Concepts

Passkey guardian

How a phone unlock becomes an on-chain veto.

What a passkey is

A passkey is a key pair your device makes for a website. The private half never leaves the device (or your synced keychain) and is unlocked with Face ID, a fingerprint or the device PIN. Technically it is a WebAuthn credential on the P-256 curve.

Why it works on Robinhood Chain

Verifying a P-256 signature in plain Solidity is expensive (hundreds of thousands of gas or more). The chain offers it as a precompile at address 0x100. We tested it live on chain 4663: a valid signature returns 1, anything else returns nothing. That makes a veto cost about 87,000 gas.

The flow

The Guardian page asks your browser to create a passkey for the site's domain. We keep only its public point (x, y).
The vault is created with that point and a hash of the domain (rpIdHash).
To refuse announcement n, the browser asks your device to sign a challenge: the EIP-712 digest Veto(id, epoch) for this vault.
The browser returns authenticatorData, clientDataJSON and an ECDSA signature (r, s).
Anyone submits vetoWithSig(id, abi.encode(Auth)). The vault checks, in order: the domain hash, the "user present" and "user verified" flags, that the type is webauthn.get, that the challenge in the client data is exactly this digest, and the P-256 signature.

struct Auth {
  bytes   authenticatorData;
  string  clientDataJSON;
  uint256 challengeIndex;   // where "challenge":" starts in clientDataJSON
  uint256 typeIndex;        // where "type":"webauthn.get" starts
  uint256 r;
  uint256 s;
}

Bound to a domain

A passkey only signs for the website it was created on. A copy of the site on another domain cannot ask your device for this passkey, and a signature from another domain is refused by the vault. The flip side: choose the domain before you create a guardian passkey and do not move the site.

User verification is mandatory

The vault refuses a signature unless the device reports that the person was verified (Face ID, fingerprint, PIN), not merely present. A stolen unlocked phone still needs the biometric or PIN to whistle.

Backup and loss

Passkeys sync through iCloud Keychain or Google Password Manager. Check that yours does before trusting it.
If you lose the guardian passkey, the vault keeps working but nobody can refuse anything. Announce a new guardian, wait, execute. Until then you are protected only by luck.
For large amounts consider an ERC-1271 guardian, such as a Safe with two of three signers.

Relaying

The signature is bound to a specific vault, announcement and epoch, so a relayer can do nothing with it except submit the veto you meant. If a relayer ignores it, submit it yourself with the CLI (marmot relay) or from your wallet on the Lookout page.

Marmot docs