XChain Platform Action - NODEPROOF

This action records an on-chain, quorum-signed verdict asserting which validators answered a periodic possession challenge, proving they run a real coin full node rather than mirroring the decoder/indexer DBs via xchain-sync.

PARAMS

Formats

Version 0 - Verdict (validator-broadcast)

• VERSION|CHALLENGE_ID|EPOCH_HEIGHT|PASS_COUNT|PASS_PK_1|...|PASS_PK_n|SIG_COUNT|PUBKEY_1|SIG_1|...|PUBKEY_m|SIG_m

Examples

NODEPROOF|0|a3f1e9...c42|1008|2|aa01bb...|cc02dd...|3|aa01bb...|sig1aaa...|cc02dd...|sig2bbb...|ee03ff...|sig3ccc...
A verdict for epoch height 1008, listing 2 passing validators, signed by 3 eligible verifiers

NODEPROOF|0|7b8d2a...f91|2016|1|ff00aa...|2|ff00aa...|sig4ddd...|bb11cc...|sig5eee...
A verdict for epoch height 2016 with 1 passing validator, signed by 2 verifiers (meeting 2f+1 quorum from 2 eligible)

Rules

• Available on BTC chain only; scope extends to other chains in a later version
• EPOCH_HEIGHT must be a multiple of CHALLENGE_INTERVAL_BLOCKS and must not exceed the current BLOCK_INDEX
• BLOCK_INDEX - EPOCH_HEIGHT must not exceed VERDICT_ACCEPT_WINDOW_BLOCKS; verdicts must land promptly to bound replay and reorg exposure
• The challenge target block is EPOCH_HEIGHT - CONFIRM_DEPTH; the target must be non-negative
• CHALLENGE_ID must equal SHA256(NETWORK ":" EPOCH_HEIGHT ":" ledger_hash(EPOCH_HEIGHT) ":" target) as recomputed by the indexer; any mismatch or unknown epoch block causes rejection
• Eligible verifiers at snapshotBlock = EPOCH_HEIGHT are verified_full_nodes(EPOCH_HEIGHT) union GENESIS_VERIFIERS; quorum is floor(2 * V / 3) + 1 where V is the eligible count; V == 0 causes rejection (feature dormant until GENESIS_VERIFIERS is seeded)
• Each signature must come from a member of the eligible set, must be a valid Ed25519 signature over the canonical message, and must not be duplicated; the total count of valid, deduplicated signers must meet quorum
• For each PASS_PK that holds full_node capability stake at EPOCH_HEIGHT, the indexer writes a full_node_verifications row; the insert is idempotent on (epoch_height, signing_pubkey)
• This action is validator-broadcast; it is not VM-emitted and not meaningful for end users
• This action writes no ledger rows; reward eligibility is derived later during PRICE finalization

Canonical signed message

The canonical message signed by each verifier is:

CHALLENGE_ID "|" EPOCH_HEIGHT "|" sort(lower(PASS_PK)) joined by ","

At and above the EQUIV flag-day, this canonical string is wrapped with the uniform header (TAG = XNODEPROOF, ROUND_ID = CHALLENGE_ID, VIEW = 0). Below the flag-day, the bare bytes are used. The indexer _buildCanonical and the hub publisher must agree byte-for-byte; this is consensus-critical.

Reward eligibility

A staking source earns the full-node reward tranche at block B only if:

• It has a passed full_node_verifications row with block_index in (B - PROOF_WINDOW_BLOCKS, B] and still holds full_node capability stake at B (this makes the node verifier-eligible: able to vouch in later verdicts)
• Over the trailing REWARD_PASS_WINDOW_BLOCKS, the source answered at least MIN_PASS_RATE_BPS basis points of the challenge epochs that actually produced a verdict (the denominator counts only epochs the federation ran, so an outage never costs anyone)

The gate uses integer math: passed_epochs * 10000 >= MIN_PASS_RATE_BPS * total_epochs. The verified set is deduplicated by staking source (one operator, one full node, one share). A validator that does not run a full node simply fails the challenges, earns nothing from the full-node tranche, and is never penalised: its full_node bond is untouched and there is no failed-challenge slash.

