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--------------------------------------------------
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BIP: 16
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Title: Pay to Script Hash
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Author: Gavin Andresen <gavinandresen@gmail.com>
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Status: Final
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Type: Standards Track
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Created: 2012-01-03
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--------------------------------------------------
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[[abstract]]
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Abstract
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~~~~~~~~
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This BIP describes a new "standard" transaction type for the Bitcoin
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scripting system, and defines additional validation rules that apply
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only to the new transactions.
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[[motivation]]
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Motivation
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~~~~~~~~~~
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The purpose of pay-to-script-hash is to move the responsibility for
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supplying the conditions to redeem a transaction from the sender of the
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funds to the redeemer.
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The benefit is allowing a sender to fund any arbitrary transaction, no
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matter how complicated, using a fixed-length 20-byte hash that is short
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enough to scan from a QR code or easily copied and pasted.
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[[specification]]
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Specification
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~~~~~~~~~~~~~
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A new standard transaction type that is relayed and included in mined
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blocks is defined:
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` OP_HASH160 [20-byte-hash-value] OP_EQUAL`
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[20-byte-hash-value] shall be the push-20-bytes-onto-the-stack opcode
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(0x14) followed by exactly 20 bytes.
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This new transaction type is redeemed by a standard scriptSig:
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` ...signatures... {serialized script}`
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Transactions that redeem these pay-to-script outpoints are only
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considered standard if the _serialized script_ - also referred to as the
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_redeemScript_ - is, itself, one of the other standard transaction
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types.
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The rules for validating these outpoints when relaying transactions or
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considering them for inclusion in a new block are as follows:
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1. Validation fails if there are any operations other than "push data"
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operations in the scriptSig.
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2. Normal validation is done: an initial stack is created from the
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signatures and \{serialized script}, and the hash of the script is
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computed and validation fails immediately if it does not match the hash
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in the outpoint.
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3. \{serialized script} is popped off the initial stack, and the
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transaction is validated again using the popped stack and the
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deserialized script as the scriptPubKey.
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These new rules should only be applied when validating transactions in
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blocks with timestamps >= 1333238400 (Apr 1 2012)
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footnote:[https://github.com/bitcoin/bitcoin/commit/8f188ece3c82c4cf5d52a3363e7643c23169c0ff[Remove
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-bip16 and -paytoscripthashtime command-line arguments]]. There are
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transaction earlier than 13333238400 in the block chain that fail these
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new validation rules.
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footnote:[http://blockexplorer.com/tx/6a26d2ecb67f27d1fa5524763b49029d7106e91e3cc05743073461a719776192[Transaction
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6a26d2ecb67f27d1fa5524763b49029d7106e91e3cc05743073461a719776192]].
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Older transactions must be validated under the old rules. (see the
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Backwards Compatibility section for details).
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For example, the scriptPubKey and corresponding scriptSig for a
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one-signature-required transaction is:
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` scriptSig: [signature] {[pubkey] OP_CHECKSIG}` +
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` scriptPubKey: OP_HASH160 [20-byte-hash of {[pubkey] OP_CHECKSIG} ] OP_EQUAL`
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Signature operations in the \{serialized script} shall contribute to the
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maximum number allowed per block (20,000) as follows:
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1. OP_CHECKSIG and OP_CHECKSIGVERIFY count as 1 signature operation,
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whether or not they are evaluated.
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2. OP_CHECKMULTISIG and OP_CHECKMULTISIGVERIFY immediately preceded by
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OP_1 through OP_16 are counted as 1 to 16 signature operation, whether
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or not they are evaluated.
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3. All other OP_CHECKMULTISIG and OP_CHECKMULTISIGVERIFY are counted as
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20 signature operations.
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Examples:
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+3 signature operations:
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` {2 [pubkey1] [pubkey2] [pubkey3] 3 OP_CHECKMULTISIG}`
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+22 signature operations
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` {OP_CHECKSIG OP_IF OP_CHECKSIGVERIFY OP_ELSE OP_CHECKMULTISIGVERIFY OP_ENDIF}`
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[[rationale]]
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Rationale
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~~~~~~~~~
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This BIP replaces BIP 12, which proposed a new Script opcode ("OP_EVAL")
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to accomplish everything in this BIP and more.
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The Motivation for this BIP (and BIP 13, the pay-to-script-hash address
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type) is somewhat controversial; several people feel that it is
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unnecessary, and complex/multisignature transaction types should be
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supported by simply giving the sender the complete \{serialized script}.
