diff --git a/core/iovec/iovec.go b/core/iovec/iovec.go
index 7700aac..eacc4b8 100644
--- a/core/iovec/iovec.go
+++ b/core/iovec/iovec.go
@@ -33,6 +33,9 @@ func (self IoVec) Concat() []byte {
 }
 
 func (self *IoVec) Take(s []byte) *IoVec {
+	if len(s) == 0 {
+		return self
+	}
 	*self = append(*self, s)
 	return self
 }
@@ -69,6 +72,31 @@ func (self *IoVec) Consume() []byte {
 	return b.Bytes()
 }
 
+func (self IoVec) LastByte() (byte, error) {
+	l := len(self)
+	if l == 0 {
+		return 0, fmt.Errorf("This IoVec is empty")
+	}
+	k := len(self[l-1])
+	return self[l-1][k-1], nil
+}
+
+func (self *IoVec) Drop(s int) error {
+	l := len(*self)
+	c := 0
+	for i := l - 1; i >= 0; i-- {
+		v := (*self)[i]
+		vl := len(v)
+		if c+vl >= s {
+			(*self) = (*self)[:i]
+			self.Take(v[:c+vl-s])
+			return nil
+		}
+		c += vl
+	}
+	return fmt.Errorf("Unable to drop %d bytes", s)
+}
+
 func (self IoVec) At(i int) (byte, error) {
 	c := 0
 	for _, v := range self {
@@ -91,17 +119,19 @@ func (self IoVec) Peek(i int) ([]byte, error) {
 	return nil, fmt.Errorf("Index %d out of bound", i)
 }
 
-func (self *IoVec) Drop(i int) error {
+func (self *IoVec) Split(i int) (tail IoVec) {
 	c := 0
 	for k, v := range *self {
 		if i >= c && i < c+len(v) {
-			*self = (*self)[:k]
-			if i-c != 0 {
-				*self = append(*self, v[:i-c])
+			tail.Take(v[i-c:])
+			if len((*self)[k+1:]) != 0 {
+				tail = append(tail, (*self)[k+1:]...)
 			}
-			return nil
+			*self = (*self)[:k]
+			self.Take(v[:i-c])
+			return
 		}
 		c += len(v)
 	}
-	return fmt.Errorf("Index %d out of bound", i)
+	return
 }
diff --git a/core/iovec/iovec_test.go b/core/iovec/iovec_test.go
index 223c62e..605d6ba 100644
--- a/core/iovec/iovec_test.go
+++ b/core/iovec/iovec_test.go
@@ -32,7 +32,7 @@ func TestDrop(t *testing.T) {
 	var v IoVec
 	v.Take([]byte("hello"))
 	v.Take([]byte("bar"))
-	if err := v.Drop(5); err != nil {
+	if err := v.Drop(3); err != nil {
 		t.Fail()
 	}
 	s := string(v.Consume())
@@ -45,7 +45,7 @@ func TestDrop2(t *testing.T) {
 	var v IoVec
 	v.Take([]byte("hello"))
 	v.Take([]byte("bar"))
-	if err := v.Drop(6); err != nil {
+	if err := v.Drop(2); err != nil {
 		t.Fail()
 	}
 	s := string(v.Consume())
@@ -54,6 +54,66 @@ func TestDrop2(t *testing.T) {
 	}
 }
 
+func TestSplit(t *testing.T) {
+	var v IoVec
+	v.Take([]byte("hello"))
+	v.Take([]byte("bar"))
+	tail := v.Split(0)
+	s := string(tail.Consume())
+	if s != "hellobar" {
+		t.Fail()
+	}
+	sv := string(v.Consume())
+	if sv != "" {
+		t.Fail()
+	}
+}
+
+func TestSplit1(t *testing.T) {
+	var v IoVec
+	v.Take([]byte("hello"))
+	v.Take([]byte("bar"))
+	tail := v.Split(5)
+	s := string(tail.Consume())
+	if s != "bar" {
+		t.Fail()
+	}
+	sv := string(v.Consume())
+	if sv != "hello" {
+		t.Fail()
+	}
+}
+
+func TestSplit2(t *testing.T) {
+	var v IoVec
+	v.Take([]byte("hello"))
+	v.Take([]byte("bar"))
+	tail := v.Split(6)
+	s := string(tail.Consume())
+	if s != "ar" {
+		t.Fail()
+	}
+	sv := string(v.Consume())
+	if sv != "hellob" {
+		t.Fail()
+	}
+}
+
+func TestSplit3(t *testing.T) {
+	var v IoVec
+	v.Take([]byte("hello"))
+	v.Take([]byte("bar"))
+	tail := v.Split(3)
+	s := string(tail.Consume())
+	if s != "lobar" {
+		t.Fail()
+	}
+	sv := string(v.Consume())
+	if sv != "hel" {
+		t.Fail()
+	}
+}
+
 func TestConcat(t *testing.T) {
 	var v IoVec
 	v.Take([]byte("hello"))
diff --git a/passes/passes.go b/passes/passes.go
index aa5b170..ef1f3dc 100644
--- a/passes/passes.go
+++ b/passes/passes.go
@@ -208,12 +208,11 @@ type OBFSDecoder struct {
 }
 
 func (self *OBFSDecoder) Run(b *iovec.IoVec) error {
-	l := b.Len()
-	t, err := b.At(l - 1)
+	t, err := b.LastByte()
 	if err != nil {
 		return err
 	}
-	if err := b.Drop(l - (1 + int(t))); err != nil {
+	if err := b.Drop(1 + int(t)); err != nil {
 		return err
 	}
 	return WrapLegacyPass(self, b)