keyChain & identity添加部分注释

src/main/java/minsecurity/crypto/sm2/SM2Exception.java

@@ -1,5 +1,7 @@
 package minsecurity.crypto.sm2;
 
+import minsecurity.crypto.AsymKeyException;
+
 /*
  * @Author: hongyu guo
  * @Description: sm2的自定义异常
@@ -7,7 +9,7 @@ package minsecurity.crypto.sm2;
  * @Date: 19:00 2021/03/03
  * @Copyright: MIN-Group；国家重大科技基础设施——未来网络北大实验室；深圳市信息论与未来网络重点实验室
  */
-public class SM2Exception extends java.lang.Exception {
+public class SM2Exception extends AsymKeyException {
     public SM2Exception(String msg){
         super(msg);
     }

src/main/java/minsecurity/crypto/sm2/SM2PublicKey.java

@@ -49,7 +49,7 @@ public class SM2PublicKey implements PublicKeyInterface {
     public byte[] getBytes() {
         if(publicKey != null)
             return publicKey.getQ().getEncoded(false); //不压缩
-        return new byte[0];
+        return null;
     }
 
     @Override