script.go

// Copyright (c) 2013-2016 The btcsuite developers
// Copyright (c) 2015-2016 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package txscript

import (
	"bytes"
	"encoding/binary"
	"fmt"

	"github.com/decred/dcrd/chaincfg"
	"github.com/decred/dcrd/chaincfg/chainhash"
	"github.com/decred/dcrd/wire"
)

// SigHashType represents hash type bits at the end of a signature.
type SigHashType byte

// Hash type bits from the end of a signature.
const (
	SigHashOld          SigHashType = 0x0
	SigHashAll          SigHashType = 0x1
	SigHashNone         SigHashType = 0x2
	SigHashSingle       SigHashType = 0x3
	SigHashAllValue     SigHashType = 0x4
	SigHashAnyOneCanPay SigHashType = 0x80

	// sigHashMask defines the number of bits of the hash type which is used
	// to identify which outputs are signed.
	sigHashMask = 0x1f
)

// These are the constants specified for maximums in individual scripts.
const (
	MaxOpsPerScript       = 255  // Max number of non-push operations.
	MaxPubKeysPerMultiSig = 20   // Multisig can't have more sigs than this.
	MaxScriptElementSize  = 2048 // Max bytes pushable to the stack.
)

// isSmallInt returns whether or not the opcode is considered a small integer,
// which is an OP_0, or OP_1 through OP_16.
func isSmallInt(op *opcode) bool {
	if op.value == OP_0 || (op.value >= OP_1 && op.value <= OP_16) {
		return true
	}
	return false
}

// IsPayToScriptHash returns true if the script is in the standard
// pay-to-script-hash (P2SH) format, false otherwise.
func IsPayToScriptHash(script []byte) bool {
	pops, err := parseScript(script)
	if err != nil {
		return false
	}
	return isScriptHash(pops)
}

// isPushOnly returns true if the script only pushes data, false otherwise.
func isPushOnly(pops []parsedOpcode) bool {
	// NOTE: This function does NOT verify opcodes directly since it is
	// internal and is only called with parsed opcodes for scripts that did
	// not have any parse errors.  Thus, consensus is properly maintained.

	for _, pop := range pops {
		// All opcodes up to OP_16 are data push instructions.
		// NOTE: This does consider OP_RESERVED to be a data push
		// instruction, but execution of OP_RESERVED will fail anyways
		// and matches the behavior required by consensus.
		if pop.opcode.value > OP_16 {
			return false
		}
	}
	return true
}

// IsPushOnlyScript returns whether or not the passed script only pushes data.
//
// False will be returned when the script does not parse.
func IsPushOnlyScript(script []byte) bool {
	pops, err := parseScript(script)
	if err != nil {
		return false
	}
	return isPushOnly(pops)
}

// HasP2SHScriptSigStakeOpCodes returns an error is the p2sh script has either
// stake opcodes or if the pkscript cannot be retrieved.
func HasP2SHScriptSigStakeOpCodes(version uint16, scriptSig,
	scriptPubKey []byte) error {
	class := GetScriptClass(version, scriptPubKey)
	if IsStakeOutput(scriptPubKey) {
		class, _ = GetStakeOutSubclass(scriptPubKey)
	}
	if class == ScriptHashTy {
		// Obtain the embedded pkScript from the scriptSig of the
		// current transaction. Then, ensure that it does not use
		// any stake tagging OP codes.
		pData, err := PushedData(scriptSig)
		if err != nil {
			return fmt.Errorf("error retrieving pushed data "+
				"from script: %v", err)
		}
		if len(pData) == 0 {
			return fmt.Errorf("script has no pushed data")
		}

		// The pay-to-hash-script is the final data push of the
		// signature script.
		shScript := pData[len(pData)-1]

		hasStakeOpCodes, err := ContainsStakeOpCodes(shScript)
		if err != nil {
			return fmt.Errorf("unexpected error checking pkscript "+
				"from p2sh transaction: %v", err.Error())
		}
		if hasStakeOpCodes {
			return ErrP2SHStakeOpCodes
		}
	}

	return nil
}

// parseScriptTemplate is the same as parseScript but allows the passing of the
// template list for testing purposes.  When there are parse errors, it returns
// the list of parsed opcodes up to the point of failure along with the error.
func parseScriptTemplate(script []byte, opcodes *[256]opcode) ([]parsedOpcode,
	error) {
	retScript := make([]parsedOpcode, 0, len(script))
	for i := 0; i < len(script); {
		instr := script[i]
		op := &opcodes[instr]
		pop := parsedOpcode{opcode: op}

		// Parse data out of instruction.
		switch {
		// No additional data.  Note that some of the opcodes, notably
		// OP_1NEGATE, OP_0, and OP_[1-16] represent the data
		// themselves.
		case op.length == 1:
			i++

