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# Certificate System

**Date:** December 2025
**Technologies:** TypeScript, MongoDB, OpenBadges, Elysia.js

---

## Project Overview

The Certificate System is a template-driven engine that generates verifiable digital certificates for hackathon participants, event attendees, and course completers. It replaces manual certificate creation with an automated pipeline that resolves recipient data from team memberships, inherits design defaults from parent hackathons, and exports credentials compatible with the OpenBadges v2 standard.

The core insight is that certificates are rarely created in isolation. A hackathon organizer defines a template once (design, signatures, date range, title), and the system generates individual certificates for every team member automatically. When the hackathon date changes or a team gains new members, the template synchronizes all linked certificates without manual intervention.

The system also supports manual certificate creation for one-off awards, with the same OpenBadges compliance and revocation capabilities.

## Technical Stack

- **Backend**: Elysia.js with MongoDB native driver
- **Schema Validation**: Elysia `t` schema for request/response contracts
- **Badge Standard**: OpenBadges v2 (Issuer, BadgeClass, Assertion endpoints)
- **Image Processing**: Sharp for signature image optimization
- **Hashing**: SHA-256 for recipient identity verification in badge assertions

## Architecture Highlights

### Template-to-Certificate Synchronization

The `CertificateTemplateRepository` maintains bidirectional consistency between templates and their generated certificates. When a template is created or updated, the `generateCertificates` private method:

1. Fetches all existing certificates for the template
2. Deletes certificates for removed recipients (hard delete, as requested by the organization)
3. Updates existing certificates with new template values
4. Creates new certificates for added recipients

This ensures the certificate collection always reflects the current state of the template, eliminating stale data without requiring manual reconciliation.

<div style="max-width: 600px; margin: 0 auto;">

![Certificate example from GDG UAM's Smart Cities Hackathon.](../../../../assets/projects/gdguam/website/certificate-system/certificate.webp)

</div>

### Fallback Value Resolution

Certificates support a three-tier fallback chain for properties like `designId`, `title`, `signatures`, `startDate`, and `endDate`:

1. **Certificate-level override**: Stored directly on the certificate document
2. **Template-level value**: Inherited from the linked `CertificateTemplate`
3. **Hackathon defaults**: Resolved from `hackathon.certificateDefaults` or `hackathon.title`/`hackathon.date`

The `CertificateRepository.findById` method implements this via MongoDB aggregation with `$lookup` joins and `$ifNull` chains:

```typescript
$addFields: {
    designId: {
        $ifNull: ["$designId", "$template.designId", "$hackathon.certificateDefaults.designId", 0]
    },
    title: {
        $ifNull: ["$title", "$template.title", "$hackathon.certificateDefaults.title", "$hackathon.title", ""]
    }
}
```

This allows administrators to override specific fields on individual certificates while maintaining sensible defaults from the organizational context.

### Recipient Resolution from Teams

When a template is linked to a team, the `syncWithEntities` method resolves recipients by iterating over `team.users` and querying the `UserRepository` for each member. The resulting recipient list contains display names and user IDs, enabling both named certificates and badge assertions tied to verified accounts.

If a user cannot be found (e.g., a string identifier without a matching document), the system falls back to using the raw string as the recipient name, ensuring certificates are never blocked by data inconsistencies.

### OpenBadges v2 Compliance

The system exposes three public endpoints for badge verification:

- **`/badges/issuer`**: Organization metadata (name, URL, email, logo)
- **`/badges/class/:id`**: BadgeClass describing the achievement (title, description, criteria, issuer reference)
- **`/badges/assertion/:id`**: Assertion linking a recipient to a badge class with hashed email identity and issuance date

Badge assertions use SHA-256 hashing of the recipient's email address for privacy-preserving verification. Revoked certificates return 404 on all badge endpoints, preventing verification of invalidated credentials.

### Revocation and Soft Deletion

Certificates support both soft deletion (`isActive: false`) and revocation (`revoked: true`). Soft deletion hides certificates from public listings but preserves them for audit purposes. Revocation explicitly invalidates the credential, causing badge assertion endpoints to return 404.

The repository's `delete` method accepts a `soft` parameter, defaulting to `true` for safety. Bulk operations like `deleteByTemplateId` support both modes for administrative flexibility.