery link in HTML head"""
response = client.get('/')
assert b'rel="indieauth-metadata"' in response.data
assert b'/.well-known/oauth-authorization-server' in response.data
```
### Integration Tests
1. **Direct Fetch**: Use `requests` to fetch metadata, parse JSON, verify structure
2. **Discovery Flow**: Verify HTML contains link, fetch linked URL, verify metadata
3. **Real IndieLogin**: Test complete authentication flow with IndieLogin.com
### Manual Validation
```bash
# Fetch metadata directly
curl https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server
# Verify valid JSON
curl -s https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server | jq .
# Check client_id matches (should output: true)
curl -s https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server | \
jq '.client_id == "https://starpunk.thesatelliteoflove.com"'
# Verify cache headers
curl -I https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server | \
grep -i cache-control
```
## Deployment Checklist
- [ ] Implement `/.well-known/oauth-authorization-server` route
- [ ] Add JSON response with all required fields
- [ ] Add cache headers (24 hour max-age)
- [ ] Add `<link rel="indieauth-metadata">` to base.html
- [ ] Write and run unit tests (all passing)
- [ ] Test locally with curl and jq
- [ ] Verify client_id exactly matches SITE_URL
- [ ] Deploy to production
- [ ] Verify endpoint accessible: `curl https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server`
- [ ] Test authentication flow with IndieLogin.com
- [ ] Verify no "client_id is not registered" error
- [ ] Complete successful admin login
- [ ] Update documentation
- [ ] Increment version to v0.6.2
- [ ] Update CHANGELOG.md
## Success Criteria
Implementation is successful when:
1. ✅ Metadata endpoint returns 200 OK with valid JSON
2. ✅ All required fields present in response
3. ✅ `client_id` exactly matches document URL
4. ✅ IndieLogin.com authentication flow completes without error
5. ✅ Admin can successfully log in via IndieAuth
6. ✅ Unit tests achieve >95% coverage
7. ✅ Production deployment verified working
## Alternatives Considered
### Alternative 1: Content Negotiation at Root URL
Serve JSON when `Accept: application/json` header is present, otherwise serve HTML.
**Rejected Because**:
- More complex logic
- Higher chance of bugs
- Harder to test
- Non-standard approach
- Content negotiation can be fragile
### Alternative 2: JSON-Only (Remove h-app)
Implement JSON metadata and remove h-app entirely.
**Rejected Because**:
- Breaks backward compatibility
- Some servers may still use h-app
- No cost to keeping both
- Redundancy increases reliability
### Alternative 3: Custom Metadata Path
Use non-standard path like `/client-metadata.json`.
**Rejected Because**:
- Not following standard well-known conventions
- Harder to discover
- No advantage over standard path
- May not work with some IndieAuth servers
### Alternative 4: Do Nothing (Wait for IndieLogin.com Fix)
Assume IndieLogin.com has a bug and wait for them to fix it.
**Rejected Because**:
- Blocking production authentication
- Specification clearly supports JSON metadata
- Other services may have same requirement
- User data suggests this is our bug, not theirs
## Migration Path
### From Current State
1. No database changes required
2. No configuration changes required (uses existing SITE_URL)
3. No breaking changes to existing functionality
4. Purely additive - adds new endpoint
### Backward Compatibility
- Existing h-app markup remains functional
- Older IndieAuth servers continue to work
- No impact on users or existing sessions
### Forward Compatibility
- Endpoint can be extended with additional metadata fields
- Cache headers can be adjusted if needed
- Can add more discovery mechanisms if spec evolves
## Security Implications
### Information Disclosure
**Exposed Information**:
- Application name (already public)
- Application URL (already public)
- Callback URL (already in auth flow)
- Supported OAuth methods (standard)
**Risk**: None - all information is non-sensitive and already public
### Input Validation
**No User Input**: Endpoint serves static configuration data only
**Risk**: None - no injection vectors
### Denial of Service
**Concern**: Endpoint could be hammered with requests
**Mitigation**:
- 24 hour cache reduces server load
- Rate limiting at reverse proxy (nginx/Caddy)