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The author believes that this BIP will minimize the changes needed to
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all of the supporting infrastructure that has already been created to
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send funds to a base58-encoded-20-byte bitcoin addresses, allowing
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merchants and exchanges and other software to start supporting
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multisignature transactions sooner.
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Recognizing one 'special' form of scriptPubKey and performing extra
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validation when it is detected is ugly. However, the consensus is that
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the alternatives are either uglier, are more complex to implement,
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and/or expand the power of the expression language in dangerous ways.
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The signature operation counting rules are intended to be easy and quick
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to implement by statically scanning the \{serialized script}. Bitcoin
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imposes a maximum-number-of-signature-operations per block to prevent
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denial-of-service attacks on miners. If there was no limit, a rogue
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miner might broadcast a block that required hundreds of thousands of
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ECDSA signature operations to validate, and it might be able to get a
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head start computing the next block while the rest of the network worked
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to validate the current one.
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There is a 1-confirmation attack on old implementations, but it is
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expensive and difficult in practice. The attack is:
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1. Attacker creates a pay-to-script-hash transaction that is valid as
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seen by old software, but invalid for new implementation, and sends
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themselves some coins using it.
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2. Attacker also creates a standard transaction that spends the
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pay-to-script transaction, and pays the victim who is running old
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software.
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3. Attacker mines a block that contains both transactions.
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If the victim accepts the 1-confirmation payment, then the attacker wins
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because both transactions will be invalidated when the rest of the
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network overwrites the attacker's invalid block.
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The attack is expensive because it requires the attacker create a block
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that they know will be invalidated by the rest of the network. It is
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difficult because creating blocks is difficult and users should not
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accept 1-confirmation transactions for higher-value transactions.
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[[backwards-compatibility]]
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Backwards Compatibility
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~~~~~~~~~~~~~~~~~~~~~~~
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These transactions are non-standard to old implementations, which will
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(typically) not relay them or include them in blocks.
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Old implementations will validate that the \{serialize script}'s hash
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value matches when they validate blocks created by software that fully
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support this BIP, but will do no other validation.
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Avoiding a block-chain split by malicious pay-to-script transactions
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requires careful handling of one case:
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* A pay-to-script-hash transaction that is invalid for new
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clients/miners but valid for old clients/miners.
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To gracefully upgrade and ensure no long-lasting block-chain split
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occurs, more than 50% of miners must support full validation of the new
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transaction type and must switch from the old validation rules to the
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new rules at the same time.
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To judge whether or not more than 50% of hashing power supports this
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BIP, miners are asked to upgrade their software and put the string
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"/P2SH/" in the input of the coinbase transaction for blocks that they
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create.
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On February 1, 2012, the block-chain will be examined to determine the
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number of blocks supporting pay-to-script-hash for the previous 7 days.
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If 550 or more contain "/P2SH/" in their coinbase, then all blocks with
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timestamps after 15 Feb 2012, 00:00:00 GMT shall have their
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pay-to-script-hash transactions fully validated. Approximately 1,000
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blocks are created in a week; 550 should, therefore, be approximately
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55% of the network supporting the new feature.
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If a majority of hashing power does not support the new validation
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rules, then rollout will be postponed (or rejected if it becomes clear
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that a majority will never be achieved).
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[[byte-limitation-on-serialized-script-size]]
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520-byte limitation on serialized script size
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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As a consequence of the requirement for backwards compatiblity the
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serialized script is itself subject to the same rules as any other
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PUSHDATA operation, including the rule that no data greater than 520
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bytes may be pushed to the stack. Thus is it not possible to spend a
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P2SH output if the redemption script it refers to is >520 bytes in
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length. For instance while the OP_CHECKMULTISIG opcode can itself accept
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up to 20 pubkeys, with 33-byte compressed pubkeys it is only possible to
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spend a P2SH output requiring a maximum of 15 pubkeys to redeem: 3 bytes
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+ 15 pubkeys * 34 bytes/pubkey = 513 bytes.
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[[reference-implementation]]
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Reference Implementation
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~~~~~~~~~~~~~~~~~~~~~~~~
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https://gist.github.com/gavinandresen/3966071
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[[see-also]]
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See Also
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~~~~~~~~
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* https://bitcointalk.org/index.php?topic=46538
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* The link:bip-0013.mediawiki[Address format for Pay to Script Hash BIP]
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* M-of-N Multisignature Transactions link:bip-0011.mediawiki[BIP 11]
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* link:bip-0016/qa.mediawiki[Quality Assurance test checklist]
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[[references]]
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References
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~~~~~~~~~~
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