new token system

[madebygps]

This pull request introduces a comprehensive verification token system to the CTF platform, enabling participants to export a cryptographically signed completion certificate that can be validated externally. The changes include modifications to the setup, export, and testing scripts, as well as documentation updates and the addition of a Python script to verify tokens. This system ensures that CTF completions are securely tied to a user's GitHub identity and can be independently validated.

The most important changes are:

Verification Token System Implementation:

• Added generation of unique instance_id and a derived verification_secret during CTF setup, storing them in /etc/ctf/ for use in token signing (ctf_setup.sh). [1] [2]
• Enhanced the verify export command to require a GitHub username and generate a signed, base64-encoded verification token containing the username, instance ID, challenge count, and timestamp, with clear instructions for users. [1] [2] [3] [4] [5]
• Added a standalone verify_token.py script that verifies the exported token by checking the signature, payload fields, and matching the GitHub username, mimicking the production verification process.

Testing and Validation Enhancements:

• Expanded the test suite to include verification token tests, checking for secret file existence, format, and validating the structure and content of exported tokens.

Documentation Updates:

• Updated user and contributor documentation (README.md, .github/skills/ctf-testing/SKILL.md, CONTRIBUTING.md) to describe the new verification token workflow, including precise instructions for exporting and submitting the completion certificate. [1] [2] [3]

These changes collectively provide a secure, user-friendly, and verifiable way for participants to prove CTF completion, tightly integrating the process with GitHub authentication.