txscript: add support to re-derive output's PkScript from input

In this commit, we extend the txscript package to support re-deriving
the PkScript of an output by looking at the input's signature
script/witness attempting to spend it. As of this commit, the only
supported types are P2SH, v0 P2WSH, and v0 P2WPKH.

This will serve useful to detect when a particular script has been spent
on-chain.

A set of test vectors has also been added for the supported script types
to ensure its correctness.

txscript/pkscript.go

@@ -0,0 +1,260 @@
+package txscript
+
+import (
+	"crypto/sha256"
+	"errors"
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcutil"
+	"golang.org/x/crypto/ripemd160"
+)
+
+const (
+	// pubKeyHashSigScriptLen is the length of a signature script attempting
+	// to spend a P2PKH script. The only other possible length value is 107
+	// bytes, due to the signature within it. This length is determined by
+	// the following:
+	//   0x47 or 0x48 (71 or 72 byte data push) | <71 or 72 byte sig> |
+	//   0x21 (33 byte data push) | <33 byte compressed pubkey>
+	pubKeyHashSigScriptLen = 106
+
+	// compressedPubKeyLen is the length in bytes of a compressed public
+	// key.
+	compressedPubKeyLen = 33
+
+	// pubKeyHashLen is the length of a P2PKH script.
+	pubKeyHashLen = 25
+
+	// witnessV0PubKeyHashLen is the length of a P2WPKH script.
+	witnessV0PubKeyHashLen = 22
+
+	// scriptHashLen is the length of a P2SH script.
+	scriptHashLen = 23
+
+	// witnessV0ScriptHashLen is the length of a P2WSH script.
+	witnessV0ScriptHashLen = 34
+
+	// maxLen is the maximum script length supported by ParsePkScript.
+	maxLen = witnessV0ScriptHashLen
+)
+
+var (
+	// ErrUnsupportedScriptType is an error returned when we attempt to
+	// parse/re-compute an output script into a PkScript struct.
+	ErrUnsupportedScriptType = errors.New("unsupported script type")
+)
+
+// PkScript is a wrapper struct around a byte array, allowing it to be used
+// as a map index.
+type PkScript struct {
+	// class is the type of the script encoded within the byte array. This
+	// is used to determine the correct length of the script within the byte
+	// array.
+	class ScriptClass
+
+	// script is the script contained within a byte array. If the script is
+	// smaller than the length of the byte array, it will be padded with 0s
+	// at the end.
+	script [maxLen]byte
+}
+
+// ParsePkScript parses an output script into the PkScript struct.
+// ErrUnsupportedScriptType is returned when attempting to parse an unsupported
+// script type.
+func ParsePkScript(pkScript []byte) (PkScript, error) {
+	var outputScript PkScript
+	scriptClass, _, _, err := ExtractPkScriptAddrs(
+		pkScript, &chaincfg.MainNetParams,
+	)
+	if err != nil {
+		return outputScript, fmt.Errorf("unable to parse script type: "+
+			"%v", err)
+	}
+
+	if !isSupportedScriptType(scriptClass) {
+		return outputScript, ErrUnsupportedScriptType
+	}
+
+	outputScript.class = scriptClass
+	copy(outputScript.script[:], pkScript)
+
+	return outputScript, nil
+}
+
+// isSupportedScriptType determines whether the script type is supported by the
+// PkScript struct.
+func isSupportedScriptType(class ScriptClass) bool {
+	switch class {
+	case PubKeyHashTy, WitnessV0PubKeyHashTy, ScriptHashTy,
+		WitnessV0ScriptHashTy:
+		return true
+	default:
+		return false
+	}
+}
+
+// Class returns the script type.
+func (s PkScript) Class() ScriptClass {
+	return s.class
+}
+
+// Script returns the script as a byte slice without any padding.
+func (s PkScript) Script() []byte {
+	var script []byte
+
+	switch s.class {
+	case PubKeyHashTy:
+		script = make([]byte, pubKeyHashLen)
+		copy(script, s.script[:pubKeyHashLen])
+
+	case WitnessV0PubKeyHashTy:
+		script = make([]byte, witnessV0PubKeyHashLen)
+		copy(script, s.script[:witnessV0PubKeyHashLen])
+
+	case ScriptHashTy:
+		script = make([]byte, scriptHashLen)
+		copy(script, s.script[:scriptHashLen])
+
+	case WitnessV0ScriptHashTy:
+		script = make([]byte, witnessV0ScriptHashLen)
+		copy(script, s.script[:witnessV0ScriptHashLen])
+
+	default:
+		// Unsupported script type.
+		return nil
+	}
+
+	return script
+}
+
