BA-12 · LESSON 01

OP_RETURN and data commitments

KEY DEFINITIONS

Terms used in this lesson

OP_RETURN: A Bitcoin Script operation commonly used to create an unspendable output carrying a small amount of data.
UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.
UTF-8: A standard variable-length byte encoding for Unicode text.
OP_PUSHDATA1: A Bitcoin Script data-push operation whose following byte states the length of an item from 76 through 255 bytes.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
ASCII: A standard mapping between common text characters and byte values.

OP_RETURN outputs carry provably unspendable data but do not themselves enforce later spending conditions.

35 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · data commitmentExternal meaning is anchored to exact Bitcoin bytesSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered validation pipeline

01
PAYLOADTyped external meaning
An application defines a payload, event, inscription, or token instruction.
02
ENCODECanonical bytes
Ordering, lengths, tags, and version rules must be unambiguous.
03
COMMITOP_RETURN and data commitments
The OP_RETURN and data commitments mechanism places or commits those bytes in a Bitcoin transaction.
04
BITCOINValid transaction
Bitcoin nodes enforce only Bitcoin consensus rules for the carrier transaction.
05
INDEXReconstructed state
Application software scans the chain and applies its own versioned state-transition rules.

Rule shown: Bitcoin validates transaction bytes and Script; indexers apply additional protocol meaning deterministically.

STAGE 1 OF 5PAYLOAD: Typed external meaning

An application defines a payload, event, inscription, or token instruction.

Governing relationship: Bitcoin validates transaction bytes and Script; indexers apply additional protocol meaning deterministically. Complete path: 1. PAYLOAD, Typed external meaning. An application defines a payload, event, inscription, or token instruction. 2. ENCODE, Canonical bytes. Ordering, lengths, tags, and version rules must be unambiguous. 3. COMMIT, OP_RETURN and data commitments. The OP_RETURN and data commitments mechanism places or commits those bytes in a Bitcoin transaction. 4. BITCOIN, Valid transaction. Bitcoin nodes enforce only Bitcoin consensus rules for the carrier transaction. 5. INDEX, Reconstructed state. Application software scans the chain and applies its own versioned state-transition rules.

1. State transitionWhat exists, what changes, and what results

BEFORE

Script bytes are 6a 04 74 65 73 74. The local node's teaching policy permits up to 80 payload bytes and enables data-carrier relay.

MECHANISM

An application defines its own prefix, version, and payload grammar. Bitcoin does not assign business meaning to arbitrary bytes.

AFTER

Data commitment, local relay policy, and application-state enforcement are separate layers.

Text equivalent: Starting from Script bytes are 6a 04 74 65 73 74. The local node's teaching policy permits up to 80 payload bytes and enables data-carrier relay. The mechanism begins when An application defines its own prefix, version, and payload grammar. Bitcoin does not assign business meaning to arbitrary bytes. The resulting lesson is Data commitment, local relay policy, and application-state enforcement are separate layers.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Encode application bytes

An application defines its own prefix, version, and payload grammar. Bitcoin does not assign business meaning to arbitrary bytes.

Complete path: 1. Encode application bytes: An application defines its own prefix, version, and payload grammar. Bitcoin does not assign business meaning to arbitrary bytes. 2. Create an unspendable output: The script begins OP_RETURN followed by data pushes. Its value is normally zero because no future input can spend it. 3. Apply relay policy: Nodes may limit data-carrier script forms and sizes for mempool relay. Current policy is implementation and configuration dependent, not a permanent consensus constant. 4. Commit through the transaction: The bytes affect transaction serialization and txid, then block commitments if mined. 5. Interpret externally: Indexers identify the application prefix and parse payload rules. Bitcoin nodes need not know or agree with that interpretation.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The complete pushed-byte count selects direct push or OP_PUSHDATA1, the declared length must equal carried bytes, the transaction commits the Script, and local relay policy is evaluated separately from consensus unspendability.

CHANGED CONDITION

Malformed length, nonminimal push, relay disabled, or a local byte limit can block relay; executing OP_RETURN still fails as a spend, while policy refusal alone cannot invalidate a mined transaction.

Important distinction: Putting a rule after OP_RETURN makes Bitcoin enforce that rule later. OP_RETURN makes the output unspendable. Later application meaning comes from external parsing unless another spendable output explicitly commits enforcement.

Text equivalent: The accepted path is enforced by The complete pushed-byte count selects direct push or OP_PUSHDATA1, the declared length must equal carried bytes, the transaction commits the Script, and local relay policy is evaluated separately from consensus unspendability. If the required condition changes, Malformed length, nonminimal push, relay disabled, or a local byte limit can block relay; executing OP_RETURN still fails as a spend, while policy refusal alone cannot invalidate a mined transaction.

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Learn the idea

OP_RETURN is a Bitcoin Script opcode that terminates execution as failure. Outputs beginning with it are provably unspendable, so nodes do not add them to the UTXO set. UTF-8 means Unicode Transformation Format, 8-bit, the byte encoding used by the text case. OP_PUSHDATA1 is the Script length prefix used when the next pushed item is too long for a one-byte direct-push opcode. Relay policy permits common bounded data-carrier forms, while consensus only requires the whole transaction and block remain valid under consensus limits.

GUIDED EXPLANATION1/5 ideas inspected

Encode application bytes

An application defines its own prefix, version, and payload grammar. Bitcoin does not assign business meaning to arbitrary bytes.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Putting a rule after OP_RETURN makes Bitcoin enforce that rule later.”

OP_RETURN makes the output unspendable. Later application meaning comes from external parsing unless another spendable output explicitly commits enforcement.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.