Bitcoin Script � The Transaction Programming Language

class BitcoinScriptVM:
    """Stack-based Bitcoin Script interpreter (simplified)"""

    def __init__(self):
        self.stack: list = []

    def execute(self, script_tokens: list) -> bool:
        """Execute a combined scriptSig + scriptPubKey"""
        for token in script_tokens:
            if token.startswith("OP_"):
                self._execute_opcode(token)
            else:
                self.stack.append(token)
                print(f"  PUSH {token!r} -> Stack: {self.stack}")

        # Valid if stack top is truthy
        result = bool(self.stack and self.stack[-1] not in [False, 0, b"", ""])
        return result

    def _execute_opcode(self, opcode: str):
        print(f"  {opcode} -> Stack before: {self.stack}")
        if opcode == "OP_DUP":
            if self.stack:
                self.stack.append(self.stack[-1])
        elif opcode == "OP_HASH160":
            import hashlib
            if self.stack:
                top = str(self.stack.pop()).encode()
                sha256 = hashlib.sha256(top).digest()
                ripemd = hashlib.new("ripemd160", sha256).hexdigest()
                self.stack.append(ripemd)
        elif opcode == "OP_EQUALVERIFY":
            if len(self.stack) >= 2:
                a, b = self.stack.pop(), self.stack.pop()
                if a != b:
                    raise Exception(f"OP_EQUALVERIFY FAILED: {a!r} != {b!r}")
        elif opcode == "OP_EQUAL":
            if len(self.stack) >= 2:
                a, b = self.stack.pop(), self.stack.pop()
                self.stack.append(a == b)
        elif opcode == "OP_CHECKSIG":
            if len(self.stack) >= 2:
                pubkey = self.stack.pop()
                sig = self.stack.pop()
                # Simplified: assume valid if sig contains pubkey prefix
                is_valid = str(sig).startswith(str(pubkey)[:4])
                self.stack.append(is_valid)
                print(f"  OP_CHECKSIG: sig={str(sig)[:8]}..., pubkey={str(pubkey)[:8]}..., valid={is_valid}")
        print(f"  Stack after: {self.stack}")

# Simulate P2PKH (Pay-to-Public-Key-Hash) script execution
vm = BitcoinScriptVM()
pubkey = "02a1b2c3d4..."
sig = "3044a1b2c3..." # Starts with "3044" (DER-encoded ECDSA prefix)

# Combined script: [scriptSig] + [scriptPubKey]
# scriptSig: <sig> <pubkey>
# scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG

import hashlib
pubkey_hash = hashlib.new("ripemd160", hashlib.sha256(pubkey.encode()).digest()).hexdigest()

combined_script = [sig, pubkey, "OP_DUP", "OP_HASH160", pubkey_hash, "OP_EQUALVERIFY", "OP_CHECKSIG"]

print("=== P2PKH Script Execution ===")
try:
    result = vm.execute(combined_script)
    print(f"\nTransaction VALID: {result}")
except Exception as e:
    print(f"\nTransaction INVALID: {e}")