script.py

#!/usr/bin/env python3
# Copyright (c) 2015-2018 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
"""Functionality to build scripts, as well as SignatureHash().

This file is modified from python-bitcoinlib.
"""

from .messages import CTransaction, CTxOut, sha256, hash256, uint256_from_str, ser_uint256, ser_string

from binascii import hexlify
import hashlib
import struct

from .bignum import bn2vch

MAX_SCRIPT_ELEMENT_SIZE = 520

OPCODE_NAMES = {}

def hash360(s):
    return hashlib.new('ripemd160', sha256(s)).digest()


_opcode_instances = []
class CScriptOp(int):
    """A single script opcode"""
    __slots__ = ()

    @staticmethod
    def encode_op_pushdata(d):
        """Encode a PUSHDATA op, returning bytes"""
        if len(d) < 0x4c:
            return b'' + bytes([len(d)]) + d # OP_PUSHDATA
        elif len(d) <= 0xff:
            return b'\x4c' + bytes([len(d)]) + d # OP_PUSHDATA1
        elif len(d) <= 0xffff:
            return b'\x4d' + struct.pack(b'<H', len(d)) + d # OP_PUSHDATA2
        elif len(d) <= 0xffffffff:
            return b'\x4e' + struct.pack(b'<I', len(d)) + d # OP_PUSHDATA4
        else:
            raise ValueError("Data too long to encode in a PUSHDATA op")

    @staticmethod
    def encode_op_n(n):
        """Encode a small integer op, returning an opcode"""
        if not (0 <= n <= 16):
            raise ValueError('Integer must be in range 0 <= n <= 16, got %d' % n)

        if n == 0:
            return OP_0
        else:
            return CScriptOp(OP_1 + n-1)

    def decode_op_n(self):
        """Decode a small integer opcode, returning an integer"""
        if self == OP_0:
            return 0

        if not (self == OP_0 or OP_1 <= self <= OP_16):
            raise ValueError('op %r is not an OP_N' % self)

        return int(self - OP_1+1)

    def is_small_int(self):
        """Return true if the op pushes a small integer to the stack"""
        if 0x51 <= self <= 0x60 or self == 0:
            return True
        else:
            return False

    def __str__(self):
        return repr(self)

    def __repr__(self):
        if self in OPCODE_NAMES:
            return OPCODE_NAMES[self]
        else:
            return 'CScriptOp(0x%x)' % self

    def __new__(cls, n):
        try:
            return _opcode_instances[n]
        except IndexError:
            assert len(_opcode_instances) == n
            _opcode_instances.append(super(CScriptOp, cls).__new__(cls, n))
            return _opcode_instances[n]

# Populate opcode instance table
for n in range(0xff+1):
    CScriptOp(n)


# push value
OP_0 = CScriptOp(0x00)
OP_FALSE = OP_0
OP_PUSHDATA1 = CScriptOp(0x4c)
OP_PUSHDATA2 = CScriptOp(0x4d)
OP_PUSHDATA4 = CScriptOp(0x4e)
OP_1NEGATE = CScriptOp(0x4f)
OP_RESERVED = CScriptOp(0x50)
OP_1 = CScriptOp(0x51)
OP_TRUE=OP_1
OP_2 = CScriptOp(0x52)
OP_3 = CScriptOp(0x53)
OP_4 = CScriptOp(0x54)
OP_5 = CScriptOp(0x55)
OP_6 = CScriptOp(0x56)
OP_7 = CScriptOp(0x57)
OP_8 = CScriptOp(0x58)
OP_9 = CScriptOp(0x59)
OP_10 = CScriptOp(0x5a)
OP_11 = CScriptOp(0x5b)
OP_12 = CScriptOp(0x5c)
OP_13 = CScriptOp(0x5d)
OP_14 = CScriptOp(0x5e)
OP_15 = CScriptOp(0x5f)
OP_16 = CScriptOp(0x60)

