How CTV+CSFS improves BitVM bridges

Protocol Design
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14

This example I created seems to deduplicate the signatures to work with OP_CODESEPARATOR

#!/usr/bin/env python3
"""
Test‑bed for the script pair:

    scriptSig   = <sig> OP_DUP OP_CODESEPARATOR <pk> OP_CHECKSIG
    scriptPubKey= <pk> OP_CHECKSIG

The code builds a fake funding UTXO, spends it with the
non‑standard scriptSig above, and verifies consensus acceptance
(regtest only – policy rules would reject this on mainnet).
"""

from bitcointx.core import (
    COutPoint, CTxIn, CTxOut, CMutableTransaction, b2x
)
from bitcointx.core.script import (
    CScript, OP_DUP, OP_CODESEPARATOR, OP_CHECKSIGVERIFY, OP_TRUE
)
from bitcointx.core.key import CPubKey
from bitcointx.wallet import CBitcoinSecret
from bitcointx.core.scripteval import VerifyScript,MANDATORY_SCRIPT_VERIFY_FLAGS
from bitcointx.core.script import SignatureHash, SIGHASH_ALL


# --------------------------------------------------------------------------
# 1. Network & key‑pair
# --------------------------------------------------------------------------
seckey  = CBitcoinSecret.from_secret_bytes(b"\x01"*32)
pubkey  = CPubKey(seckey.pub)

# --------------------------------------------------------------------------
# 2. Fake funding tx : 1 BTC to "<pk> OP_CHECKSIG"
# --------------------------------------------------------------------------
funding_tx = CMutableTransaction([], [
    CTxOut(100_000_000, CScript([pubkey, OP_CHECKSIGVERIFY]))
])
funding_tx.GetHash()                                  # gives it a txid

# --------------------------------------------------------------------------
# 3. Spending tx skeleton
# --------------------------------------------------------------------------
vin  = CTxIn(COutPoint(funding_tx.GetTxid(), 0))
vout = CTxOut(90_000_000, CScript([OP_DUP]))         # arbitrary output
spend_tx = CMutableTransaction([vin], [vout])

# --------------------------------------------------------------------------
# 4. Signature – note the script *after* OP_CODESEPARATOR
# --------------------------------------------------------------------------
# For OP_CHECKSIG inside the scriptSig, only the bytes *after* the last
# OP_CODESEPARATOR – "[pubkey] OP_CHECKSIG" – are hashed.  That sequence
# is byte‑for‑byte identical to the funding output’s scriptPubKey, so a
# single SIGHASH_ALL signature works for both checks.

subscript = CScript([pubkey, OP_CHECKSIGVERIFY])           # bytes after CODESEP
sighash   = SignatureHash(subscript, spend_tx, 0, SIGHASH_ALL)

sig = seckey.sign(sighash) + bytes([SIGHASH_ALL])


print(len(sig), sig)
# --------------------------------------------------------------------------
# 5. Assemble full scriptSig
#    <sig> OP_DUP OP_CODESEPARATOR <pk> OP_CHECKSIG
# --------------------------------------------------------------------------
spend_tx.vin[0].scriptSig = CScript([
    OP_TRUE,
    sig,
    OP_DUP,
    OP_CODESEPARATOR,
    pubkey,
    OP_CHECKSIGVERIFY
])

spend_tx.GetHash()

# --------------------------------------------------------------------------
# 6. Consensus verification (policy flags kept minimal)
# --------------------------------------------------------------------------
VerifyScript(
    spend_tx.vin[0].scriptSig,
    CScript([pubkey, OP_CHECKSIGVERIFY]),
    spend_tx, 0,
    flags=MANDATORY_SCRIPT_VERIFY_FLAGS,
)

print("✓ script validated under consensus rules")
print("Funding txid :", b2x(funding_tx.GetTxid()))
print("Spending tx  :", b2x(spend_tx.serialize()))

output:

✓ script validated under consensus rules
Funding txid : a9114135fa063561c2f931374a411328225e4069f53bdbb57326ff919feef0d4
Spending tx  : 0200000001a9114135fa063561c2f931374a411328225e4069f53bdbb57326ff919feef0d4000000006e51473044022064c6b87eed1936f2c10ed05ef410f967aecea28acf1dbb58dcb5a643afb5c71f02202fbef2d81754f46a19583948d1c3fb38c25ea1ce5429affc4386c99ad2039e320176ab21031b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078fadffffffff01804a5d0500000000017600000000

p.s., if you wanted the script to have a scriptSig that was a bit tighter, you could do something like this to restrict the scriptSig and scriptPubKey to have a single item on the stack that gets dup’d, and then checksigverify it. Less garbage data possible

POST_SEP = [
    OP_DEPTH,
    OP_1,
    OP_EQUALVERIFY,
    OP_DUP,
    pubkey,
    OP_CHECKSIGVERIFY]
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