'GPS-verified crypto' sounds like a privacy nightmare: a public ledger plus everyone's location. Done naively, it would be. This post walks through how DropHood verifies presence while making sure exact locations never reach the chain — because the interesting engineering is in what we DON'T store.
Step 1: the browser shares a position — once
When you claim, your browser asks for your location with a standard permission prompt, and sends it over TLS to the verifier for a single check. There's no background tracking, no location history, no SDK following you around. One claim, one position, one check.
Step 2: a pass/fail radius check
The verifier computes the great-circle (haversine) distance between your position and the drop's center. Inside the radius → pass; outside → fail. Crucially, a failed check never tells you how far away you are — returning distances would let an attacker probe from three points and triangulate the private center.
Step 3: what goes on-chain is a salted hash
The claim voucher commits to a locationHash: a one-way hash of the drop's center, radius and a secret salt. The chain can verify claims are bound to the right campaign without the campaign's coordinates ever being public. Even the drop's own listing only shows coordinates rounded to roughly a kilometre — enough to find the neighborhood, never the point.
Step 4: the physical-world check GPS can't fake
GPS can be spoofed by a motivated attacker — every location app knows this. That's why drops carry an on-site code displayed only at the venue: a fake GPS position without the code claims nothing. The code turns 'my phone says I'm here' into 'I read something that only exists here.'
What the claimer's privacy looks like
- Your exact coordinates are used for one distance check and are never written to the chain.
- The public record (the Proof of Hood NFT) attests you were inside a zone — not where exactly, not where you went after.
- No advertising SDKs, no location resale, no cross-site tracking. The analytics on the site are cookieless and aggregate.
Read the full architecture in the whitepaper → drophood.xyz/whitepaper