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AIP-0005: FlightPath — Trustless Multi‑Chain SPV Verification for Avian

AIP:     0005
Title:   FlightPath — Trustless Multi-Chain SPV Verification for Avian
Author:  Craig Donnachie <craig.donnachie@gmail.com>
Status:  Draft
Type:    Consensus (soft fork)
Created: 2025-08-10
License: BSD-2-Clause

Abstract

FlightPath adds new Avian Script opcodes that enable on‑chain Simple Payment Verification (SPV) of transactions from external UTXO blockchains. With these opcodes, an Avian contract can trustlessly verify that a transaction on another chain (BTC/LTC/BCH/DOGE/RVN/EVR) paid an exact amount to an exact script and release AVN accordingly. This AIP covers trustless methods only (SPV/HTLC); custodian flows are out‑of‑scope.

Motivation

Avian currently supports HTLC‐style swaps but cannot validate external‑chain payments in‑script. FlightPath enables asymmetric, one‑contract swaps (e.g., BTC→AVN) where Avian verifies the external payment directly. This reduces coordination and expands interoperability without relying on third parties.

Specification

New Opcodes

  • OP_CHECKCHAINHEADERS Stack (top → bottom): headers_blob · min_height · min_chainwork · chain_tag Semantics:

    1. Parse headers_blob as contiguous 80‑byte headers.
    2. Verify PoW per chain_tag (target from nBits), and header linkage from the chain’s consensus anchor (height+hash) compiled into Avian.
    3. Accumulate chainwork; require ≥ min_chainwork and tip height ≥ min_height.
    4. Push tip merkle root (32B) then tip block hash (32B). Policy limits: max headers relayed 160 KiB; window caps MAY vary by chain.

  • OP_MERKLEVERIFY Stack: txid · merkle_root · branch_blob · leaf_index Recompute the merkle path using Bitcoin‐style double‑SHA256; push 1 on success, 0 otherwise. Policy: ≤32 siblings.

  • OP_CHECKBTCTXOUT Stack: raw_tx · vout_index · expected_amount · expected_scriptpubkey Minimally parse raw_tx; require output vout_index matches amount and scriptPubKey bytes; push 1/0. Policy: ≤200 KiB tx.

  • (Optional) OP_CHECKAUXPOW For AuxPoW chains (e.g., DOGE), validates linkage from the parent chain coinbase to the child merkle root (coinbase contains child merkle & chain‑id), including merkle‑of‑merkle path.

Chain Tags

Tag Chain PoW Difficulty/DAA Stage
1 BTC SHA256d Bitcoin DAA 0
2 LTC Scrypt Litecoin DAA 1
3 BCH SHA256d BCH DAA 1
4 DOGE Scrypt + AuxPoW Digishield 2
5 RVN KAWPOW RVN DAA 3
6 EVR KAWPOW EVR DAA 3

Script Template (External→AVN success path)

The redeem script releases AVN if the external chain payment is proven; otherwise, sender can refund after timeout.


OP_IF
  # Stack before:
  # raw_tx vout expected_amount expected_spk
  # txid branch_blob leaf_index
  # headers_blob min_height min_chainwork chain_tag
  OP_CHECKCHAINHEADERS      # → tip_merkle_root tip_block_hash
  OP_SWAP OP_DROP           # keep merkle_root
  OP_MERKLEVERIFY OP_VERIFY
  OP_CHECKBTCTXOUT OP_VERIFY
  OP_SHA256 <swap_id> OP_EQUALVERIFY
  OP_DUP OP_HASH160 <RecipientPKH> OP_EQUALVERIFY
  OP_CHECKSIG
OP_ELSE
  <locktime> OP_CHECKLOCKTIMEVERIFY OP_DROP
  OP_DUP OP_HASH160 <SenderPKH> OP_EQUALVERIFY
  OP_CHECKSIG
OP_ENDIF

Anchors

Each supported external chain has a consensus‑level anchor (height+hash). Nodes MUST verify that the first provided header links to the anchor. Anchors are set per release and may be updated periodically.

Deployment

  • VersionBits: use bit 28 (bit 27 reserved for dual‑algo 0=X16RT / 1=MinotaurX).
  • Threshold: 90% over a 2016‑block window; activation per BIP9‑style state machine.
  • Standardness: proofs preferred in witness data (P2WSH) with policy caps noted above.

Security Considerations

  • Anchors prevent validating adversarial alt‑forks.
  • Proof size limits and strict parsing mitigate DoS.
  • Callers should include ≥6 confirmations (or chain‑appropriate equivalents) in provided headers.
  • Exact amount and scriptPubKey matching prevents redirect attacks.

Rationale

A small, purpose‑built opcode set gives Avian trustless inbound swaps from multiple UTXO chains, aligns with FlightDeck (wallet) and FlightPlans (contracts), and avoids general‑purpose string ops.

Staged Rollout Plan

Phase 1 — LTC & BCH Implement LTC (scrypt) and BCH (sha256d) header/DAA rules and anchors. Phase 2 — DOGE (AuxPoW) Add AuxPoW verification (parent LTC headers, coinbase proof, merkle‑of‑merkle path) via OP_CHECKAUXPOW or chain‑specific branch inside OP_CHECKCHAINHEADERS. Phase 3 — RVN & EVR (KAWPOW) Bounded KAWPOW header verification (compact‑target check), chain‑specific DAAs, faster‑block policy adjustments.

Reference Implementation Notes

  • Consensus params: add ExtChainTag, PowAlgo, and per‑chain anchors in Params.
  • Interpreter: implement opcode cases; compute chainwork; reuse existing SHA256 primitives; add scrypt and bounded KAWPOW checks.
  • Policy: constants for max headers/branch/tx sizes; miners may enforce stricter relay.

Appendix A: Anchor Selection

Use each chain’s CLI to pick a recent, stable anchor and record height+hash:


bitcoin-cli   getblockhash <H_BTC>
litecoin-cli  getblockhash <H_LTC>
ravend-cli    getblockhash <H_RVN>
evrmore-cli   getblockhash <H_EVR>
bitcoin-cli   getblockcount  (to choose age)

Appendix B: C++ Stubs

Include enums (ExtChainTag, PowAlgo), ExtChainParams, and a helper EvalCheckChainHeaders(...) wired into OP_CHECKCHAINHEADERS; optional OP_CHECKAUXPOW verifier for DOGE.

Diagram

center|700px

End of AIP‑0005