See db.getFullNodeParticipation and PRICE for the reward derivation.

The Derived Challenge

The challenge is derived deterministically from the chain, not broadcast. Every node (full and light, including the indexer) can recompute a challenge’s identity from on-chain data; only a node with a real coin full node can compute the answer. This avoids leader election, per-challenge fees, and the non-determinism of “who broadcasts the challenge.”

For each epoch height E where E % CHALLENGE_INTERVAL_BLOCKS == 0:

• seed = ledger_hash(E), the indexer’s stored per-block ledger hash at E (deterministic and identical across every honest node)
• target = E - CONFIRM_DEPTH, a buried block (reorg-stable) whose contents the possession query targets
• challenge_id = SHA256(NETWORK ":" E ":" seed ":" target)

Full-node verifiers map seed to a concrete query inside the target block: tx_index = int(seed) mod txcount, vout = int(seed[16:]) mod voutcount, answer = that output’s scriptPubKey (hex). This datum is provably absent from a synced mirror: the decoder stores no scriptPubKey, no raw tx/block bytes, no non-XChain transactions, and no UTXO set. The within-block mapping and the answer are resolved entirely by the verifiers (who have the block); the indexer never needs them and only recomputes challenge_id, trusting the quorum.

sequenceDiagram
    participant Chain
    participant Verifiers as Full-node verifiers
    participant Indexer

    Chain->>Chain: epoch height E reached, E mod CHALLENGE_INTERVAL_BLOCKS = 0
    Note over Chain: seed = ledger_hash(E), target = E - CONFIRM_DEPTH,<br>challenge_id = SHA256(NETWORK, E, seed, target)
    Verifiers->>Verifiers: map seed to target block, tx_index = int(seed) mod txcount,<br>vout = int(seed[16:]) mod voutcount
    Verifiers->>Verifiers: answer = that output's scriptPubKey, read from a real full node
    Verifiers->>Verifiers: sign canonical message, CHALLENGE_ID, EPOCH_HEIGHT, sorted PASS_PK list
    Verifiers->>Indexer: NODEPROOF v0 verdict, PASS_PK list plus verifier signatures
    Indexer->>Indexer: recompute challenge_id, verify each signature against the eligible verifier set
    Indexer->>Indexer: check quorum, floor(2 x V / 3) + 1 of eligible verifiers
    alt quorum met
        Indexer->>Indexer: record full_node_verifications row for each passing PASS_PK
    else quorum not met
        Indexer->>Indexer: reject verdict
    end

Config (STAKING.CAPABILITIES.full_node, FULLNODE)

Reorg Safety

full_node_verifications rows carry the verdict’s action_index and block_index. The generic action-rollback removes them on reorg exactly like any other action’s rows. The (epoch_height, signing_pubkey) unique key makes re-recording on replay idempotent.

Bootstrap and Degradation

With no seeded GENESIS_VERIFIERS and no verified nodes, no verdict can reach quorum, so the verified set is empty and the full-node tranche rolls back into the base tranche (see PRICE). Behavior is identical to pre-feature. The mechanism activates as the operator seeds genesis verifiers and independent full nodes join and get verified, at which point verified nodes (not just genesis) can vouch for new joiners.

Notes

• The verified set earns the full-node tranche of the oracle-round reward; light mirrors keep the base tranche only
• The only thing that reaches the wire is the federation’s signed verdict over who answered correctly, exactly the trust model of ATTEST v1 (the indexer verifies signatures and schedule; the federation verifies the off-chain fact)
• PUBKEY is the consensus token name for verifier public keys (consistent with ANCHOR, ATTEST, and PRICE)
• See ATTEST for the signature-verification and responsible-set template this models on
• See PRICE for oracle-round reward derivation (two-tranche split) and validator_rewards / COLLECT for the reward ledger and claim flow

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Based on XChain Platform by Dankest, LLC – https://dankest.llc

Licensed under the GNU Affero General Public License v3.0 (AGPL-3.0-or-later) with a commercial license available for proprietary use.

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