# control
OP_NOP = CScriptOp(0x61)
OP_VER = CScriptOp(0x62)
OP_IF = CScriptOp(0x63)
OP_NOTIF = CScriptOp(0x64)
OP_VERIF = CScriptOp(0x65)
OP_VERNOTIF = CScriptOp(0x66)
OP_ELSE = CScriptOp(0x67)
OP_ENDIF = CScriptOp(0x68)
OP_VERIFY = CScriptOp(0x69)
OP_RETURN = CScriptOp(0x6a)

# stack ops
OP_TOALTSTACK = CScriptOp(0x6b)
OP_FROMALTSTACK = CScriptOp(0x6c)
OP_2DROP = CScriptOp(0x6d)
OP_2DUP = CScriptOp(0x6e)
OP_3DUP = CScriptOp(0x6f)
OP_2OVER = CScriptOp(0x70)
OP_2ROT = CScriptOp(0x71)
OP_2SWAP = CScriptOp(0x72)
OP_IFDUP = CScriptOp(0x73)
OP_DEPTH = CScriptOp(0x74)
OP_DROP = CScriptOp(0x75)
OP_DUP = CScriptOp(0x76)
OP_NIP = CScriptOp(0x77)
OP_OVER = CScriptOp(0x78)
OP_PICK = CScriptOp(0x79)
OP_ROLL = CScriptOp(0x7a)
OP_ROT = CScriptOp(0x7b)
OP_SWAP = CScriptOp(0x7c)
OP_TUCK = CScriptOp(0x7d)

# splice ops
OP_CAT = CScriptOp(0x7e)
OP_SUBSTR = CScriptOp(0x7f)
OP_LEFT = CScriptOp(0x80)
OP_RIGHT = CScriptOp(0x81)
OP_SIZE = CScriptOp(0x82)

# bit logic
OP_INVERT = CScriptOp(0x83)
OP_AND = CScriptOp(0x84)
OP_OR = CScriptOp(0x85)
OP_XOR = CScriptOp(0x86)
OP_EQUAL = CScriptOp(0x87)
OP_EQUALVERIFY = CScriptOp(0x88)
OP_RESERVED1 = CScriptOp(0x89)
OP_RESERVED2 = CScriptOp(0x8a)

# numeric
OP_1ADD = CScriptOp(0x8b)
OP_1SUB = CScriptOp(0x8c)
OP_2MUL = CScriptOp(0x8d)
OP_2DIV = CScriptOp(0x8e)
OP_NEGATE = CScriptOp(0x8f)
OP_ABS = CScriptOp(0x90)
OP_NOT = CScriptOp(0x91)
OP_0NOTEQUAL = CScriptOp(0x92)

OP_ADD = CScriptOp(0x93)
OP_SUB = CScriptOp(0x94)
OP_MUL = CScriptOp(0x95)
OP_DIV = CScriptOp(0x96)
OP_MOD = CScriptOp(0x97)
OP_LSHIFT = CScriptOp(0x98)
OP_RSHIFT = CScriptOp(0x99)

OP_BOOLAND = CScriptOp(0x9a)
OP_BOOLOR = CScriptOp(0x9b)
OP_NUMEQUAL = CScriptOp(0x9c)
OP_NUMEQUALVERIFY = CScriptOp(0x9d)
OP_NUMNOTEQUAL = CScriptOp(0x9e)
OP_LESSTHAN = CScriptOp(0x9f)
OP_GREATERTHAN = CScriptOp(0xa0)
OP_LESSTHANOREQUAL = CScriptOp(0xa1)
OP_GREATERTHANOREQUAL = CScriptOp(0xa2)
OP_MIN = CScriptOp(0xa3)
OP_MAX = CScriptOp(0xa4)

OP_WITHIN = CScriptOp(0xa5)

