Prevent asInt() from modifying stack data.

The stack data is normally sliced from the actual script and btcscript is not
supposed to ever change the tx passed into it.

Add a test (and fix the other leading zeros tests) to stop this happening again.

stack.go

@@ -19,10 +19,11 @@ func asInt(v []byte) *big.Int {
 	if msb&0x80 == 0x80 {
 		negative = true
 		// remove sign bit
-		v[len(v)-1] &= 0x7f
+		msb &= 0x7f
 	}
+	start := 0
 	// trim leading 0 bytes
-	for ; msb == 0; msb = v[len(v)-1] {
+	for ; msb == 0; msb, start = v[len(v)-1], start+1 {
 		v = v[:len(v)-1]
 		if len(v) == 0 {
 			break
@@ -31,8 +32,13 @@ func asInt(v []byte) *big.Int {
 	// reverse bytes with a copy since stack is immutable.
 	intArray := make([]byte, len(v))
 	for i := range v {
+		// Mask off the sign bit without changing original array.
+		if i == 0 && start == 0 && negative {
+			intArray[len(v)-i -1] = v[i] & 0x7f
+		} else {
 			intArray[len(v)-i-1] = v[i]
 		}
+	}
 
 	num := new(big.Int).SetBytes(intArray)
 	if negative {

stack_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"errors"
 	"github.com/conformal/btcscript"
+	"fmt"
 	"math/big"
 	"testing"
 )
@@ -220,13 +221,14 @@ var stackTests = []stackTest{
 	},
 	{
 		"popInt 1 leading 0",
-		[][]byte{{0x00000001}},
+		[][]byte{{0x01, 0x00, 0x00, 0x00}},
 		func(stack *btcscript.Stack) error {
 			v, err := stack.PopInt()
 			if err != nil {
 				return err
 			}
 			if v.Cmp(big.NewInt(1)) != 0 {
+				fmt.Printf("%v != %v\n", v, big.NewInt(1))
 				return errors.New("1 != 1 on popInt")
 			}
 			return nil
@@ -234,6 +236,41 @@ var stackTests = []stackTest{
 		nil,
 		[][]byte{},
 	},
+	{
+		"popInt -1 leading 0",
+		[][]byte{{0x01,0x00, 0x00, 0x80}},
+		func(stack *btcscript.Stack) error {
+			v, err := stack.PopInt()
+			if err != nil {
+				return err
+			}
+			if v.Cmp(big.NewInt(-1)) != 0 {
+				fmt.Printf("%v != %v\n", v, big.NewInt(-1))
+				return errors.New("-1 != -1 on popInt")
+			}
+			return nil
+		},
+		nil,
+		[][]byte{},
+	},
+	// Confirm that the asInt code doesn't modify the base data.
+	{
+		"peekint nomodify -1",
+		[][]byte{{0x01,0x00, 0x00, 0x80}},
+		func(stack *btcscript.Stack) error {
+			v, err := stack.PeekInt(0)
+			if err != nil {
+				return err
+			}
+			if v.Cmp(big.NewInt(-1)) != 0 {
+				fmt.Printf("%v != %v\n", v, big.NewInt(-1))
+				return errors.New("-1 != -1 on popInt")
+			}
+			return nil
+		},
+		nil,
+		[][]byte{{0x01,0x00, 0x00, 0x80}},
+	},
 	{
 		"PushInt 0",
 		[][]byte{},