diff --git a/sm4/block.go b/sm4/block.go
index fbdbdbe..71fc5e6 100644
--- a/sm4/block.go
+++ b/sm4/block.go
@@ -11,8 +11,10 @@ type convert func(uint32) uint32
 
 // Encrypt one block from src into dst, using the expanded key xk.
 func encryptBlockGo(xk []uint32, dst, src []byte) {
-	_ = src[15] // early bounds check
-	_ = dst[15] // early bounds check
+	_ = src[15]    // early bounds check
+	dst = dst[:16] // early bounds check
+	_ = xk[31]     // bounds check elimination hint
+
 	var b0, b1, b2, b3 uint32
 	b0 = binary.BigEndian.Uint32(src[0:4])
 	b1 = binary.BigEndian.Uint32(src[4:8])
@@ -68,16 +70,22 @@ func encryptBlockGo(xk []uint32, dst, src []byte) {
 // Key expansion algorithm.
 func expandKeyGo(key []byte, enc, dec []uint32) {
 	// Encryption key setup.
+	enc = enc[:rounds-1]
 	var i int
-	var mk []uint32
+	var mk [4]uint32
 	var k [rounds + 4]uint32
-	nk := len(key) / 4
-	mk = make([]uint32, nk)
-	for i = 0; i < nk; i++ {
-		mk[i] = binary.BigEndian.Uint32(key[4*i:])
-		k[i] = mk[i] ^ fk[i]
-	}
 
+	key = key[:KeySize]
+	mk[0] = binary.BigEndian.Uint32(key)
+	k[0] = mk[0] ^ fk[0]
+	mk[1] = binary.BigEndian.Uint32(key[4:])
+	k[1] = mk[1] ^ fk[1]
+	mk[2] = binary.BigEndian.Uint32(key[8:])
+	k[2] = mk[2] ^ fk[2]
+	mk[3] = binary.BigEndian.Uint32(key[12:])
+	k[3] = mk[3] ^ fk[3]
+
+	_ = enc[rounds-1]
 	for i = 0; i < rounds; i++ {
 		k[i+4] = k[i] ^ t2(k[i+1]^k[i+2]^k[i+3]^ck[i])
 		enc[i] = k[i+4]
@@ -87,6 +95,8 @@ func expandKeyGo(key []byte, enc, dec []uint32) {
 	if dec == nil {
 		return
 	}
+
+	dec = dec[:rounds-1]
 	for i = 0; i < rounds; i++ {
 		dec[i] = enc[rounds-1-i]
 	}
@@ -97,12 +107,12 @@ func decryptBlockGo(xk []uint32, dst, src []byte) {
 	encryptBlockGo(xk, dst, src)
 }
 
-//L(B)
+// L(B)
 func l(b uint32) uint32 {
 	return b ^ bits.RotateLeft32(b, 2) ^ bits.RotateLeft32(b, 10) ^ bits.RotateLeft32(b, 18) ^ bits.RotateLeft32(b, 24)
 }
 
-//L'(B)
+// L'(B)
 func l2(b uint32) uint32 {
 	return b ^ bits.RotateLeft32(b, 13) ^ bits.RotateLeft32(b, 23)
 }
@@ -116,12 +126,12 @@ func _t(in uint32, fn convert) uint32 {
 	return fn(binary.BigEndian.Uint32(bytes[:]))
 }
 
-//T
+// T
 func t(in uint32) uint32 {
 	return _t(in, l)
 }
 
-//T'
+// T'
 func t2(in uint32) uint32 {
 	return _t(in, l2)
 }
diff --git a/sm4/cbc_cipher_asm.go b/sm4/cbc_cipher_asm.go
index d0da0a6..82691ba 100644
--- a/sm4/cbc_cipher_asm.go
+++ b/sm4/cbc_cipher_asm.go
@@ -78,8 +78,8 @@ func (x *cbc) CryptBlocks(dst, src []byte) {
 
 	for start > 0 {
 		x.b.DecryptBlocks(temp, src[start:end])
-		copy(batchSrc, src[start-BlockSize:])
-		subtle.XORBytes(dst[start:], temp, batchSrc)
+		subtle.XORBytes(temp, temp, src[start-BlockSize:end-BlockSize])
+		copy(dst[start:], temp)
 		end = start
 		start -= x.b.blocksSize
 	}
diff --git a/sm4/const.go b/sm4/const.go
index 0fed83b..742aae3 100644
--- a/sm4/const.go
+++ b/sm4/const.go
@@ -214,3 +214,5 @@ var ck = [32]uint32{
 var fk = [4]uint32{
 	0xa3b1bac6, 0x56aa3350, 0x677d9197, 0xb27022dc,
 }
+
+const KeySize = 16
\ No newline at end of file