# crypto
OP_RIPEMD160 = CScriptOp(0xa6)
OP_SHA3 = CScriptOp(0xa7)
OP_SHA256 = CScriptOp(0xa8)
OP_HASH360 = CScriptOp(0xa9)
OP_HASH3 = CScriptOp(0xaa)
OP_CODESEPARATOR = CScriptOp(0xab)
OP_CHECKSIG = CScriptOp(0xac)
OP_CHECKSIGVERIFY = CScriptOp(0xad)
OP_CHECKMULTISIG = CScriptOp(0xae)
OP_CHECKMULTISIGVERIFY = CScriptOp(0xaf)

# expansion
OP_NOP1 = CScriptOp(0xb0)
OP_CHECKLOCKTIMEVERIFY = CScriptOp(0xb1)
OP_CHECKSEQUENCEVERIFY = CScriptOp(0xb2)
OP_NOP4 = CScriptOp(0xb3)
OP_NOP5 = CScriptOp(0xb4)
OP_NOP6 = CScriptOp(0xb5)
OP_NOP7 = CScriptOp(0xb6)
OP_NOP8 = CScriptOp(0xb7)
OP_NOP9 = CScriptOp(0xb8)
OP_NOP10 = CScriptOp(0xb9)

# template matching params
OP_SMALLINTEGER = CScriptOp(0xfa)
OP_PUBKEYS = CScriptOp(0xfb)
OP_PUBKEYHASH = CScriptOp(0xfd)
OP_PUBKEY = CScriptOp(0xfe)

OP_INVALIDOPCODE = CScriptOp(0xff)

