Snapshot Fraud Proofs

A posted snapshot root is an optimistic claim. It is final only if unchallenged for SNAPSHOT_CHALLENGE_WINDOW_SECS (1 day) OR survives every challenge. Fraud is proven on-chain by one of five predicates in the FraudProof enum.

Bond economics

Actor	Bond	Outcome on valid fraud	Outcome on false challenge
Poster (on post_snapshot)	1 × max-appeal-fee	forfeited (+ challenger's bond → challenger)	returned (on finalize_snapshot)
Challenger (on challenge_snapshot)	equal to poster's	returned + receives poster's bond	forfeited → poster

Both sweeps are PDA-signed out of the Subaccord vault. Wrong root ⇒ Snapshot.status = Voided (never drawable). False challenge ⇒ snapshot stays Posted (window still open).

Predicates

#	Predicate	What it catches	Witness	Resolution
1	Duplicate	Two leaves at different indices with the same juror pubkey, both verifying against the root.	leaf_a, proof_a, index_a, leaf_b, proof_b, index_b	void
2	Omission	The snapshot dropped a staked juror.	Two adjacent sorted leaves bracketing challenger.key() (leaf_lo.juror < challenger < leaf_hi.juror, consecutive indices) + challenger's JurorStake (remaining_accounts[0]) showing last_change_slot < anchor_slot && amount > 0.	void
3	WrongStake	A leaf's stake ≠ the juror's actual anchor-time stake.	leaf, proof, index + the juror's JurorStake (remaining_accounts[0]) with last_change_slot < anchor_slot ⇒ live amount is the anchor-time amount; require amount != leaf.stake.	void
4	Inflation (at draw)	Leaf overstates a juror's stake.	Enforced in draw, not challenge_snapshot: JurorStake.amount ≥ leaf.stake. Reads live state — race-immune.	draw reverts InflatedStake
5	NotSorted	Tree not sorted by juror pubkey ascending (breaks omission proofs).	Two leaves index_lo < index_hi with leaf_lo.juror > leaf_hi.juror, both verifying against the root.	void

Predicates 1, 2, 3, 5 fire in challenge_snapshot (window-gated). Predicate 4 fires in draw (every draw, every leaf).

Anchor-slot witness

JurorStake.last_change_slot is the slot of the most recent stake/unstake. If last_change_slot < Snapshot.anchor_slot, the live JurorStake.amount equals the anchor-time amount — the live account is its own historical witness. No ring buffer, no epoch snapshot. Closes the TOCTOU gap for predicates 2 and 3.

sequenceDiagram
    participant P as Poster
    participant V as Vault
    participant C as Challenger
    participant S as Snapshot
    P->>V: post_snapshot (bond 1×max-appeal-fee)
    S-->>S: status=Posted, anchor_slot=now
    C->>V: challenge_snapshot (equal bond) + FraudProof
    alt valid fraud
        V->>C: 2× bond (poster's + challenger's)
        S-->>S: status=Voided
    else false challenge
        V->>P: challenger's bond
        S-->>S: status stays Posted
    end

Why: ADR-0003, ADR-0008. Sortition consumption: sortition & VRF.