This BIP describes a new opcode (OP_CHECKHASHVERIFY) for the Bitcoin scripting system, and a new 'standard' transaction type that uses it to enables the receiver of bitcoins to specify the transaction type needed to re-spend them.
\nThis BIP is licensed under the BSD 2-clause license.
\nThe purpose of pay-to-script-hash is to move the responsibility for supplying the conditions to redeem a transaction from the sender of the funds to the redeemer.
\nThe benefit is allowing a sender to fund any arbitrary transaction, no matter how complicated, using a fixed-length 20-byte hash that is short enough to scan from a QR code or easily copied and pasted.
\nOP_CHECKHASHVERIFY will re-define the existing OP_NOP2 opcode, and will function as follows when executed:
\nThis opcode reassignment shall be applied when validating transactions in blocks only with timestamps after February 23, 2012 (see the Backwards Compatibility section for details).
\nA new standard transaction type that is relayed and included in mined blocks is defined:
\n[20-byte-hash-value] OP_CHECKHASHVERIFY OP_DROP
\n[20-byte-hash-value] shall be the push-20-bytes-onto-the-stack opcode (0x14) followed by exactly 20 bytes.
\nThis new transaction type is redeemed by a standard scriptSig:
\n...signatures... OP_CODESEPARATOR {script}
\nTransactions that redeem these pay-to-script outpoints are only considered standard if they contain exactly one OP_CODESEPARATOR and the appended script is, itself, one of the other standard transaction types.
\nFor example, the scriptPubKey and corresponding scriptSig for a one-signature-required transaction is:
\nscriptSig: [signature] OP_CODESEPARATOR [pubkey] OP_CHECKSIG scriptPubKey: [20-byte-hash of {[pubkey] OP_CHECKSIG} ] OP_CHECKHASHVERIFY OP_DROP
\n2-of-3:
\nscriptSig: [signatures...] OP_CODESEPARATOR 2 [pubkey1] [pubkey2] [pubkey3] 3 OP_CHECKMULTISIG scriptPubKey: [20-byte-hash of {2 [pubkey1] [pubkey2] [pubkey3] 3 OP_CHECKMULTISIG} ] OP_CHECKHASHVERIFY OP_DROP
\nThis BIP replaces BIP 12 and BIP 16, which propose evaluating a Script from the stack after verifying its hash.
\nThe Motivation for this BIP (and BIP 13, the pay-to-script-hash address type) is somewhat controversial; several people feel that it is unnecessary, and complex/multisignature transaction types should be supported by simply giving the sender the complete {serialized script}. The author believes that this BIP will minimize the changes needed to all of the supporting infrastructure that has already been created to send funds to a base58-encoded-20-byte bitcoin addresses, allowing merchants and exchanges and other software to start supporting multisignature transactions sooner.
\nThere is a 1-confirmation attack on old implementations, but it is expensive and difficult in practice. The attack is:
\nIf the victim accepts the 1-confirmation payment, then the attacker wins because both transactions will be invalidated when the rest of the network overwrites the attacker's invalid block.
\nThe attack is expensive because it requires the attacker create a block that they know will be invalidated by the rest of the network. It is difficult because creating blocks is difficult and users should not accept 1-confirmation transactions for higher-value transactions.
\nThese transactions are non-standard to old implementations, which will (typically) not relay them nor include them in blocks.
\nOld implementations will not validate that the {script}'s hash value matches when they validate blocks created by software that fully support this BIP.
\nAvoiding a block-chain split by malicious pay-to-script transactions requires careful handling of one case:
\nTo gracefully upgrade and ensure no long-lasting block-chain split occurs, more than 50% of miners must support full validation of the new transaction type and must switch from the old validation rules to the new rules at the same time.
\nTo judge whether or not more than 50% of hashing power supports this BIP, miners are asked to upgrade their software and put the string \"p2sh/CHV\" in the input of the coinbase transaction for blocks that they create.
\nOn February 8, 2012, the block-chain will be examined to determine the number of blocks supporting pay-to-script-hash for the previous 7 days. If at least 60% contain \"p2sh/CHV\" in their coinbase, then all blocks with timestamps after 23 Feb 2012, 00:00:00 GMT shall have their pay-to-script-hash transactions validated.
\nIf a majority of hashing power does not support the new validation rules, then rollout will be postponed (or rejected if it becomes clear that a majority will never be achieved).
\nOP_NOP2 is used, so existing OP_EVAL (BIP 12) transactions in the block chain can still be redeemed.
\nValidation, sending, and receiving for bitcoind git master
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