diff --git a/sm3/sm3.go b/sm3/sm3.go
index 828f006..2ad99fa 100644
--- a/sm3/sm3.go
+++ b/sm3/sm3.go
@@ -129,17 +129,19 @@ func (d *digest) Sum(in []byte) []byte {
 func (d *digest) checkSum() [Size]byte {
 	len := d.len
 	// Padding. Add a 1 bit and 0 bits until 56 bytes mod 64.
-	var tmp [64]byte
+	var tmp [64 + 8]byte // padding + length buffer
 	tmp[0] = 0x80
+	var t uint64
 	if len%64 < 56 {
-		d.Write(tmp[0 : 56-len%64])
+		t = 56 - len%64
 	} else {
-		d.Write(tmp[0 : 64+56-len%64])
+		t = 64 + 56 - len%64
 	}
 	// Length in bits.
 	len <<= 3
-	binary.BigEndian.PutUint64(tmp[:], len)
-	d.Write(tmp[0:8])
+	padlen := tmp[:t+8]
+	binary.BigEndian.PutUint64(padlen[t:], len)
+	d.Write(padlen)
 
 	if d.nx != 0 {
 		panic("d.nx != 0")