+// Address encodes the script into an address for the given chain.
+func (s PkScript) Address(chainParams *chaincfg.Params) (btcutil.Address, error) {
+	_, addrs, _, err := ExtractPkScriptAddrs(s.Script(), chainParams)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse address: %v", err)
+	}
+
+	return addrs[0], nil
+}
+
+// String returns a hex-encoded string representation of the script.
+func (s PkScript) String() string {
+	str, _ := DisasmString(s.Script())
+	return str
+}
+
+// ComputePkScript computes the pkScript of an transaction output by looking at
+// the transaction input's signature script or witness.
+//
+// NOTE: Only P2PKH, P2SH, P2WSH, and P2WPKH redeem scripts are supported.
+func ComputePkScript(sigScript []byte, witness wire.TxWitness) (PkScript, error) {
+	var pkScript PkScript
+
+	// Ensure that either an input's signature script or a witness was
+	// provided.
+	if len(sigScript) == 0 && len(witness) == 0 {
+		return pkScript, ErrUnsupportedScriptType
+	}
+
+	// We'll start by checking the input's signature script, if provided.
+	switch {
+	// If a signature script is provided with a length long enough to
+	// represent a P2PKH script, then we'll attempt to parse the compressed
+	// public key from it.
+	case len(sigScript) == pubKeyHashSigScriptLen ||
+		len(sigScript) == pubKeyHashSigScriptLen+1:
+
+		// The public key should be found as the last part of the
+		// signature script. We'll attempt to parse it to ensure this is
+		// a P2PKH redeem script.
+		pubKey := sigScript[len(sigScript)-compressedPubKeyLen:]
+		if btcec.IsCompressedPubKey(pubKey) {
+			pubKeyHash := hash160(pubKey)
+			script, err := payToPubKeyHashScript(pubKeyHash)
+			if err != nil {
+				return pkScript, err
+			}
+
+			pkScript.class = PubKeyHashTy
+			copy(pkScript.script[:], script)
+			return pkScript, nil
+		}
+
+		// If it isn't, we'll assume it is a P2SH signature script.
+		fallthrough
+
+	// If we failed to parse a compressed public key from the script in the
+	// case above, or if the script length is not that of a P2PKH one, and
+	// our redeem script is only composed of data pushed, we can assume it's
+	// a P2SH signature script.
+	case len(sigScript) > 0 && IsPushOnlyScript(sigScript):
+		// The redeem script will always be the last data push of the
+		// signature script, so we'll parse the script into opcodes to
+		// obtain it.
+		parsedOpcodes, err := parseScript(sigScript)
+		if err != nil {
+			return pkScript, err
+		}
+		redeemScript := parsedOpcodes[len(parsedOpcodes)-1].data
+
+		scriptHash := hash160(redeemScript)
+		script, err := payToScriptHashScript(scriptHash)
+		if err != nil {
+			return pkScript, err
+		}
+
+		pkScript.class = ScriptHashTy
+		copy(pkScript.script[:], script)
+		return pkScript, nil
+
+	case len(sigScript) > 0:
+		return pkScript, ErrUnsupportedScriptType
+	}
+
+	// If a witness was provided instead, we'll use the last item of the
+	// witness stack to determine the proper witness type.
+	lastWitnessItem := witness[len(witness)-1]
+
+	switch {
+	// If the witness stack has a size of 2 and its last item is a
+	// compressed public key, then this is a P2WPKH witness.
+	case len(witness) == 2 && len(lastWitnessItem) == compressedPubKeyLen:
+		pubKeyHash := hash160(lastWitnessItem)
+		script, err := payToWitnessPubKeyHashScript(pubKeyHash)
+		if err != nil {
+			return pkScript, err
+		}
+
+		pkScript.class = WitnessV0PubKeyHashTy
+		copy(pkScript.script[:], script)
+		return pkScript, nil
+
+	// For any other witnesses, we'll assume it's a P2WSH witness.
+	default:
+		scriptHash := sha256.Sum256(lastWitnessItem)
+		script, err := payToWitnessScriptHashScript(scriptHash[:])
+		if err != nil {
+			return pkScript, err
+		}
+
+		pkScript.class = WitnessV0ScriptHashTy
+		copy(pkScript.script[:], script)
+		return pkScript, nil
+	}
+}
+
+// hash160 returns the RIPEMD160 hash of the SHA-256 HASH of the given data.
+func hash160(data []byte) []byte {
+	h := sha256.Sum256(data)
+	return ripemd160h(h[:])
+}
+
+// ripemd160h returns the RIPEMD160 hash of the given data.
+func ripemd160h(data []byte) []byte {
+	h := ripemd160.New()
+	h.Write(data)
+	return h.Sum(nil)
+}