		// Data pushes of specific lengths -- OP_DATA_[1-75].
		case op.length > 1:
			if len(script[i:]) < op.length {
				return retScript, ErrStackShortScript
			}

			// Slice out the data.
			pop.data = script[i+1 : i+op.length]
			i += op.length

		// Data pushes with parsed lengths -- OP_PUSHDATAP{1,2,4}.
		case op.length < 0:
			var l uint
			off := i + 1

			if len(script[off:]) < -op.length {
				return retScript, ErrStackShortScript
			}

			// Next -length bytes are little endian length of data.
			switch op.length {
			case -1:
				l = uint(script[off])
			case -2:
				l = ((uint(script[off+1]) << 8) |
					uint(script[off]))
			case -4:
				l = ((uint(script[off+3]) << 24) |
					(uint(script[off+2]) << 16) |
					(uint(script[off+1]) << 8) |
					uint(script[off]))
			default:
				return retScript,
					fmt.Errorf("invalid opcode length %d",
						op.length)
			}

			// Move offset to beginning of the data.
			off += -op.length

			// Disallow entries that do not fit script or were
			// sign extended.
			if int(l) > len(script[off:]) || int(l) < 0 {
				return retScript, ErrStackShortScript
			}

			pop.data = script[off : off+int(l)]
			i += 1 - op.length + int(l)
		}

		retScript = append(retScript, pop)
	}

	return retScript, nil
}

// parseScript preparses the script in bytes into a list of parsedOpcodes while
// applying a number of sanity checks.
func parseScript(script []byte) ([]parsedOpcode, error) {
	return parseScriptTemplate(script, &opcodeArray)
}

// unparseScript reversed the action of parseScript and returns the
// parsedOpcodes as a list of bytes
func unparseScript(pops []parsedOpcode) ([]byte, error) {
	script := make([]byte, 0, len(pops))
	for _, pop := range pops {
		b, err := pop.bytes()
		if err != nil {
			return nil, err
		}
		script = append(script, b...)
	}
	return script, nil
}

// DisasmString formats a disassembled script for one line printing.  When the
// script fails to parse, the returned string will contain the disassembled
// script up to the point the failure occurred along with the string '[error]'
// appended.  In addition, the reason the script failed to parse is returned
// if the caller wants more information about the failure.
func DisasmString(buf []byte) (string, error) {
	var disbuf bytes.Buffer
	opcodes, err := parseScript(buf)
	for _, pop := range opcodes {
		disbuf.WriteString(pop.print(true))
		disbuf.WriteByte(' ')
	}
	if disbuf.Len() > 0 {
		disbuf.Truncate(disbuf.Len() - 1)
	}
	if err != nil {
		disbuf.WriteString("[error]")
	}
	return disbuf.String(), err
}

// removeOpcode will remove any opcode matching ``opcode'' from the opcode
// stream in pkscript
func removeOpcode(pkscript []parsedOpcode, opcode byte) []parsedOpcode {
	retScript := make([]parsedOpcode, 0, len(pkscript))
	for _, pop := range pkscript {
		if pop.opcode.value != opcode {
			retScript = append(retScript, pop)
		}
	}
	return retScript
}

// canonicalPush returns true if the object is either not a push instruction
// or the push instruction contained wherein is matches the canonical form
// or using the smallest instruction to do the job. False otherwise.
func canonicalPush(pop parsedOpcode) bool {
	opcode := pop.opcode.value
	data := pop.data
	dataLen := len(pop.data)
	if opcode > OP_16 {
		return true
	}

	if opcode < OP_PUSHDATA1 && opcode > OP_0 && (dataLen == 1 && data[0] <= 16) {
		return false
	}
	if opcode == OP_PUSHDATA1 && dataLen < OP_PUSHDATA1 {
		return false
	}
	if opcode == OP_PUSHDATA2 && dataLen <= 0xff {
		return false
	}
	if opcode == OP_PUSHDATA4 && dataLen <= 0xffff {
		return false
	}
	return true
}

// removeOpcodeByData will return the script minus any opcodes that would push
// the passed data to the stack.
func removeOpcodeByData(pkscript []parsedOpcode, data []byte) []parsedOpcode {
	retScript := make([]parsedOpcode, 0, len(pkscript))
	for _, pop := range pkscript {
		if !canonicalPush(pop) || !bytes.Contains(pop.data, data) {
			retScript = append(retScript, pop)
		}
	}
	return retScript

