I'm attempting to redeem the following htlc ( the python script below ) via timeout, however the redemption tx errors out upon validation with the error below:

bitcoin.rpc.VerifyRejectedError: {'code': -26, 'message': 'non-mandatory-script-verify-flag (Locktime requirement not satisfied) (code 64)'}

Deserializing the funding tx has a transaction locktime of 422 while the current blockheight is 433. The final tx has a locktime of 434, so I'm unsure what the issue could be.

htlc test script using python-bitcoinlib:

import sys
if sys.version_info.major < 3:
    sys.stderr.write('Sorry, Python 3.x required by this example.\n')

import bitcoin
from bitcoin import rpc
from bitcoin import SelectParams
from bitcoin.core import b2x, lx, b2lx, COIN, COutPoint, CMutableTxOut, CMutableTxIn, CMutableTransaction, Hash160
from bitcoin.core.script import CScript, OP_DUP, OP_IF, OP_ELSE, OP_ENDIF, OP_HASH160, OP_EQUALVERIFY, OP_CHECKSIG, SignatureHash, SIGHASH_ALL
from bitcoin.core.script import OP_DROP, OP_CHECKLOCKTIMEVERIFY, OP_SHA256
from bitcoin.core.scripteval import SCRIPT_VERIFY_P2SH # VerifyScript
from bitcoin.wallet import CBitcoinAddress, CBitcoinSecret

import hashlib
from utils import VerifyScript
from bitcoin.core.script import *
from bitcoin.core.scripteval import *
from bitcoin.core.scripteval import _CheckExec, _ISA_BINOP, _ISA_UNOP, _CastToBool, _CheckSig

proxy = bitcoin.rpc.Proxy()

# Set the preimage. This is the secret Bob will need to redeem the htlc
preimage = bytes(b'preimage'.hex(), 'utf8')

# Hashed version of the preimage which we can safely store on the blockchain
# and use to verify against what Bob offers
h = hashlib.sha256(preimage).digest()

# Bob
recipientpubkey = proxy.getnewaddress()

# Alice
senderpubkey = proxy.getnewaddress()

# privkey of Alice, used to sign the redeemTx
sender_seckey = proxy.dumpprivkey(senderpubkey)

# privkey of Bob, used to construct the htlc
recipient_seckey = proxy.dumpprivkey(recipientpubkey)

# How long the htlc will last until it times out
lockduration = 1
blocknum = proxy.getblockcount()
redeemblocknum = blocknum + lockduration

# We construct the locking script in the form of a standard bip199 htlc
txin_redeemScript = CScript([
        OP_SHA256, h, OP_EQUALVERIFY,OP_DUP, OP_HASH160, bytes(Hash160(recipient_seckey.pub)),
        redeemblocknum, OP_CHECKLOCKTIMEVERIFY, OP_DROP, OP_DUP, OP_HASH160, bytes(Hash160(sender_seckey.pub)),

# Generate a P2SH address from the locking script
txin_scriptPubKey = txin_redeemScript.to_p2sh_scriptPubKey()
txin_p2sh_address = CBitcoinAddress.from_scriptPubKey(txin_scriptPubKey)
p2sh = str(txin_p2sh_address)

# Load the htlc
amount = 1.0*COIN
fund_tx = proxy.sendtoaddress(txin_p2sh_address, amount)

# Fast forward time
print('before height ' + str(proxy.getblockheader(proxy.getbestblockhash(), True)['height']))
print('locktime ' + str(lockduration))
proxy.generatetoaddress(lockduration + 10, proxy.getnewaddress())
print('after height ' + str(proxy.getblockheader(proxy.getbestblockhash(), True)['height']))

# Construct the transaction which will redeem the htlc via the first payment route
txinfo = proxy.gettransaction(fund_tx)
txin = CMutableTxIn(COutPoint(fund_tx, txinfo['details'][0]['vout']))
default_fee = 0.001*COIN
txout = CMutableTxOut(amount - default_fee, senderpubkey.to_scriptPubKey())
tx = CMutableTransaction([txin], [txout])
tx.nLockTime = proxy.getblockheader(proxy.getbestblockhash(), True)['height'] + lockduration
# Sign the redeem script with Alice's private key ( for whose address the first payment path
# is set up exclusively for )
sighash = SignatureHash(txin_redeemScript, tx, 0, SIGHASH_ALL)
sig = sender_seckey.sign(sighash) + bytes([SIGHASH_ALL])

# Load the script sig of Bob's redemption transaction with the appropriate values
txin.scriptSig = CScript([

# Verify
VerifyScript(txin.scriptSig, txin_scriptPubKey, tx, 0, (SCRIPT_VERIFY_P2SH,))

# Send
txid = proxy.sendrawtransaction(tx)
  • You're setting the tx.nLockTime to the current block height + how long you want to lock it. That's obviously still in the future. Commented Apr 18, 2020 at 8:15
  • Wouldn't that value only apply to the tx that seeks to redeem that transaction? ( that transaction being which redeemed the original htlc ). I went ahead and tried changing tx.nLockTime to the expected current block height and to a block height in the past - the same error persists
    – arshbot
    Commented Apr 18, 2020 at 8:43
  • A transaction with an nLockTime in the future cannot be included in the blockchain. An OP_CHECKLOCKTIME oocode checks the nLockTime field of the transaction spending the output in which the script appears (it does not check the block height itself) Commented Apr 18, 2020 at 8:49


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