HarmonyOS sm4加密后的Uint8Array怎么转化base64格式

Uint8Array转base64。请参考以下代码demo案例：

// ss为base64的String
let ss = "2N+ZMvs8Dn4HVylFmddFa4COJnkoz7PkgPsR7iGDgnU53S5TW5eMen7BIqc59bXt7kEDbqNUe+PQUmbRMoU9gJ9hcv79QzN0iQJPux5q1rM="
// 生成 util base64Help
let base64Helper = new util.Base64Helper();
// 对base64的string 解码为 Uint8Array
let keySS = base64Helper.decodeSync(ss);
console.log("keySS : "+keySS.toString());
// 对Uint8Array 转码为base64 的String
let keyNN = base64Helper.encodeToStringSync(keySS)
console.log("keyNN : "+keyNN.toString());
输出如下 // Uint8Array com.example.demo0003 I keySS : 216,223,153,50,251,60,14,126,7,87,41,69,153,215,69,107,128,142,38,121,40,207,179,228,128,251,17,238,33,131,130,117,57,221,46,83,91,151,140,122,126,193,34,167,57,245,181,237,238,65,3,110,163,84,123,227,208,82,102,209,50,133,61,128,159,97,114,254,253,67,51,116,137,2,79,187,30,106,214,179 // base64 com.example.demo0003 I keyNN : 2N+ZMvs8Dn4HVylFmddFa4COJnkoz7PkgPsR7iGDgnU53S5TW5eMen7BIqc59bXt7kEDbqNUe+PQUmbRMoU9gJ9hcv79QzN0iQJPux5q1rM=