M of 2 DRAFT: Difference between revisions

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==Security==
==Security==


Although it may appear insecure totalling the sum of the public keys this is not actually the case. The resulting number is a massively large unit256.
Although it may appear insecure totalling the sum of the public keys this is not actually the case. The resulting number is still a massively large unit256.


For example if you wanted to find a SHA256 public key matching a target number you could theoretically brute force it as follows:
For example if you wanted to find a SHA256 public key matching a target number you could theoretically brute force it as follows:

Revision as of 22:18, 15 January 2012

  BIP: Unassigned
  Title: M-of-2 scriptPubKey
  Author: Ben Reeves <support@pi.uk.com>
  Status: Draft
  Type: Standards Track
  Created: 15-01-2012

Abstract

This BIP describes a new standard type of scriptPubKey designed to allow immediate use of split key wallets.

Motivation

The purpose of this proposal is to provide split key wallet functionality without requiring changes to block validation rules or risking a fork in the blockchain.

Specification

A new standard scriptPubKey will be defined: 

  OP_2OVER OP_2OVER OP_ADD OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG

Redeemed by a new standard scriptSig 

  OP_0  <sig> 1 [pubkey] [pubkey] 2

This proposal only supports A + B or A or B split key transactions.

Use Cases

  • Split key wallets
  • Escrow Services - The author does not believe that (A + B) or C scripts are required for escrow transactions. Both parties must trust the holder of C anyway so that party can instead hold both A and B. The holders of A and B can either reach an agreement themselves or party C can act as the final arbitrator using A and B together.

Example Execution

Stack Code
<sig> 1 [pubkey] [pubkey] 2 OP_2OVER OP_2OVER OP_ADD OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 OP_2OVER OP_2OVER OP_ADD OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 1 [pubkey] OP_2OVER OP_ADD OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 1 [pubkey] [pubkey] 2 OP_ADD OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 1 [pubkey] <sumA> OP_ADD OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 1 <sumB> OP_ADD OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 <sumC> OP_HASH160 <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 <scriptHashA> <scriptHash> OP_EQUALVERIFY OP_CHECKMULTISIG
<sig> 1 [pubkey] [pubkey] 2 OP_CHECKMULTISIG
1

Where scriptHash must be constructed in the following manor:

scriptHash = Hash160((1 + CastToBigNum(pubKey1) + CastToBigNum(pubKey2) + 2);

Security

Although it may appear insecure totalling the sum of the public keys this is not actually the case. The resulting number is still a massively large unit256.

For example if you wanted to find a SHA256 public key matching a target number you could theoretically brute force it as follows: 

uint256 target = 0;
uint256 nonce = 0;
while(SHA256(nonce) != target) {
  ++nonce;
}

Just because you can have two nonces doesn't make it any easier. 

uint256 target = 0;
uint256 nonce = 0;
uint256 nonceTwo = 0;
while(SHA256(nonce) + SHA256(nonceTwo) != target) {
  ++nonce;
  nonceTwo = rand();
}

What does affect security is ability to adjust M (either 1 or 2) without needing to generate a new SHA256 pair. The effectively halves the key space to 2^255 keys however it is still far far out of the realm of brute force possibility.

Rationale

This BIP replaces BIP 12 ("OP_EVAL") and BIP 16, ("/P2SH/").

There is a general consensus that multi signature transactions need to be implemented ASAP without requiring the use extremely long "script addresses". A number of potential issues have been found with the proposals thus far:

  • CHV requires the scriptPubKey interacts with data from scriptSig which has not been push onto the stack
  • OP_EVAL essentially makes the scripting language turing complete, something which Satoshi deliberately avoided during its design.
  • P2SH requires that standard templates become a mandatory part of the scripting language, meaning they can never be fully depreciated in future.

All of the these solutions risk a fork in the blockchain and require at least 50% miners approval. This proposal requires no changes to the block validation rules and can be implemented immediately. However it only allows for the most common use cases of pay to script transactions and has limited flexibility. Additionally the resulting scriptPubKey is 4 bytes larger than the standard "pay to address" scriptPubKey in common use, the author does not believe this will significantly effect transaction fees.

Backwards Compatibility

This proposal is fully backwards compatible.

See Also

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