Function opcodeCheckSig

txscript/opcode.go:1977–2102

opcodeCheckSig treats the top 2 items on the stack as a public key and a signature and replaces them with a bool which indicates if the signature was successfully verified. The process of verifying a signature requires calculating a signature hash in the same way the transaction signer did. It inv

(op *opcode, data []byte, vm *Engine)

//
// Stack transformation: [... signature pubkey] -> [... bool]
func opcodeCheckSig(op *opcode, data []byte, vm *Engine) error {
	pkBytes, err := vm.dstack.PopByteArray()
	if err != nil {
		return err
	}

	fullSigBytes, err := vm.dstack.PopByteArray()
	if err != nil {
		return err
	}

	// The signature actually needs to be longer than this, but at
	// least 1 byte is needed for the hash type below.  The full length is
	// checked depending on the script flags and upon parsing the signature.
	//
	// This only applies if tapscript verification isn't active, as this
	// check is done within the sighash itself.
	if vm.taprootCtx == nil && len(fullSigBytes) < 1 {
		if err := vm.checkPubKeyEncoding(pkBytes); err != nil {
			return err
		}

		vm.dstack.PushBool(false)
		return nil
	}

	var sigVerifier signatureVerifier
	switch {
	// If no witness program is active, then we're verifying under the
	// base consensus rules.
	case vm.witnessProgram == nil:
		sigVerifier, err = newBaseSigVerifier(
			pkBytes, fullSigBytes, vm,
		)
		if err != nil {
			var scriptErr Error
			if errors.As(err, &scriptErr) {
				return err
			}

			vm.dstack.PushBool(false)
			return nil
		}

	// If the base segwit version is active, then we'll create the verifier
	// that factors in those new consensus rules.
	case vm.isWitnessVersionActive(BaseSegwitWitnessVersion):
		sigVerifier, err = newBaseSegwitSigVerifier(
			pkBytes, fullSigBytes, vm,
		)
		if err != nil {
			var scriptErr Error
			if errors.As(err, &scriptErr) {
				return err
			}

			vm.dstack.PushBool(false)
			return nil