Offline License Verify

The bug this exists to prevent

Licence checks are usually a request to your server. That works until any of these, all of which punish the paying customer:

• Your licence service is down, so nobody who paid can open the app.
• The customer is on a plane, or on a locked-down network, or behind a proxy that eats the call.
• You shut the service off in three years. Every copy sold stops working, including for
• people who paid once and were promised perpetual use.

Signature verification moves the check to the client permanently. You hold the private key and issue tokens; the app holds only the public key and can verify without asking anyone.

Why publishing this does not help a pirate

The security is the private key, not the algorithm. Anyone can read this code; nobody can forge a token without the key you never ship. That is the whole point of asymmetric signing, and it is why a verifier is safe to publish while an obfuscated home-rolled scheme is not.

What it does not stop is someone patching the check out of your binary. No client-side licence system can. This raises the effort from "share a key" to "modify and redistribute the app," which is the honest ceiling for offline verification — and worth stating plainly rather than selling this as protection it cannot provide.

The three failures the tests pin

• A tampered payload is rejected. Editing the expiry or tier after signing invalidates the
• signature — that is the entire mechanism, so it is asserted directly.
• A token signed by a different key is rejected, not merely a token with a bad signature. A
• verifier that checks structure but not provenance accepts anyone's correctly-formed licence.
• Junk never throws. Truncated tokens, wrong encoding, empty strings and nonsense all return
• a clean negative. A licence check that throws on malformed input becomes a crash on startup,
• which is a worse outcome than the unlicensed state it was guarding.

Entitlements and trials are carried in the signed payload, so they cannot be extended by editing local state.

Before you ship it

The bundled public key is a placeholder and the source says so at the point of definition. Shipping a build with it verifies nothing, because the matching private key is public too. Replace it with the one paired with your own key.

What this does NOT do

No key generation, no token issuing, no payment integration, no revocation. Revocation in particular is fundamentally at odds with offline verification — a client that cannot reach you cannot learn a licence was revoked, so use short expiries if you need that.

Verified

7 tests: a validly signed token returning its payload, a post-signing payload tamper rejected, a token from a different key rejected, junk handled without throwing, and entitlement/trial fields surviving verification.

Not covered: no test of clock skew or timezone handling around expiry, and nothing verifies behaviour when the host's Web Crypto implementation is absent.