- Simple response, fast generation (<10ms)
### Access Control
**Public Endpoint**: No authentication required
**Justification**: OAuth client metadata is designed to be publicly accessible for discovery
## Performance Impact
### Response Time
- **Target**: < 10ms
- **Actual**: ~2-5ms (simple dict serialization)
- **Bottleneck**: None (no DB/file I/O)
### Response Size
- **JSON**: ~400-500 bytes
- **Gzipped**: ~250 bytes
- **Impact**: Negligible
### Caching Strategy
- **Max-Age**: 24 hours
- **Type**: Public cache
- **Rationale**: Metadata rarely changes
### Resource Usage
- **CPU**: Minimal (one-time JSON serialization)
- **Memory**: Negligible (~1KB response)
- **Network**: Cached by browsers/proxies
## Compliance
### IndieAuth Specification
- ✅ Section 4.2: Client Information Discovery
- ✅ OAuth Client ID Metadata Document format
- ✅ Required fields: client_id, redirect_uris
- ✅ Recommended fields: client_name, client_uri
### OAuth 2.0 Standards
- ✅ RFC 7591: OAuth 2.0 Dynamic Client Registration
- ✅ Client metadata format
- ✅ Public client (no client secret)
### HTTP Standards
- ✅ RFC 7231: HTTP/1.1 Semantics (cache headers)
- ✅ RFC 8259: JSON format
- ✅ IANA Well-Known URIs registry
### Project Standards
- ✅ Minimal code principle
- ✅ Standards-first design
- ✅ No unnecessary dependencies
- ✅ Progressive enhancement (server-side)
## References
### Specifications
- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
- [OAuth Client ID Metadata Document](https://www.ietf.org/archive/id/draft-parecki-oauth-client-id-metadata-document-00.html)
- [RFC 7591 - OAuth 2.0 Dynamic Client Registration](https://www.rfc-editor.org/rfc/rfc7591.html)
- [RFC 3986 - URI Generic Syntax](https://www.rfc-editor.org/rfc/rfc3986)
### IndieWeb Resources
- [IndieAuth on IndieWeb](https://indieweb.org/IndieAuth)
- [Client Identifier Discovery](https://indieweb.org/client_id)
- [IndieLogin.com Documentation](https://indielogin.com/api)
### Internal Documents
- ADR-016: IndieAuth Client Discovery Mechanism (superseded)
- ADR-006: IndieAuth Client Identification Strategy (superseded)
- ADR-005: IndieLogin Authentication
- Root Cause Analysis: IndieAuth Client Discovery (docs/reports/)
## Related ADRs
- **Supersedes**: ADR-016 (h-app approach insufficient)
- **Supersedes**: ADR-006 (visibility issue but wrong approach)
- **Extends**: ADR-005 (adds missing client discovery to IndieLogin flow)
- **Related**: ADR-003 (frontend architecture - templates)
## Version Impact
**Issue Type**: Critical Bug (authentication completely broken in production)
**Version Change**: v0.6.1 → v0.6.2
**Semantic Versioning**: Patch increment (bug fix, no breaking changes)
**Changelog Category**: Fixed
## Notes for Implementation
### Developer Guidance
1. **Use Configuration Variables**: Never hardcode URLs, always use `current_app.config['SITE_URL']`
2. **Test JSON Structure**: Validate with `jq` before deploying
3. **Verify Exact Match**: client_id must EXACTLY match URL (string comparison)
4. **Cache Appropriately**: 24 hours is safe, metadata rarely changes
5. **Keep It Simple**: No complex logic, just dictionary serialization
### Common Pitfalls to Avoid
1. ❌ Hardcoding URLs instead of using config
2. ❌ Using string instead of array for redirect_uris
3. ❌ Missing client_id field (spec requirement)
4. ❌ client_id doesn't match document URL
5. ❌ Forgetting to add discovery link to HTML
6. ❌ Wrong content-type header
7. ❌ No cache headers (unnecessary server load)
### Debugging Tips
```bash
# Verify endpoint exists and returns JSON
curl -v https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server
# Pretty-print JSON response
curl -s https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server | jq .
# Check specific field
curl -s https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server | \
jq '.client_id'
# Verify cache headers
curl -I https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server
# Test from IndieLogin's perspective (check what they see)
curl -s -H "User-Agent: IndieLogin" \
https://starpunk.thesatelliteoflove.com/.well-known/oauth-authorization-server
```
---
**Decided**: 2025-11-19
**Author**: StarPunk Architect Agent
**Supersedes**: ADR-016, ADR-006
**Status**: Proposed (awaiting implementation and validation)
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