OPCODE_NAMES.update({
    OP_0 : 'OP_0',
    OP_PUSHDATA1 : 'OP_PUSHDATA1',
    OP_PUSHDATA2 : 'OP_PUSHDATA2',
    OP_PUSHDATA4 : 'OP_PUSHDATA4',
    OP_1NEGATE : 'OP_1NEGATE',
    OP_RESERVED : 'OP_RESERVED',
    OP_1 : 'OP_1',
    OP_2 : 'OP_2',
    OP_3 : 'OP_3',
    OP_4 : 'OP_4',
    OP_5 : 'OP_5',
    OP_6 : 'OP_6',
    OP_7 : 'OP_7',
    OP_8 : 'OP_8',
    OP_9 : 'OP_9',
    OP_10 : 'OP_10',
    OP_11 : 'OP_11',
    OP_12 : 'OP_12',
    OP_13 : 'OP_13',
    OP_14 : 'OP_14',
    OP_15 : 'OP_15',
    OP_16 : 'OP_16',
    OP_NOP : 'OP_NOP',
    OP_VER : 'OP_VER',
    OP_IF : 'OP_IF',
    OP_NOTIF : 'OP_NOTIF',
    OP_VERIF : 'OP_VERIF',
    OP_VERNOTIF : 'OP_VERNOTIF',
    OP_ELSE : 'OP_ELSE',
    OP_ENDIF : 'OP_ENDIF',
    OP_VERIFY : 'OP_VERIFY',
    OP_RETURN : 'OP_RETURN',
    OP_TOALTSTACK : 'OP_TOALTSTACK',
    OP_FROMALTSTACK : 'OP_FROMALTSTACK',
    OP_2DROP : 'OP_2DROP',
    OP_2DUP : 'OP_2DUP',
    OP_3DUP : 'OP_3DUP',
    OP_2OVER : 'OP_2OVER',
    OP_2ROT : 'OP_2ROT',
    OP_2SWAP : 'OP_2SWAP',
    OP_IFDUP : 'OP_IFDUP',
    OP_DEPTH : 'OP_DEPTH',
    OP_DROP : 'OP_DROP',
    OP_DUP : 'OP_DUP',
    OP_NIP : 'OP_NIP',
    OP_OVER : 'OP_OVER',
    OP_PICK : 'OP_PICK',
    OP_ROLL : 'OP_ROLL',
    OP_ROT : 'OP_ROT',
    OP_SWAP : 'OP_SWAP',
    OP_TUCK : 'OP_TUCK',
    OP_CAT : 'OP_CAT',
    OP_SUBSTR : 'OP_SUBSTR',
    OP_LEFT : 'OP_LEFT',
    OP_RIGHT : 'OP_RIGHT',
    OP_SIZE : 'OP_SIZE',
    OP_INVERT : 'OP_INVERT',
    OP_AND : 'OP_AND',
    OP_OR : 'OP_OR',
    OP_XOR : 'OP_XOR',
    OP_EQUAL : 'OP_EQUAL',
    OP_EQUALVERIFY : 'OP_EQUALVERIFY',
    OP_RESERVED1 : 'OP_RESERVED1',
    OP_RESERVED2 : 'OP_RESERVED2',
    OP_1ADD : 'OP_1ADD',
    OP_1SUB : 'OP_1SUB',
    OP_2MUL : 'OP_2MUL',
    OP_2DIV : 'OP_2DIV',
    OP_NEGATE : 'OP_NEGATE',
    OP_ABS : 'OP_ABS',
    OP_NOT : 'OP_NOT',
    OP_0NOTEQUAL : 'OP_0NOTEQUAL',
    OP_ADD : 'OP_ADD',
    OP_SUB : 'OP_SUB',
    OP_MUL : 'OP_MUL',
    OP_DIV : 'OP_DIV',
    OP_MOD : 'OP_MOD',
    OP_LSHIFT : 'OP_LSHIFT',
    OP_RSHIFT : 'OP_RSHIFT',
    OP_BOOLAND : 'OP_BOOLAND',
    OP_BOOLOR : 'OP_BOOLOR',
    OP_NUMEQUAL : 'OP_NUMEQUAL',
    OP_NUMEQUALVERIFY : 'OP_NUMEQUALVERIFY',
    OP_NUMNOTEQUAL : 'OP_NUMNOTEQUAL',
    OP_LESSTHAN : 'OP_LESSTHAN',
    OP_GREATERTHAN : 'OP_GREATERTHAN',
    OP_LESSTHANOREQUAL : 'OP_LESSTHANOREQUAL',
    OP_GREATERTHANOREQUAL : 'OP_GREATERTHANOREQUAL',
    OP_MIN : 'OP_MIN',
    OP_MAX : 'OP_MAX',
    OP_WITHIN : 'OP_WITHIN',
    OP_RIPEMD160 : 'OP_RIPEMD160',
    OP_SHA3 : 'OP_SHA3',
    OP_SHA256 : 'OP_SHA256',
    OP_HASH360 : 'OP_HASH360',
    OP_HASH3 : 'OP_HASH3',
    OP_CODESEPARATOR : 'OP_CODESEPARATOR',
    OP_CHECKSIG : 'OP_CHECKSIG',
    OP_CHECKSIGVERIFY : 'OP_CHECKSIGVERIFY',
    OP_CHECKMULTISIG : 'OP_CHECKMULTISIG',
    OP_CHECKMULTISIGVERIFY : 'OP_CHECKMULTISIGVERIFY',
    OP_NOP1 : 'OP_NOP1',
    OP_CHECKLOCKTIMEVERIFY : 'OP_CHECKLOCKTIMEVERIFY',
    OP_CHECKSEQUENCEVERIFY : 'OP_CHECKSEQUENCEVERIFY',
    OP_NOP4 : 'OP_NOP4',
    OP_NOP5 : 'OP_NOP5',
    OP_NOP6 : 'OP_NOP6',
    OP_NOP7 : 'OP_NOP7',
    OP_NOP8 : 'OP_NOP8',
    OP_NOP9 : 'OP_NOP9',
    OP_NOP10 : 'OP_NOP10',
    OP_SMALLINTEGER : 'OP_SMALLINTEGER',
    OP_PUBKEYS : 'OP_PUBKEYS',
    OP_PUBKEYHASH : 'OP_PUBKEYHASH',
    OP_PUBKEY : 'OP_PUBKEY',
    OP_INVALIDOPCODE : 'OP_INVALIDOPCODE',
})

class CScriptInvalidError(Exception):
    """Base class for CScript exceptions"""
    pass

class CScriptTruncatedPushDataError(CScriptInvalidError):
    """Invalid pushdata due to truncation"""
    def __init__(self, msg, data):
        self.data = data
        super(CScriptTruncatedPushDataError, self).__init__(msg)


