What is a Standard vs Non-Standard Script?

Bitcoin distinguishes between standard scripts and non-standard scripts. This distinction does not affect consensus validity — both types can appear in valid blocks — but it determines whether most nodes will relay a transaction through the peer-to-peer network.

Why the Distinction Exists

The Bitcoin P2P network uses relay policies to protect against spam and denial-of-service attacks. If any arbitrary script could be relayed freely, attackers could flood the network with complex, expensive-to-validate scripts. Standard script rules are a defense layer that predates the block size limit as a spam prevention mechanism.

The Five Original Standard Script Types

Bitcoin Core's IsStandard() function (in src/policy/policy.cpp) recognizes these output script types:

1. P2PK:    <pubkey> OP_CHECKSIG

2. P2PKH:   OP_DUP OP_HASH160 <hash> OP_EQUALVERIFY OP_CHECKSIG

3. P2SH:    OP_HASH160 <hash> OP_EQUAL

4. P2WPKH:  OP_0 <20-byte-hash>

5. P2WSH:   OP_0 <32-byte-hash>

6. P2TR:    OP_1 <32-byte-x-only-pubkey>

7. OP_RETURN: OP_RETURN <data up to 80 bytes>  (data carrier)

8. Bare multisig: OP_M <pubkeys> OP_N OP_CHECKMULTISIG (limited)

What Happens to Non-Standard Scripts

If a transaction contains a non-standard script:

1. Default nodes will not relay it to their peers

2. It can still be mined — there is no consensus rule against non-standard scripts

3. Miners must receive it directly (out-of-band) to include it in a block

Some miners offer "non-standard transaction" services for a premium fee. Once a non-standard transaction is mined into a block, all nodes accept it as consensus-valid.

Checking Standardness

# Pseudocode for standardness check:
def is_standard_output(scriptPubKey):
    if is_p2pkh(scriptPubKey):      return True
    if is_p2pk(scriptPubKey):       return True
    if is_p2sh(scriptPubKey):       return True
    if is_p2wpkh(scriptPubKey):     return True
    if is_p2wsh(scriptPubKey):      return True
    if is_p2tr(scriptPubKey):       return True
    if is_op_return(scriptPubKey):  return True  # data carrier
    if is_bare_multisig(scriptPubKey):
        return m <= 3  # Only up to 3-of-3 bare multisig
    return False  # Non-standard

The Evolution of Standard Scripts

The set of standard scripts has grown over time:

• 2009: P2PK, P2PKH, bare multisig

• 2012: P2SH added (BIP 16)

• 2014: OP_RETURN data carrier added

• 2017: P2WPKH, P2WSH added (SegWit BIP 141)

• 2021: P2TR added (Taproot BIP 341)

Each addition required a coordinated upgrade of Bitcoin Core's policy rules, separate from any consensus changes.