security: QR signature uses hex digest — migrate to constant-length base64url to prevent length-extension attacks

[BigBen-7]

Description

compute_signature in src/utils.py returns hmac.new(...).hexdigest() — a hex string. While HMAC-SHA256 is not directly vulnerable to length-extension attacks (unlike raw SHA256), using a hex digest doubles the output size unnecessarily and diverges from the JWT/PASETO convention of base64url encoding. More importantly, the validate-qr endpoint accepts the raw JSON-encoded QR string and extracts sig by key name, making it possible to craft a valid-looking payload that passes JSON parsing but includes extra unsigned fields.

Requirements & context

• Migrate compute_signature to return base64.urlsafe_b64encode(hmac_bytes).decode() instead of hexdigest()
• Update validate_qr to rebuild the unsigned dict by explicitly whitelisting known fields (ticket_id, event, user) rather than using {k: v for k, v in data.items() if k != "sig"} — unknown extra fields must be rejected
• Add a version field to the QR payload ("v": 2) so old hex-signed QR codes can be detected and rejected with a clear error message
• Write tests: extra unsigned field is rejected, old v1 hex signature is rejected with 200+isValid: false

Suggested execution

git checkout -b security/qr-signature-hardening

• Update src/utils.py compute_signature
• Update src/main.py validate_qr handler
• Bump QR payload version
• Update all related tests

Guidelines

• This is a breaking change for existing QR codes — document the migration in the PR
• PR must include: Closes #[issue_id]
• Timeframe: 72 hours

[PatrickKish1]

@PatrickKish1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

The issue calls out a real cryptographic foot-gun: hex digests and length-extension class concerns. I am applying because I am comfortable reading security-sensitive Python, designing a backward-compatible migration path, and adding tests that prove old and new verification behavior so ticket fraud and QR workflows stay trustworthy.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @PatrickKish1 to this issue.