I cannot broadcast my raw tx (P2PKH) on testnet: Signature must be zero for failed CHECK(MULTI)SIG operation

Question

I am trying to broadcast my transaction on the test network with no success so far. I get always this error:

sendrawtransaction RPC error: {"code":-26,"message":"mandatory-script-verify-flag-failed (Signature must be zero for failed CHECK(MULTI)SIG operation)"}

I have reviewed my code at least 10 times the last two hours and can´t find what is wrong with it. It looks like everything is okay but obviously not.

Could someone spot the cause of the issue?

This is the raw transaction in hex:

010000000119385e43e5d69ce6dd8aa465cdfc897c80782d1b44fcc07239bf17e000fdf7db000000006a47304402207fd3680d14fe8de7ef71d35d9af0c60b2f878532cdd912fd3b293ae30c8bd3a902203de1b6330657487d4fe115a90916d55fbea0445433eb206b04073594ef5bcdd1012102ce96cbb4508b6555c0968a90cc2e24e511e7431e78e69a04144f5017390c9612ffffffff0150dc0000000000001976a914655b68715774105f48b475dc2a15026ade9e013e88ac00000000

And here the same hex deconstructed:

version: 01000000

tx in count: 01

prev_tx_id (flipped): 19385e43e5d69ce6dd8aa465cdfc897c80782d1b44fcc07239bf17e000fdf7db

v_out: 00000000

sigscript size: 6a

sigscript: 47304402207fd3680d14fe8de7ef71d35d9af0c60b2f878532cdd912fd3b293ae30c8bd3a902203de1b6330657487d4fe115a90916d55fbea0445433eb206b04073594ef5bcdd1012102ce96cbb4508b6555c0968a90cc2e24e511e7431e78e69a04144f5017390c9612

sequence: ffffffff

tx_out_count: 01

amount: 50dc000000000000

scriptPubKey_out_size: 19

scriptPubKey_out: 76a914655b68715774105f48b475dc2a15026ade9e013e88ac

lock time: 00000000

This is my code on Python. I just removed the private_key_hex field, anything else remains untouched. Also, notice that I got no change, the difference between the full UTXO amount and the value sent is the fee for the miners.

import base58
import hashlib
import ecdsa


def hash160(x): # Both accepts & returns bytes
    return hashlib.new('ripemd160', hashlib.sha256(x).digest()).digest()

def doublesha256(x):
    return hashlib.sha256(hashlib.sha256(x).digest()).digest()

def flip(string):
    flipped = "".join(reversed([string[i:i+2] for i in range(0, len(string), 2)]))
    return flipped

def hashed_pubkey(pubkey):
    return base58.b58decode_check(pubkey)[1:].hex()



private_key_hex = ''
pub_address_in = 'mpfmNgZAkSdhDoP5UJKhh6wahfGboXhFmj'
hashed_pubkey_in = hashed_pubkey(pub_address_in)

version = 1
tx_input_count = 1
tx_output_count = 1

#TX_IN_VARIABLES
prev_tx_id = 'dbf7fd00e017bf3972c0fc441b2d78807c89fccd65a48adde69cd6e5435e3819'
v_out = 0
scriptPubKey_in = bytes.fromhex("76a914{}88ac".format(hashed_pubkey_in))
scriptPubKey_in_size = len(scriptPubKey_in)

#TX_OUT_VARIABLES
amount = 56400
pub_address_out = 'mpkt3ZpTfZJDeN9p5LgkAHj3NmVMaND7vR'
hashed_pubkey_out = hashed_pubkey(pub_address_out)
scriptPubKey_out = bytes.fromhex("76a914{}88ac".format(hashed_pubkey_out))
scriptPubKey_out_size = len(scriptPubKey_out)                          



class raw_tx:
    version                             = (version).to_bytes(4, byteorder='little')
    tx_in_count                         = (tx_input_count).to_bytes(1, byteorder = 'little')
    tx_input                            = {}
    tx_out_count                        = (tx_output_count).to_bytes(1, byteorder = 'little')
    tx_output                           = {}
    lock_time                           = (0).to_bytes(4, byteorder = 'little')

rtx = raw_tx()


