docs(dip27): version 2 Asset Unlock transactions with stable txids

[PastaPastaPasta]

Amends DIP-0027 (plus the DIP-2 registry and DIP-4) to introduce version 2 Asset Unlock transactions, activating with the v24 hard fork. Goal: make spends of unmined Platform withdrawals valid across Platform's re-signs and eligible for InstantSend, so Platform→Core transfers become rapidly respendable.

Design ("segwit-for-unlocks"): a version 2 Asset Unlock is serialized identically to version 1, but its transaction hash is computed with the signHeight, quorumHash, and quorumSig fields zeroed — exactly the fields Platform changes when it re-signs an expired withdrawal. Every re-signed instance of one withdrawal is therefore the same transaction with one txid: outputs are tracked and spent under it like any other transaction (no parallel identifier, no special spending rules for wallets), and children plus their InstantSend locks survive expiry and re-signing. Signature validity rules are unchanged from version 1, and the signed message hash still commits to signHeight and quorumHash.

Supporting changes:

• DIP-4 / CbTx version 4: merkleRootAssetUnlocks commits to the instance hashes (full-serialization hashes) of the block's version 2 Asset Unlocks, restoring the commitment the block merkle root no longer provides; mismatches are treated as block mutation.
• Relay: instances are announced by instance hash via a new MSG_ASSET_UNLOCK inventory type, since a txid-based announcement can never propagate a re-sign; nodes keep the instance with the higher signHeight and retain expired instances awaiting a replacement.
• Worked txid examples plus a reference script (dip-0027/dip-0027-txid-calc.py); vectors are pinned byte-for-byte against the Dash Core implementation's unit tests.
• Registry updates: Asset Unlock payload version 2, CbTx payload version 4.
• Two wording fixes on amended lines where the spec said "asset lock" but meant "Asset Unlock".

Implementation: dashpay/dash#7639.

🤖 Generated with Claude Code

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}