}

// calcSignatureHash will, given a script and hash type for the current script
// engine instance, calculate the signature hash to be used for signing and
// verification.
func calcSignatureHash(script []parsedOpcode, hashType SigHashType,
	tx *wire.MsgTx, idx int, cachedPrefix *chainhash.Hash) ([]byte, error) {
	// The SigHashSingle signature type signs only the corresponding input
	// and output (the output with the same index number as the input).
	//
	// Since transactions can have more inputs than outputs, this means it
	// is improper to use SigHashSingle on input indices that don't have a
	// corresponding output.
	//
	// A bug in the original Satoshi client implementation means specifying
	// an index that is out of range results in a signature hash of 1 (as a
	// uint256 little endian).  The original intent appeared to be to
	// indicate failure, but unfortunately, it was never checked and thus is
	// treated as the actual signature hash.  This buggy behavior is now
	// part of the consensus and a hard fork would be required to fix it.
	//
	// Due to this, care must be taken by software that creates transactions
	// which make use of SigHashSingle because it can lead to an extremely
	// dangerous situation where the invalid inputs will end up signing a
	// hash of 1.  This in turn presents an opportunity for attackers to
	// cleverly construct transactions which can steal those coins provided
	// they can reuse signatures.
	//
	// Decred mitigates this by actually returning an error instead.
	if hashType&sigHashMask == SigHashSingle && idx >= len(tx.TxOut) {
		return nil, ErrSighashSingleIdx
	}

	// Remove all instances of OP_CODESEPARATOR from the script.
	script = removeOpcode(script, OP_CODESEPARATOR)

	// Make a deep copy of the transaction, zeroing out the script for all
	// inputs that are not currently being processed.
	txCopy := tx.Copy()
	for i := range txCopy.TxIn {
		if i == idx {
			// UnparseScript cannot fail here because removeOpcode
			// above only returns a valid script.
			sigScript, _ := unparseScript(script)
			txCopy.TxIn[idx].SignatureScript = sigScript
		} else {
			txCopy.TxIn[i].SignatureScript = nil
		}
	}

	switch hashType & sigHashMask {
	case SigHashNone:
		txCopy.TxOut = txCopy.TxOut[0:0] // Empty slice.
		for i := range txCopy.TxIn {
			if i != idx {
				txCopy.TxIn[i].Sequence = 0
			}
		}

	case SigHashSingle:
		// Resize output array to up to and including requested index.
		txCopy.TxOut = txCopy.TxOut[:idx+1]

		// All but current output get zeroed out.
		for i := 0; i < idx; i++ {
			txCopy.TxOut[i].Value = -1
			txCopy.TxOut[i].PkScript = nil
		}

		// Sequence on all other inputs is 0, too.
		for i := range txCopy.TxIn {
			if i != idx {
				txCopy.TxIn[i].Sequence = 0
			}
		}

	default:
		// Consensus treats undefined hashtypes like normal SigHashAll
		// for purposes of hash generation.
		fallthrough
	case SigHashOld:
		fallthrough
	case SigHashAllValue:
		fallthrough
	case SigHashAll:
		// Nothing special here.
	}
	if hashType&SigHashAnyOneCanPay != 0 {
		txCopy.TxIn = txCopy.TxIn[idx : idx+1]

		// Future code that may need the index
		// should consider resetting it.
		// idx = 0
	}

	// The final hash (message to sign) is the hash of:
	// 1) the hash type (encoded as a 4-byte little-endian value)
	// 2) hash of the prefix ||
	// 3) hash of the witness for signing ||
	var wbuf bytes.Buffer
	wbuf.Grow(chainhash.HashSize*2 + 4)
	binary.Write(&wbuf, binary.LittleEndian, uint32(hashType))

	// Optimization for SIGHASH_ALL. In this case, the prefix hash is
	// the same as the transaction hash because only the inputs have
	// been modified, so don't bother to do the wasteful O(N^2) extra
	// hash here.
	// The caching only works if the "anyone can pay flag" is also
	// disabled.
	var prefixHash chainhash.Hash
	if cachedPrefix != nil &&
		(hashType&sigHashMask == SigHashAll) &&
		(hashType&SigHashAnyOneCanPay == 0) &&
		chaincfg.SigHashOptimization {
		prefixHash = *cachedPrefix
	} else {
		prefixHash = txCopy.TxHash()
	}

	// If the ValueIn is to be included in what we're signing, sign
	// the witness hash that includes it. Otherwise, just sign the
	// prefix and signature scripts.
	var witnessHash chainhash.Hash
	if hashType&sigHashMask != SigHashAllValue {
		witnessHash = txCopy.TxHashWitnessSigning()
	} else {
		witnessHash = txCopy.TxHashWitnessValueSigning()
	}
	wbuf.Write(prefixHash[:])
	wbuf.Write(witnessHash[:])
	return chainhash.HashB(wbuf.Bytes()), nil
}

