Proof of location
Proof of location is evidence that a device was at a place at a time, produced or checked by parties other than the device itself, so a claim does not rest only on self-reported GPS. Approaches include network timing measured by witnesses, radio contact with nearby devices, and trusted beacons. How much it proves depends on who the witnesses are and what they measure.
How it works
A device makes a claim, such as a coordinate or a coarse area and a time. One or more witnesses measure something they can observe on their own, such as the round-trip time of a network exchange, and sign what they measured. A verifier checks the signatures and decides whether the evidence is enough for its purpose.
Physics sets the limits. A round trip cannot be faster than the speed of light allows, so timing can rule out claims that are too far away, but a device can always add delay, so timing narrows a claim rather than pinning it. Colluding witnesses are a separate risk, which is why a system should say who its witnesses are.
Why it matters
GPS fixes are reported by the device and can be spoofed with inexpensive hardware, so any reward, payment, or access rule based on where someone was invites false claims. Independent evidence raises the cost of lying.
Location is also sensitive. Good designs record the least they need, such as a coarse area instead of a trajectory, and say plainly what becomes public and what is kept.
How it relates to Offline Protocol
Offline Protocol runs Proof of Location on Ethereum Sepolia testnet: the backend commits a coarse geohash, a cell of about 5 km, to an EigenLayer AVS contract, and a witness operator times a network round trip to the device and signs an EIP-191 attestation that your backend verifies.
It is witness-attested evidence, not a zero-knowledge proof or proof that a person was present. The default operator list has one witness, and slashing is not implemented. The security boundaries page in the docs lists the limits.
Frequently asked questions
Is proof of location the same as GPS?
No. GPS gives a position that the device computes and reports itself. Proof of location adds evidence from parties other than the device, so a verifier does not have to take the device's word for it.
Can proof of location be faked?
Some claims can. Timing-based witnesses rule out claims the speed of light makes impossible, but a device can add delay to make a farther claim fit, and colluding witnesses can sign false statements. Treat it as evidence, not certainty.
Does Offline Protocol offer proof of location?
Yes, on Ethereum Sepolia testnet: witness operators time a round trip and sign EIP-191 attestations, and a coarse geohash cell and time are public. Every plan includes verifications, 250 a month on Free.
