Example: embed a hash

OP_RETURN script:
0x6a → OP_RETURN (marks immediate failure for any spending attempt)
<data>

Example:
6a 14 48656c6c6f2c20426974636f696e2057726c6421

Decoded:
6a → OP_RETURN
14 → push 20 bytes
48656c6c6f2c20426974636f696e20576f726c6421 → "Hello, Bitcoin World!"

def create_op_return_output(data: bytes) -> bytes:
    """
    Create an OP_RETURN output script.
    data: up to 80 bytes of arbitrary data
    """
    assert len(data) <= 80, "OP_RETURN data must be <= 80 bytes"

    script = bytes([0x6a])  # OP_RETURN

    if len(data) <= 75:
        # Direct push: length byte + data
        script += bytes([len(data)]) + data
    elif len(data) <= 255:
        # OP_PUSHDATA1
        script += bytes([0x4c, len(data)]) + data

    return script

# Example: embed a hash
document_hash = sha256(b"My important document")  # 32 bytes
op_return_script = create_op_return_output(document_hash)
# Result: 6a 20 <32-byte-hash>
# This proves the document existed at the time this transaction was confirmed

# 1. OpenTimestamps (timestamping service):
# Format: 0x6a 0x08 <OTS_MAGIC> <merkle_root>
OTS_MAGIC = bytes.fromhex("0x4f70656e54696d65")  # "OpenTime"

# 2. Omni Layer (USDT was built on this):
# Format: 0x6a 0x14 "omni" <omni_data>
OMNI_HEADER = b"omni"

# 3. Colored Coins / RGB protocol:
# Format: 0x6a <protocol_magic> <commitment>

# 4. Counterparty protocol:
# Format: 0x6a 0x28 "CNTRPRTY" <encrypted_data>
COUNTERPARTY_PREFIX = b"CNTRPRTY"

# 5. Ordinals inscriptions (content hash):
# Ordinals uses witness data, not OP_RETURN, but some use OP_RETURN for metadata

# Generic protocol identifier pattern:
def create_protocol_op_return(protocol_id: bytes, data: bytes) -> bytes:
    """
    Standard pattern for protocol-tagged OP_RETURN.
    protocol_id: 4-8 bytes identifying the protocol
    data: protocol-specific payload
    """
    payload = protocol_id + data
    assert len(payload) <= 80
    return create_op_return_output(payload)

Normal output lifecycle:
Created → Added to UTXO set → Spent → Removed from UTXO set

OP_RETURN output lifecycle:
Created → NEVER added to UTXO set → Never spent

Why this matters:

- Bitcoin full nodes must keep the entire UTXO set in memory (fast access)

- As of 2026: UTXO set is ~5GB and growing

- OP_RETURN data does NOT bloat the UTXO set

- It only adds ~100 bytes to the blockchain history (not RAM-resident index)

- This is why OP_RETURN is the accepted way to embed data
  (vs fake P2PKH outputs to random addresses, which pollute the UTXO set)