// CalcSignatureHash computes the signature hash for the specified input of
// the target transaction observing the desired signature hash type.  The
// cached prefix parameter allows the caller to optimize the calculation by
// providing the prefix hash to be reused in the case of SigHashAll without the
// SigHashAnyOneCanPay flag set.
func CalcSignatureHash(script []byte, hashType SigHashType, tx *wire.MsgTx, idx int, cachedPrefix *chainhash.Hash) ([]byte, error) {
	pops, err := parseScript(script)
	if err != nil {
		return nil, err
	}

	return calcSignatureHash(pops, hashType, tx, idx, cachedPrefix)
}

// asSmallInt returns the passed opcode, which must be true according to
// isSmallInt(), as an integer.
func asSmallInt(op *opcode) int {
	if op.value == OP_0 {
		return 0
	}

	return int(op.value - (OP_1 - 1))
}

// getSigOpCount is the implementation function for counting the number of
// signature operations in the script provided by pops. If precise mode is
// requested then we attempt to count the number of operations for a multisig
// op. Otherwise we use the maximum.
func getSigOpCount(pops []parsedOpcode, precise bool) int {
	nSigs := 0
	for i, pop := range pops {
		switch pop.opcode.value {
		case OP_CHECKSIG:
			fallthrough
		case OP_CHECKSIGVERIFY:
			fallthrough
		case OP_CHECKSIGALT:
			fallthrough
		case OP_CHECKSIGALTVERIFY:
			nSigs++
		case OP_CHECKMULTISIG:
			fallthrough
		case OP_CHECKMULTISIGVERIFY:
			// If we are being precise then look for familiar
			// patterns for multisig, for now all we recognize is
			// OP_1 - OP_16 to signify the number of pubkeys.
			// Otherwise, we use the max of 20.
			if precise && i > 0 &&
				pops[i-1].opcode.value >= OP_1 &&
				pops[i-1].opcode.value <= OP_16 {
				nSigs += asSmallInt(pops[i-1].opcode)
			} else {
				nSigs += MaxPubKeysPerMultiSig
			}
		default:
			// Not a sigop.
		}
	}

	return nSigs
}

// GetSigOpCount provides a quick count of the number of signature operations
// in a script. a CHECKSIG operations counts for 1, and a CHECK_MULTISIG for 20.
// If the script fails to parse, then the count up to the point of failure is
// returned.
func GetSigOpCount(script []byte) int {
	// Don't check error since parseScript returns the parsed-up-to-error
	// list of pops.
	pops, _ := parseScript(script)
	return getSigOpCount(pops, false)
}

// GetPreciseSigOpCount returns the number of signature operations in
// scriptPubKey.  If bip16 is true then scriptSig may be searched for the
// Pay-To-Script-Hash script in order to find the precise number of signature
// operations in the transaction.  If the script fails to parse, then the count
// up to the point of failure is returned.
func GetPreciseSigOpCount(scriptSig, scriptPubKey []byte, bip16 bool) int {
	// Don't check error since parseScript returns the parsed-up-to-error
	// list of pops.
	pops, _ := parseScript(scriptPubKey)

	// Treat non P2SH transactions as normal.
	if !(bip16 && isScriptHash(pops)) {
		return getSigOpCount(pops, true)
	}

	// The public key script is a pay-to-script-hash, so parse the signature
	// script to get the final item.  Scripts that fail to fully parse count
	// as 0 signature operations.
	sigPops, err := parseScript(scriptSig)
	if err != nil {
		return 0
	}

	// The signature script must only push data to the stack for P2SH to be
	// a valid pair, so the signature operation count is 0 when that is not
	// the case.
	if !isPushOnly(sigPops) || len(sigPops) == 0 {
		return 0
	}

	// The P2SH script is the last item the signature script pushes to the
	// stack.  When the script is empty, there are no signature operations.
	shScript := sigPops[len(sigPops)-1].data
	if len(shScript) == 0 {
		return 0
	}

	// Parse the P2SH script and don't check the error since parseScript
	// returns the parsed-up-to-error list of pops and the consensus rules
	// dictate signature operations are counted up to the first parse
	// failure.
	shPops, _ := parseScript(shScript)
	return getSigOpCount(shPops, true)
}

// IsUnspendable returns whether the passed public key script is unspendable, or
// guaranteed to fail at execution.  This allows inputs to be pruned instantly
// when entering the UTXO set. In Decred, all zero value outputs are unspendable.
func IsUnspendable(amount int64, pkScript []byte) bool {
	if amount == 0 {
		return true
	}

	pops, err := parseScript(pkScript)
	if err != nil {
		return true
	}

	return len(pops) > 0 && pops[0].opcode.value == OP_RETURN
}