#TX_IN
rtx.tx_input['prev_tx_id']              = bytes.fromhex(flip(prev_tx_id))
rtx.tx_input['v_out']                   = (v_out).to_bytes(4, byteorder = 'little')
rtx.tx_input['scriptPubKey_in_size']    = scriptPubKey_in_size.to_bytes(1, byteorder = 'little')
rtx.tx_input['scriptPubKey_in']         = scriptPubKey_in
rtx.tx_input['sequence']                = bytes.fromhex('ffffffff')


#TX_OUT
rtx.tx_output['amount']                  = amount.to_bytes(8, byteorder = 'little')
rtx.tx_output['scriptPubKey_out_size']   = scriptPubKey_out_size.to_bytes(1, byteorder = 'little')
rtx.tx_output['scriptPubKey_out']        = scriptPubKey_out


raw_tx_string = (

    rtx.version
    + rtx.tx_in_count
    + rtx.tx_input["prev_tx_id"]
    + rtx.tx_input["v_out"]
    + rtx.tx_input["scriptPubKey_in_size"]
    + rtx.tx_input["scriptPubKey_in"]
    + rtx.tx_input["sequence"]
    + rtx.tx_out_count
    + rtx.tx_output["amount"]
    + rtx.tx_output["scriptPubKey_out_size"]
    + rtx.lock_time
    + (1).to_bytes(4, byteorder = 'little')#SIGHASH_ALL type of sighash

    )


#DOUBLE SHA256 OF RAW_TX
hashed_tx_to_sign = doublesha256(raw_tx_string)

#GETTING PUBLIC KEY IN BYTES FORMAT
sk = ecdsa.SigningKey.from_string(bytes.fromhex(private_key_hex), curve = ecdsa.SECP256k1)
vk = sk.verifying_key
public_key = vk.to_string()
public_key_hex = public_key.hex()

#COMPRESSING THE PUBLIC KEY
# Checking if the last byte is odd or even
if (ord(bytearray.fromhex(public_key_hex[-2:])) % 2 == 0):
    public_key_compressed = '02'
else:
    public_key_compressed = '03'
    
# Add bytes 0x02 to the X of the key if even or 0x03 if odd
public_key_compressed += public_key_hex[:64]
public_key_compressed = bytes.fromhex(public_key_compressed)


#SIGNING RAW_TX HASHED
signature = sk.sign_digest(hashed_tx_to_sign, sigencode = ecdsa.util.sigencode_der_canonize)


#CREATING SCRIPTSIG
signature_SIGHASH_ALL_byte = signature + (1).to_bytes(1, byteorder = 'little') #SIGHASH_ALL type of sighash
signature_SIGHASH_ALL_byte_size = len(signature_SIGHASH_ALL_byte)

public_key_compressed_size = len(public_key_compressed)

scriptSig = (

    signature_SIGHASH_ALL_byte_size.to_bytes(1, byteorder = 'little')
    + signature_SIGHASH_ALL_byte
    + public_key_compressed_size.to_bytes(1, byteorder = 'little')
    + public_key_compressed
        
    )

scriptSig_size = len(scriptSig)


real_tx = (
    rtx.version
    + rtx.tx_in_count
    + rtx.tx_input["prev_tx_id"]
    + rtx.tx_input["v_out"]
    + scriptSig_size.to_bytes(1, byteorder = 'little')
    + scriptSig
    + rtx.tx_input["sequence"]
    + rtx.tx_out_count
    + rtx.tx_output["amount"]
    + rtx.tx_output["scriptPubKey_out_size"]
    + rtx.tx_output["scriptPubKey_out"]
    + rtx.lock_time

    )

print(real_tx.hex())

Answer 1

The modified transaction you're hashing (raw_tx_string) is missing the output script (scriptPubKey_out), which is why the signature then fails to verify.