diff --git a/SM4-with-AESENCLAST.md b/SM4-with-AESENCLAST.md
index 1bf902f..55232b0 100644
--- a/SM4-with-AESENCLAST.md
+++ b/SM4-with-AESENCLAST.md
@@ -112,7 +112,7 @@ sm4_box_aesbox_2
 	for i := 0; i < 16; i++ {
 		v := x.bytes[i]
 		v = a1l.bytes[v&0xf] ^ a1h.bytes[v>>4]     // v = A1(x)
-		v = aes_sbox[v] ^ 0xf                      // v = AES-S(A1(x)) XOR 0x0f
+		v = aes_sbox[v] ^ 0xf                      // v = AES-S(A1(x)) XOR 0x0f, 相当于对低四位取反。
 		v = a2l.bytes[^v&0xf] ^ a2h.bytes[v>>4]    // v = A2(AES-S(A1(x)))
 		x.bytes[i] = v
 	}