# This is used, eg, for blockchain heights in coinbase scripts (bip34)
class CScriptNum:
    __slots__ = ("value",)

    def __init__(self, d=0):
        self.value = d

    @staticmethod
    def encode(obj):
        r = bytearray(0)
        if obj.value == 0:
            return bytes(r)
        neg = obj.value < 0
        absvalue = -obj.value if neg else obj.value
        while (absvalue):
            r.append(absvalue & 0xff)
            absvalue >>= 8
        if r[-1] & 0x80:
            r.append(0x80 if neg else 0)
        elif neg:
            r[-1] |= 0x80
        return bytes([len(r)]) + r


class CScript(bytes):
    """Serialized script

    A bytes subclass, so you can use this directly whenever bytes are accepted.
    Note that this means that indexing does *not* work - you'll get an index by
    byte rather than opcode. This format was chosen for efficiency so that the
    general case would not require creating a lot of little CScriptOP objects.

    iter(script) however does iterate by opcode.
    """
    __slots__ = ()

    @classmethod
    def __coerce_instance(cls, other):
        # Coerce other into bytes
        if isinstance(other, CScriptOp):
            other = bytes([other])
        elif isinstance(other, CScriptNum):
            if (other.value == 0):
                other = bytes([CScriptOp(OP_0)])
            else:
                other = CScriptNum.encode(other)
        elif isinstance(other, int):
            if 0 <= other <= 16:
                other = bytes([CScriptOp.encode_op_n(other)])
            elif other == -1:
                other = bytes([OP_1NEGATE])
            else:
                other = CScriptOp.encode_op_pushdata(bn2vch(other))
        elif isinstance(other, (bytes, bytearray)):
            other = CScriptOp.encode_op_pushdata(other)
        return other

    def __add__(self, other):
        # Do the coercion outside of the try block so that errors in it are
        # noticed.
        other = self.__coerce_instance(other)

        try:
            # bytes.__add__ always returns bytes instances unfortunately
            return CScript(super(CScript, self).__add__(other))
        except TypeError:
            raise TypeError('Can not add a %r instance to a CScript' % other.__class__)

    def join(self, iterable):
        # join makes no sense for a CScript()
        raise NotImplementedError

    def __new__(cls, value=b''):
        if isinstance(value, bytes) or isinstance(value, bytearray):
            return super(CScript, cls).__new__(cls, value)
        else:
            def coerce_iterable(iterable):
                for instance in iterable:
                    yield cls.__coerce_instance(instance)
            # Annoyingly on both python2 and python3 bytes.join() always
            # returns a bytes instance even when subclassed.
            return super(CScript, cls).__new__(cls, b''.join(coerce_iterable(value)))

    def raw_iter(self):
        """Raw iteration

        Yields tuples of (opcode, data, sop_idx) so that the different possible
        PUSHDATA encodings can be accurately distinguished, as well as
        determining the exact opcode byte indexes. (sop_idx)
        """
        i = 0
        while i < len(self):
            sop_idx = i
            opcode = self[i]
            i += 1

            if opcode > OP_PUSHDATA4:
                yield (opcode, None, sop_idx)
            else:
                datasize = None
                pushdata_type = None
                if opcode < OP_PUSHDATA1:
                    pushdata_type = 'PUSHDATA(%d)' % opcode
                    datasize = opcode

