BIP: 45 Title: Selector Script Author: Noel Tiernan <[email protected]> Status: Draft Type: Standards Track Created: 2014-04-25
This BIP describes a new "standard" transaction type for the Bitcoin scripting system
The purpose of the selector scipt is to allow a coin to be spent using more than 1 script.
Multiple scripts are provided in the sigPubKey arranged in an if-else chain. The spender selectes one of these scripts for spending.
This gives more flexibility when creating output scripts.
A new standard transaction type that is relayed and included in mined blocks is defined:
OPTION-1 OPTION-2 .... OPTION-N OPTION-CLOSE
Transaction with more than 5 options are considered non-standard.
OPTION-N is defined:
OP_DUP OP_N OP_EQUAL OP_IF OP_DROP {Standard Script} OP_ELSE
{Standard Script} is one of the standard scripts, except this type or P2SH.
Note: {Standard Script} is not serialized
OP_N is the single byte push opcode.
OPTION-CLOSE is defined:
OP_RETURN OP_ENDIF OP_ENDIF .... OP_ENDIF
The number of OP_ENDIFs is equal to the number of options.
A script with two options would be of the form:
OP_DUP
OP_1
OP_EQUAL
OP_IF
OP_DROP
{Standard Script}
OP_ELSE
OP_DUP
OP_2
OP_EQUAL
OP_IF
OP_DROP
{Standard Script}
OP_ELSE
OP_RETURN
OP_ENDIF
OP_ENDIF
The script compares compares the first item on the stack with each option in turn. Once a match is found, the index is dropped from the stack and the selected script is executed. If no matching option is found, the final else branch executes. This causes OP_RETURN to be executed.
The script is redeemed by selecting one of the options.
...signatures... OP_N
This transaction type increases flexibility. It allows the spender to choose one of many possible scripts to use for redeeming the output.
The desired redeemtion requirements can be different in different situations.
In the case of atomic cross chain transfers [1], there are two situations. If the other party completes the protocol, then the coin can be redeemed by the purchaser. If the process stalls, then the original owner of the coin can spend the coin as a refund.
In a betting situation, where an oracle determines the winner, there could be one option for each of the potential winners. The oracle would provide information, so that only one of the options can be used.
This BIP does not change the validation rules for the network, so there is no risk of network split.
Once a sufficient number of network nodes have updated, the new transactions should propagate to miners.
Only one script is executed, so the verification load is the same as a transaction which uses that script directly. Since that script must be a standard transaction, there is no additional CPU load.
Since all script options must be provided, these transactions are larger than standard transactions. Users of this transaction type would have to pay fees to cover the extra size.
TBD
[1] https://bitcointalk.org/index.php?topic=193281.0
This document is placed in the public domain.