Licensing system

Context

DataFuseAI ships to customers’ own servers. A license has to work on a machine that may never reach the internet, can’t be forged by editing a file, and has to be revocable when a contract ends. I didn’t know cryptography when the requirement arrived. These two public repositories are the companion to the work: the same design, built independently to learn and to show it.

My role

All of it: the study, the design, the server, the client, and the docs. The learning repo, cryptography-in-python-fundamentals, is the trail of how I got there.

Constraints

Offline verification. No shared secret on the client (anything on the customer’s box is readable). Tamper detection that fails closed. Revocation without a phone-home. An audit trail for who issued what, when.

What I built

Enlarge:

• Key generation on the server: an Ed25519 keypair; the private key never leaves the server.
• Signing. A license is a small signed document — customer, scope, expiry, feature flags — with the signature attached.
• Offline verification on the client: the public key ships with the client; verification is a signature check, no network.
• Tamper detection. Any edit to the license body breaks the signature; the client fails closed.
• Revocation via short-lived licenses and a revocation list the client applies when it does reach the server.
• Audit trail. Every issue, renewal, and revocation logged with actor and timestamp.
• Server in Django REST Framework; client in DRF + React.

Decisions and why

• Ed25519 over RSA: smaller keys, fast verification, fewer ways to misuse it.
• Signed documents over encrypted ones: the license isn’t secret; its integrity is what matters, and a public key on the client is safe.
• Short expiry plus a revocation list instead of always-online checks, so an offline customer keeps working and a cancelled one stops within the window.

Outcome

A working reference implementation, Apache-2.0, that a reader can run end to end. At work, the production system built on the same design shipped with the product.

What I’d change

Add key rotation from day one; retrofitting it means every client needs a new public key, which is exactly the offline problem again.

Links

The server, poc-license-server, the client, poc-offline-client, and the learning repo, cryptography-in-python-fundamentals, are linked at the top of this page.