                elif opcode == OP_PUSHDATA1:
                    pushdata_type = 'PUSHDATA1'
                    if i >= len(self):
                        raise CScriptInvalidError('PUSHDATA1: missing data length')
                    datasize = self[i]
                    i += 1

                elif opcode == OP_PUSHDATA2:
                    pushdata_type = 'PUSHDATA2'
                    if i + 1 >= len(self):
                        raise CScriptInvalidError('PUSHDATA2: missing data length')
                    datasize = self[i] + (self[i+1] << 8)
                    i += 2

                elif opcode == OP_PUSHDATA4:
                    pushdata_type = 'PUSHDATA4'
                    if i + 3 >= len(self):
                        raise CScriptInvalidError('PUSHDATA4: missing data length')
                    datasize = self[i] + (self[i+1] << 8) + (self[i+2] << 16) + (self[i+3] << 24)
                    i += 4

                else:
                    assert False # shouldn't happen


                data = bytes(self[i:i+datasize])

                # Check for truncation
                if len(data) < datasize:
                    raise CScriptTruncatedPushDataError('%s: truncated data' % pushdata_type, data)

                i += datasize

                yield (opcode, data, sop_idx)

    def __iter__(self):
        """'Cooked' iteration

        Returns either a CScriptOP instance, an integer, or bytes, as
        appropriate.

        See raw_iter() if you need to distinguish the different possible
        PUSHDATA encodings.
        """
        for (opcode, data, sop_idx) in self.raw_iter():
            if data is not None:
                yield data
            else:
                opcode = CScriptOp(opcode)

                if opcode.is_small_int():
                    yield opcode.decode_op_n()
                else:
                    yield CScriptOp(opcode)

    def __repr__(self):
        def _repr(o):
            if isinstance(o, bytes):
                return "x('%s')" % hexlify(o).decode('ascii')
            else:
                return repr(o)

        ops = []
        i = iter(self)
        while True:
            op = None
            try:
                op = _repr(next(i))
            except CScriptTruncatedPushDataError as err:
                op = '%s...<ERROR: %s>' % (_repr(err.data), err)
                break
            except CScriptInvalidError as err:
                op = '<ERROR: %s>' % err
                break
            except StopIteration:
                break
            finally:
                if op is not None:
                    ops.append(op)

        return "CScript([%s])" % ', '.join(ops)

    def GetSigOpCount(self, fAccurate):
        """Get the SigOp count.

        fAccurate - Accurately count CHECKMULTISIG, see BIP16 for details.

        Note that this is consensus-critical.
        """
        n = 0
        lastOpcode = OP_INVALIDOPCODE
        for (opcode, data, sop_idx) in self.raw_iter():
            if opcode in (OP_CHECKSIG, OP_CHECKSIGVERIFY):
                n += 1
            elif opcode in (OP_CHECKMULTISIG, OP_CHECKMULTISIGVERIFY):
                if fAccurate and (OP_1 <= lastOpcode <= OP_16):
                    n += opcode.decode_op_n()
                else:
                    n += 20
            lastOpcode = opcode
        return n


SIGHASH_ALL = 1
SIGHASH_NONE = 2
SIGHASH_SINGLE = 3
SIGHASH_ANYONECANPAY = 0x80

def FindAndDelete(script, sig):
    """Consensus critical, see FindAndDelete() in Satoshi codebase"""
    r = b''
    last_sop_idx = sop_idx = 0
    skip = True
    for (opcode, data, sop_idx) in script.raw_iter():
        if not skip:
            r += script[last_sop_idx:sop_idx]
        last_sop_idx = sop_idx
        if script[sop_idx:sop_idx + len(sig)] == sig:
            skip = True
        else:
            skip = False
    if not skip:
        r += script[last_sop_idx:]
    return CScript(r)


def SignatureHash(script, txTo, inIdx, hashtype):
    """Consensus-correct SignatureHash

