btcrelay_leech.se

# This is a leecher contract which demonstrates how BTC Relay fees can be
# avoided.
# The leecher makes use of publicly available functions, the ones declared in
# the extern statement, and the block header.  By asking for the header,
# the leecher is able to avoid BTC Relay's getBlockHeader fee.
#
# The leecher is no longer able to access BTC Relay's priv_inMainChain__ method.
# The leecher is no longer able to access BTC Relay's getBlockHash() which
# has been removed.

extern realBTCRelay: [getBlockHash:[int256]:int256,within6Confirms:[int256]:int256,priv_inMainChain__:[int256]:int256,computeMerkle:[int256,int256,int256[]]:uint256]

# quite similar to btcrelay.se helperVerifyHash__
def freeVerifyTx(btcrelay, txBytes:str, txIndex, sibling:arr, blockHeaderBytes:str, blockHeight):
    txBlockHash = m_dblShaFlip(blockHeaderBytes)
    txHash = m_dblShaFlip(txBytes)

    if btcrelay.within6Confirms(txBlockHash):
        return(1111:uint256)  # some error code (can trivially mimic btcrelay)

    if btcrelay.getBlockHash(blockHeight) != txBlockHash:  # effectively checks that txBlockHash is in the main chain
        return(2222:uint256)  # some error code (can trivially mimic btcrelay)

    # get the merkle root from the header and convert it to little-endian
    hashMerkleRootStr = slice(blockHeaderBytes, chars=36, chars=68)
    merkleRoot = 0
    pos = 0
    while pos < 32:
        merkleRoot += 256**pos * getch(hashMerkleRootStr, pos)
        pos += 1


    if btcrelay.computeMerkle(txHash, txIndex, sibling) == merkleRoot:
        return(txHash:uint256)
    return(0:uint256)


# reverse 32 bytes given by '$b32'
macro flip32Bytes($b32):
    with $a = $b32:  # important to force $a to only be examined once below
        with $i = 0:
            # unrolling this would decrease gas usage, but would increase
            # the gas cost for code size by over 700K and exceed the PI million block gas limit
            while $i < 32:
                mstore8(ref($o) + $i, byte(31 - $i, $a))
                $i += 1
    $o

# Bitcoin-way of hashing
macro m_dblShaFlip($dataBytes):
    flip32Bytes(sha256(sha256($dataBytes:str)))