Vestige protocol · v1.0.0

Proof of interaction.

Two keys co-sign a recent block. The chain records that they met, and when. Passkeys and device keys, no server to trust.

Try it in the lab Read the introduction
One address, every chain
0xc5b8A9e7175c102d51116249F121A65079224b6f
The contract
  • No owner, no admin, no upgrades
  • Verified, same CREATE2 address everywhere
  • Under 100k gas with the P-256 precompile

01 / How it works

A block hash is a timestamp nobody can fake

No one can predict a block's hash before the block exists. Sign over it and you prove you signed after that block. Get included in a later block and you prove it happened before that one. Vestige brackets an encounter between those two blocks.

The window of a co-signature A signs the hash of block n. B co-signs and the transaction is included at block n plus age. The contract accepts it only while age is at most the maxAge that A signed. block n A signs its hash n + age B co-signs, cosign() included n + maxAge window closes: StaleBlock maxAge is inside what A signs: nobody downstream can widen it

  1. The signer states

    A signs block n, its hash, and a window of maxAge blocks, bound to this chain and this contract. A can be a passkey or a raw P-256 device key.

  2. The cosigner answers

    B's passkey signs a challenge built from A's statement and A's key. It needs a fingerprint, a face or a PIN: the contract requires user verification.

  3. The chain records

    Anyone submits cosign(). The contract checks the hash, the window and both signatures, then emits Cosigned. A relayer can pay the gas but cannot change a thing.

02 / Two modes

A door that signs, or a hand to shake

The contract only knows signature formats. How you present them to people is the product.

Beacon · SignatureKind.P256

An unattended device signs every block

A kiosk, door reader or sensor holds a P-256 key and re-signs the latest block as it arrives. A visitor scans its QR code or taps its NFC tag and co-signs with a passkey. The device needs no wallet and no gas.

  • Check-ins and proof of attendance
  • Custody handovers at a fixed point
  • Device attestations from secure hardware

Handshake · SignatureKind.WebAuthn

Two people, two passkeys

A taps once to sign the statement with their passkey. B then has the window A declared to co-sign with theirs. Nobody installs a wallet; the phone's own authenticator does the cryptography.

  • Meeting someone at an event
  • Courier to recipient handoffs
  • Peer-to-peer vouching

03 / Guarantees

What a Cosigned event proves

Every check runs on-chain and reverts with a named error. The lab lets you try each attack and watch the contract refuse it.

Proves

  • After block n. B signed over A's statement, which carries a hash no one could know earlier.
  • Within A's window. The inclusion block is at most maxAge blocks later, and maxAge is signed by A.
  • These two keys. A forged hash, a swapped key, a replay or a self co-sign each fails with its own error.
  • A real person for B. Passkey assertions must carry the user-verification flag.

Does not prove

  • Physical proximity. That comes from the transport: a QR on a wall, an NFC tap, a BLE advert. Choose one that fits your threat model.
  • Who owns a key. Mapping keys to people, venues or rewards is your application's job.
  • Anything on chains with dishonest blockhash. Arbitrum, for one, returns pseudo-random hashes; Vestige is not deployed there.

04 / Integrate

Read the record, or gate on it

verify() is a view with every check and no state change; cosign() also records. Both return the cosignId and the age in blocks, so your contract can demand a tighter window than A's.

YourCheckIn.sol
// Only this venue's device, at most 10 blocks old.
(bytes32 cosignId, uint256 age) =
    vestige.verify(statement, cosigner, auth);
require(vestige.keyId(statement.signer) == venueKey);
require(age <= 10, "too old");
require(!used[cosignId]);
used[cosignId] = true;
read-record.js · ethers v6
const vestige = new ethers.Contract(
  "0xc5b8A9e7175c102d51116249F121A65079224b6f",
  abi, provider);
// Every encounter a given passkey co-signed:
const q = vestige.filters.Cosigned(null, null, keyId);
const logs = await vestige.queryFilter(q, from);
for (const log of logs) {
  const { signerKey, age } = log.args;
}

The introduction walks through building both signatures in a browser →

05 / Deployments

Live and verified

Deployed through Arachnid's CREATE2 factory with a zero salt, so the address commits to the bytecode. Anyone can put Vestige on a new EVM chain, and only this exact code can land there.

NetworkChain idExplorerSourcify
Ethereum1etherscan.io ↗exact match ↗
Base8453basescan.org ↗exact match ↗
Monad143monadscan.com ↗exact match ↗
Sepolia11155111sepolia.etherscan.io ↗exact match ↗
Base Sepolia84532sepolia.basescan.org ↗exact match ↗
Monad testnet10143testnet.monadscan.com ↗exact match ↗

solc 0.8.28 · optimizer 200 runs · cancun · runtime keccak 0xfa43f529…a1fde02e

Co-sign a block with your own passkey.

The lab runs the whole ceremony in your browser against Base Sepolia, Monad testnet or Sepolia. Signing and verify() are free; recording needs a little testnet gas in a browser wallet.

Open the lab Read the blog