TYPE 0x0681 · PLC FAMILY

COSIGN

Co-spend constraint. Requires another input in the same transaction to have a spent output whose scriptPubKey hash matches the committed HASH256. Enforces cross-input dependencies for atomic multi-UTXO operations. No Tapscript equivalent exists.

PLC Cross-Input Non-Invertible

Ladder Diagram

Fields

Field	Data Type	Size	Side	Description
script_hash	HASH256	32 B	Conditions	SHA-256 hash of the anchor's conditions scriptPubKey that must appear as a spent output in another input

No witness fields required. The transaction context provides the spent outputs of all other inputs for cross-reference.

Wire Format Breakdown

0x0681 0 1 HASH256 · 32B Conditions = 38 bytes

0x0681 0 0 Witness = 4 bytes (empty block)

Total 42 bytes

Evaluation Logic

1.Read HASH256 field. If missing or not exactly 32 bytes → ERROR

2.If no transaction context or spent_outputs unavailable → SATISFIED (structural validation only)

3.For each other input in the transaction: compute SHA256(spent_output.scriptPubKey)

4.If any computed hash matches the committed HASH256 → SATISFIED

5.No match found across all other inputs → UNSATISFIED

Return Values

Condition	Result
HASH256 field missing	ERROR
HASH256 field not exactly 32 bytes	ERROR
No transaction context (structural check)	SATISFIED
No other input's spent scriptPubKey hash matches	UNSATISFIED
At least one other input's spent scriptPubKey hash matches	SATISFIED

JSON Wire Format

Conditions (commit to anchor scriptPubKey hash)

{
  "type": "COSIGN",
  "inverted": false,
  "fields": [
    { "type": "HASH256", "hex": "b4c5d6e7...scriptPubKey SHA256...32 bytes" }
  ]
}

This creates a co-spend requirement. The transaction must include another input whose spent output's scriptPubKey, when SHA-256 hashed, matches the committed hash.

Worked Example

HASH256 = SHA256(anchor_scriptPubKey)

TX has 2 inputs. Input[1].spent_output.scriptPubKey matches → SATISFIED

TX has 3 inputs. Input[2].spent_output.scriptPubKey matches → SATISFIED

TX has 1 input (only self). No other inputs to check → UNSATISFIED

TX has 2 inputs. No scriptPubKey hash matches → UNSATISFIED

No tx context (structural only) → SATISFIED

Use Cases

Atomic Multi-UTXO Operations

Ensure two UTXOs are always spent together in the same transaction. Each UTXO commits to the other's scriptPubKey hash via COSIGN, creating a bidirectional dependency that prevents spending one without the other.

Paired Covenant Spends

Link a data-carrying UTXO to a value-carrying UTXO. The value output requires the data output as a co-spend, ensuring state transitions always include the associated data payload in the same transaction.

Cross-Input Authorization

Require that a specific authorization UTXO (identified by its scriptPubKey hash) is consumed in the same transaction. This creates a physical "key" UTXO that must be presented alongside the "lock" UTXO to authorize spending.

← RATE_LIMIT TIMELOCKED_SIG →