diff --git a/common/alloc/buffer.go b/common/alloc/buffer.go
index 896e9d36c..aa4e9ccbf 100644
--- a/common/alloc/buffer.go
+++ b/common/alloc/buffer.go
@@ -2,6 +2,7 @@
 package alloc
 
 import (
+	"hash"
 	"io"
 
 	"github.com/v2ray/v2ray-core/common/serial"
@@ -109,6 +110,12 @@ func (b *Buffer) PrependUint32(v uint32) *Buffer {
 	return b
 }
 
+func (b *Buffer) PrependHash(h hash.Hash) *Buffer {
+	b.SliceBack(h.Size())
+	h.Sum(b.Value[:0])
+	return b
+}
+
 // Bytes returns the content bytes of this Buffer.
 func (b *Buffer) Bytes() []byte {
 	return b.Value
diff --git a/proxy/vmess/io/writer.go b/proxy/vmess/io/writer.go
index ef5b407e7..42637b7e5 100644
--- a/proxy/vmess/io/writer.go
+++ b/proxy/vmess/io/writer.go
@@ -30,9 +30,7 @@ func (this *AuthChunkWriter) Release() {
 func Authenticate(buffer *alloc.Buffer) {
 	fnvHash := fnv.New32a()
 	fnvHash.Write(buffer.Value)
-
-	buffer.SliceBack(4)
-	fnvHash.Sum(buffer.Value[:0])
+	buffer.PrependHash(fnvHash)
 
 	buffer.PrependUint16(uint16(buffer.Len()))
 }
diff --git a/transport/internet/kcp/crypt.go b/transport/internet/kcp/crypt.go
index 6d620eeb0..6b8f2754c 100644
--- a/transport/internet/kcp/crypt.go
+++ b/transport/internet/kcp/crypt.go
@@ -32,9 +32,7 @@ func (this *SimpleAuthenticator) Seal(buffer *alloc.Buffer) {
 	buffer.PrependUint16(uint16(buffer.Len()))
 	fnvHash := fnv.New32a()
 	fnvHash.Write(buffer.Value)
-
-	buffer.SliceBack(4)
-	fnvHash.Sum(buffer.Value[:0])
+	buffer.PrependHash(fnvHash)
 
 	len := buffer.Len()
 	xtra := 4 - len%4