Subject: [bitcoin-dev] BIP: OP_BRIBVERIFY - the op code needed for Blind Merge Mined drivechains

From: Chris Stewart <[email-masked]>
Date: Tue, 27 Jun 2017 19:37:13 -0500

<pre>
  BIP: <BIP number>
  Layer: Consensus (Soft fork)
  Title: OP_BRIBEVERIFY
  Author: Chris Stewart <[email-masked]>
  Status: Draft
  Type: Standards Track
  Created: 2017-06-27
</pre>

==Abstract==

This BIP describes a new opcode, OP_BRIBEVERIFY, for the Bitcoin
scripting system that allows for a user to bribe a miner to include a hash
in the coinbase transaction's output.

==Summary==

BRIBEVERIFY redefines the existing NOP4 opcode. When executed, if the given
critical hash is included at the given vout index in the coinbase
transaction
the script evaluates to true. Otherwise, the script will fail.

This allows sidechains to be merged mined against
bitcoin without burdening bitcoin miners with extra resource requirements.

==Motivation==

The current political climate of bitcoin is extremely contentious. Many
community members
have different visions of what bitcoin is. This op code is meant to
enable [http://www.truthcoin.info/blog/blind-merged-mining/ Blind Merge
Mining].
This enables sidechains in Bitcoin. With OP_BRIBEVERIFY, sidechains miners
can
bribe bitcoin miners to to include their block hash in the bitcoin
blockchain. If their block
is included in the coinbase transaction's vout, it is assumed that block is
a mined block on the sidechain.

This will allow various factions of the community to realize their vision
on their own separate
blockchain that is interoperable with the bitcoin blockchain. This allows
those factions to use
bitcoin as a 'reserve currency' for their own network.

===Commitment Structure===

A new block rule is added which requires that the miner's coinbase reward
be at index 0 in the coinbase transaction's output vector.

It also fixes the witness commitment output to be at index 1 of the
coinbase transaction's output vector.

This is needed so we can reliably tell what vout corresponds to what
drivechain. For instance, the mimblewimble sidechain
could correspond to index 2 of the vector outputs on the coinbase
transaction.

The commitment is recorded in a <code>scriptPubKey</code> of the coinbase
transaction. It must be at least 34 bytes in size
   1-byte - OP_RETURN (0x6a)
   1-byte - Push the following 32 bytes (0x20)
  32-byte - block hash

the 35th byte and onward have no consensus meaning.

===OP_BRIBEVERIFY op code===

This op code reads two arguments from the stack. The stack top is expected
to be a sidechain id for which this user attempting to blind merge mine for.
The next element on the stack is expected to be a block hash. This op code
looks into the coinbase transaction's output vector at the given index
(which is derived from the sidechain id) and checks
to see if the hash in the block matches the hash inside of the BRIBEVERIFY
program. If the hashes match, the OP_BRIBEVERIFY acts as an OP_NOP. If the
comparison between the two hashes fail, the script fails.

===BRIBEVERIFY program===

A standard BRIBEVERIFY program has the format:
  1-byte - Push the following 32 bytes (0x20)
 32-byte - block hash
  1 byte - Push operation? (needed if number can't be encoded as OP_0 -
OP_16)
  1 byte - sidechain id
  1 byte - OP_BRIBEVERIFY op code

==Detailed Specification==

Refer to the reference implementation, reproduced below, for the precise
semantics and detailed rationale for those semantics.

 case OP_NOP4:
 {
    //format: block_hash sidechain_id OP_BRIBEVERIFY
    if (!(flags & SCRIPT_VERIFY_BRIBEVERIFY)) {
        // not enabled; treat as a NOP4
        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
        }
        break;
    }

    if (stack.size() < 2)
        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);

    const CScriptNum scriptNumSidechainId(stacktop(-1),fRequireMinimal);
    uint8_t nSidechainId;
    if (!checker.CheckSidechainId(scriptNumSidechainId,nSidechainId)) {
        return set_error(serror, SCRIPT_ERR_UNKNOWN_SIDECHAIN);
    }

    // Check block hash
    bool fHashCritical =
checker.CheckCriticalHash(stacktop(-2),nSidechainId);
    if (!fHashCritical) {
        return set_error(serror, SCRIPT_ERR_UNSATISFIED_BRIBE);
    }
    break;
 }

https://github.com/Christewart/bitcoin/blob/94b6f33f2278c42d4d8758a3c8ffe2078e4ec933/src/script/interpreter.cpp#L427

https://github.com/drivechain-project/bitcoin/pull/13

==Deployment==

TODO

==Credits==

Credit to Paul Sztorc for the original idea of Blind Merge Mined sidechains.

Credit to CryptAxe for writing the foundational layer of software for
drivechains so I could implement OP_BRIBEVERIFY.

==References==

Blind Merge Mined Sidechains -
http://www.truthcoin.info/blog/blind-merged-mining/

Mailing list discussion -
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014408.html

==Copyright==

This document is placed in the public domain.