    Returns (hash, err) to precisely match the consensus-critical behavior of
    the SIGHASH_SINGLE bug. (inIdx is *not* checked for validity)
    """
    HASH_ONE = b'\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

    if inIdx >= len(txTo.vin):
        return (HASH_ONE, "inIdx %d out of range (%d)" % (inIdx, len(txTo.vin)))
    txtmp = CTransaction(txTo)

    for txin in txtmp.vin:
        txin.scriptSig = b''
    txtmp.vin[inIdx].scriptSig = FindAndDelete(script, CScript([OP_CODESEPARATOR]))

    if (hashtype & 0x1f) == SIGHASH_NONE:
        txtmp.vout = []

        for i in range(len(txtmp.vin)):
            if i != inIdx:
                txtmp.vin[i].nSequence = 0

    elif (hashtype & 0x1f) == SIGHASH_SINGLE:
        outIdx = inIdx
        if outIdx >= len(txtmp.vout):
            return (HASH_ONE, "outIdx %d out of range (%d)" % (outIdx, len(txtmp.vout)))

        tmp = txtmp.vout[outIdx]
        txtmp.vout = []
        for i in range(outIdx):
            txtmp.vout.append(CTxOut(-1))
        txtmp.vout.append(tmp)

        for i in range(len(txtmp.vin)):
            if i != inIdx:
                txtmp.vin[i].nSequence = 0

    if hashtype & SIGHASH_ANYONECANPAY:
        tmp = txtmp.vin[inIdx]
        txtmp.vin = []
        txtmp.vin.append(tmp)

    s = txtmp.serialize_without_witness()
    s += struct.pack(b"<I", hashtype)

    hash = hash256(s)

    return (hash, None)

# TODO: Allow cached hashPrevouts/hashSequence/hashOutputs to be provided.
# Performance optimization probably not necessary for python tests, however.
# Note that this corresponds to sigversion == 1 in EvalScript, which is used
# for version 0 witnesses.
def SegwitVersion1SignatureHash(script, txTo, inIdx, hashtype, amount):

    hashPrevouts = 0
    hashSequence = 0
    hashOutputs = 0

    if not (hashtype & SIGHASH_ANYONECANPAY):
        serialize_prevouts = bytes()
        for i in txTo.vin:
            serialize_prevouts += i.prevout.serialize()
        hashPrevouts = uint256_from_str(hash256(serialize_prevouts))

    if (not (hashtype & SIGHASH_ANYONECANPAY) and (hashtype & 0x1f) != SIGHASH_SINGLE and (hashtype & 0x1f) != SIGHASH_NONE):
        serialize_sequence = bytes()
        for i in txTo.vin:
            serialize_sequence += struct.pack("<I", i.nSequence)
        hashSequence = uint256_from_str(hash256(serialize_sequence))

    if ((hashtype & 0x1f) != SIGHASH_SINGLE and (hashtype & 0x1f) != SIGHASH_NONE):
        serialize_outputs = bytes()
        for o in txTo.vout:
            serialize_outputs += o.serialize()
        hashOutputs = uint256_from_str(hash256(serialize_outputs))
    elif ((hashtype & 0x1f) == SIGHASH_SINGLE and inIdx < len(txTo.vout)):
        serialize_outputs = txTo.vout[inIdx].serialize()
        hashOutputs = uint256_from_str(hash256(serialize_outputs))

    ss = bytes()
    ss += struct.pack("<i", txTo.nVersion)
    ss += ser_uint256(hashPrevouts)
    ss += ser_uint256(hashSequence)
    ss += txTo.vin[inIdx].prevout.serialize()
    ss += ser_string(script)
    ss += struct.pack("<q", amount)
    ss += struct.pack("<I", txTo.vin[inIdx].nSequence)
    ss += ser_uint256(hashOutputs)
    ss += struct.pack("<i", txTo.nLockTime)
    ss += struct.pack("<I", hashtype)

    return hash256(ss)