TYPE 0x0503 · ANCHOR FAMILY

ANCHOR_POOL

VTXO pool anchor. Verifies a pool Merkle root and participant count. Used to commit the state of Ark-style virtual transaction output pools on-chain.

Anchor Invertible

Ladder Diagram

Fields

Field	Data Type	Size	Side	Description
vtxo_tree_root	HASH256	32 B	Conditions	Merkle root of the VTXO pool tree. The committed root must equal `SHA256(witness PREIMAGE)` at spend time — ANCHOR_POOL applies a hash-binding check via VerifyHashPreimageBinding.
participant_count	NUMERIC	1-4 B	Conditions	Optional. If present, must be `> 0` and decodable; absent is fine.
preimage	PREIMAGE	var	Witness	Preimage whose SHA-256 must equal `vtxo_tree_root`. Required: the witness layout is freeform (NO_IMPLICIT) but the evaluator rejects when no PREIMAGE accompanies the conditions HASH256.

Wire Format Breakdown

0x0503 0 2 HASH256 · 32B NUMERIC · 3B Conditions = 43 bytes

0x0503 0 1 PREIMAGE · var Witness ≈ 4 + 1 + len bytes

Total ≈ 48 + len bytes

Evaluation Logic

1.HasRequiredHashes(block, 1): at least one HASH256 must be present (the vtxo_tree_root); else → ERROR

2.VerifyHashPreimageBinding(block): pair each HASH256 with a witness PREIMAGE in field order; require SHA256(preimage) == hash256 for each pair. Insufficient PREIMAGEs, undersized HASH256, or any mismatch → UNSATISFIED

3.Look for an optional NUMERIC (participant_count). If present, ReadNumeric it; undecodable or value ≤ 0 → UNSATISFIED

4.All checks passed → SATISFIED

Return Values

Condition	Result
HASH256 field absent	ERROR
Witness has no PREIMAGE, or `SHA256(preimage) ≠ vtxo_tree_root`	UNSATISFIED
HASH256 not exactly 32 B	UNSATISFIED
Optional `participant_count` present and undecodable, or `≤ 0`	UNSATISFIED
Hash binding holds; `participant_count` absent or `> 0`	SATISFIED

JSON Wire Format

Conditions (VTXO pool with 128 participants — committed in the rung leaf)

{
  "type": "ANCHOR_POOL",
  "inverted": false,
  "fields": [
    { "type": "HASH256", "hex": "b3f1a7e9...vtxo tree root...32 bytes" },
    { "type": "NUMERIC", "value": 128 }
  ]
}

Witness (input)

{
  "type": "ANCHOR_POOL",
  "fields": [
    { "type": "PREIMAGE", "hex": "... preimage whose SHA256 is the vtxo_tree_root ..." }
  ]
}

Anchors a VTXO pool tree root with 128 participants. The root commits to all virtual transaction outputs in the pool; the spend reveals a preimage that hashes to the committed root.

Use Cases

Ark-style VTXO Pools

Commit the Merkle root of a virtual transaction output pool on-chain. Participants can prove their VTXO exists in the pool by providing a Merkle proof against the anchored root, enabling off-chain UTXO management with on-chain settlement guarantees.

Joinpool State Commitment

Anchor the current state of a joinpool — a shared UTXO managed by multiple participants. The tree root captures everyone's balance, while the participant count enables on-chain verification of pool size constraints.

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