Bitcoin Script

Bitcoin Script is a stack-based, Forth-like programming language used to define spending conditions for transaction outputs. It’s intentionally limited (not Turing-complete) for security.

How Script Works

When spending a UTXO:

1. scriptSig (unlocking script) executes first
2. Stack is passed to scriptPubKey (locking script)
3. If final stack is non-empty and top value is true, spend succeeds

scriptSig || scriptPubKey → Stack Execution → True/False

Stack Operations

Initial:  []
PUSH 5:   [5]
PUSH 3:   [5, 3]
OP_ADD:   [8]
PUSH 8:   [8, 8]
OP_EQUAL: [1]  // True

Common Opcodes

Constants

Opcode	Hex	Description
OP_0	0x00	Push empty array
OP_1 to OP_16	0x51-0x60	Push 1-16
OP_PUSHDATA1	0x4c	Next byte is data length
OP_PUSHDATA2	0x4d	Next 2 bytes are length

Stack Operations

Opcode	Hex	Description
OP_DUP	0x76	Duplicate top item
OP_DROP	0x75	Remove top item
OP_SWAP	0x7c	Swap top two items
OP_PICK	0x79	Copy nth item to top

Crypto Operations

Opcode	Hex	Description
OP_SHA256	0xa8	SHA-256 hash
OP_HASH160	0xa9	SHA-256 then RIPEMD-160
OP_HASH256	0xaa	Double SHA-256
OP_CHECKSIG	0xac	Verify signature
OP_CHECKMULTISIG	0xae	Verify m-of-n sigs

Flow Control

Opcode	Hex	Description
OP_IF	0x63	Execute if top is true
OP_ELSE	0x67	Else branch
OP_ENDIF	0x68	End if block
OP_VERIFY	0x69	Fail if top is false
OP_RETURN	0x6a	Mark output unspendable

Comparison

Opcode	Hex	Description
OP_EQUAL	0x87	Push 1 if equal
OP_EQUALVERIFY	0x88	OP_EQUAL + OP_VERIFY

Standard Script Types

P2PKH (Pay-to-Public-Key-Hash)

Most common legacy format.

scriptPubKey:

OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG

scriptSig:

<signature> <pubKey>

Execution:

Stack: [sig, pubKey]
OP_DUP: [sig, pubKey, pubKey]
OP_HASH160: [sig, pubKey, pubKeyHash]
<pubKeyHash>: [sig, pubKey, pubKeyHash, expected]
OP_EQUALVERIFY: [sig, pubKey]  // Verify hashes match
OP_CHECKSIG: [1]  // Verify signature

P2SH (Pay-to-Script-Hash)

Allows complex scripts with simple addresses.

scriptPubKey:

OP_HASH160 <scriptHash> OP_EQUAL

scriptSig:

<data> ... <redeemScript>

The redeemScript is revealed and executed when spending.

P2WPKH (Native SegWit)

scriptPubKey:

OP_0 <20-byte pubKeyHash>

witness:

<signature> <pubKey>

No scriptSig—witness data is separate.

P2TR (Taproot)

scriptPubKey:

OP_1 <32-byte tweaked pubKey>

Can be spent via:

1. Key path (Schnorr signature)
2. Script path (reveal script from merkle tree)

Multisig Scripts

Bare Multisig (Obsolete)

OP_2 <pubKey1> <pubKey2> <pubKey3> OP_3 OP_CHECKMULTISIG

2-of-3 multisig. Limited, rarely used.

P2SH Multisig

redeemScript:

OP_2 <pubKey1> <pubKey2> <pubKey3> OP_3 OP_CHECKMULTISIG

scriptSig:

OP_0 <sig1> <sig2> <redeemScript>

Note: OP_0 due to off-by-one bug in OP_CHECKMULTISIG.

Time Locks

Absolute Time Lock

OP_CHECKLOCKTIMEVERIFY (CLTV)

<expiry_time> OP_CHECKLOCKTIMEVERIFY OP_DROP
<normal spending conditions>

Cannot spend until block height or unix time.

Relative Time Lock

OP_CHECKSEQUENCEVERIFY (CSV)

<relative_delay> OP_CHECKSEQUENCEVERIFY OP_DROP
<normal spending conditions>

Cannot spend until N blocks/seconds after UTXO confirmation.

Hash Locks (HTLC)

Hash Time-Locked Contracts for atomic swaps and Lightning:

OP_IF
    OP_SHA256 <hash> OP_EQUALVERIFY
    <recipient_pubkey> OP_CHECKSIG
OP_ELSE
    <timeout> OP_CHECKLOCKTIMEVERIFY OP_DROP
    <sender_pubkey> OP_CHECKSIG
OP_ENDIF

Spend paths:

1. Reveal preimage + recipient signature (before timeout)
2. Sender signature (after timeout)

OP_RETURN

Creates provably unspendable output for data storage:

OP_RETURN <data up to 80 bytes>

Used for:

• Colored coins metadata
• Proof of existence
• Protocol messages

Script Validation

Standard vs Non-Standard

Nodes relay only “standard” scripts:

• P2PKH, P2SH, P2WPKH, P2WSH, P2TR
• Bare multisig (limited)
• OP_RETURN (single, ≤80 bytes)
• Null data

Non-standard scripts can still be mined but won’t relay.

Resource Limits

Limit	Value
Max script size	10,000 bytes
Max stack size	1,000 items
Max item size	520 bytes
Max ops per script	201
Max pubkeys in multisig	20

For Agents

Script Analysis

def analyze_script(script_hex):
    # Parse opcodes
    opcodes = parse_opcodes(script_hex)

    # Identify type
    if matches_p2pkh(opcodes):
        return "P2PKH"
    elif matches_p2sh(opcodes):
        return "P2SH"
    elif matches_p2wpkh(opcodes):
        return "P2WPKH"
    elif matches_p2tr(opcodes):
        return "P2TR"
    else:
        return "Non-standard"

When to Care About Script

• Verifying transactions: Understand spending conditions
• Complex contracts: Multisig, timelocks, HTLCs
• Lightning Network: Channel funding and commitment transactions
• Protocol development: Building on Bitcoin

Libraries

Language	Library
JavaScript	bitcoinjs-lib
Python	python-bitcoinlib
Rust	rust-bitcoin

---

Machine-Readable Summary

{
  "topic": "bitcoin-script",
  "type": "stack-based",
  "turing_complete": false,
  "standard_types": ["p2pkh", "p2sh", "p2wpkh", "p2wsh", "p2tr"],
  "limits": {
    "max_script_size": 10000,
    "max_stack_items": 1000,
    "max_ops": 201
  },
  "key_opcodes": ["OP_CHECKSIG", "OP_HASH160", "OP_EQUALVERIFY", "OP_IF", "OP_CHECKLOCKTIMEVERIFY"]
}