add authcode

+'use strict';
+const md5 = require('md5');
+
+const microtime = function() {
+    let unixtimeMs = new Date().getTime();
+    let sec = parseInt(unixtimeMs / 1000, 10);
+
+    return (unixtimeMs - (sec * 1000)) / 1000 + ' ' + sec;
+};
+
+const getTimestamp = function() {
+    let unixtimeMs = new Date().getTime();
+
+    return parseInt(unixtimeMs / 1000, 10);
+};
+
+module.exports = function(str, operation, key, expiry) {
+    operation = operation ? operation : 'encode';
+    key = key ? key : '';
+    expiry = expiry ? expiry : 0;
+    let tmpstr, tmp;
+
+    let ckeyLength = 4;
+
+    key = md5(key);
+
+    // 密匙a会参与加解密
+    let keya = md5(key.substr(0, 16));
+
+    // 密匙b会用来做数据完整性验证
+    let keyb = md5(key.substr(16, 16));
+
+    // 密匙c用于变化生成的密文
+    let keyc = operation === 'decode' ?
+        str.substr(0, ckeyLength) : md5(microtime()).substr(-ckeyLength);
+
+    // 参与运算的密匙
+    let cryptkey = keya + md5(keya + keyc);
+
+    let strbuf;
+
+    if (operation === 'decode') {
+        str = str.substr(ckeyLength);
+        strbuf = new Buffer(str, 'base64');
+
+        // string = b.toString();
+    } else {
+        expiry = expiry ? expiry + getTimestamp() : 0;
+        tmpstr = expiry.toString();
+        if (tmpstr.length >= 10) {
+            str = tmpstr.substr(0, 10) + md5(str + keyb).substr(0, 16) + str;
+        } else {
+            let count = 10 - tmpstr.length;
+
+            for (let i = 0; i < count; i++) {
+                tmpstr = '0' + tmpstr;
+            }
+            str = tmpstr + md5(str + keyb).substr(0, 16) + str;
+        }
+        strbuf = new Buffer(str);
+    }
+
+
+    let box = new Array(256);
+    let rndkey = [];
+
+    for (let i = 0; i < 256; i++) {
+        box[i] = i;
+
+        // 产生密匙簿
+        rndkey[i] = cryptkey.charCodeAt(i % cryptkey.length);
+    }
+
+    // 用固定的算法，打乱密匙簿，增加随机性，好像很复杂，实际上对并不会增加密文的强度
+    for (let j = 0, i = 0; i < 256; i++) {
+        j = (j + box[i] + rndkey[i]) % 256;
+        tmp = box[i];
+        box[i] = box[j];
+        box[j] = tmp;
+    }
+
+
+    // 核心加解密部分
+    let s = '';
+
+    for (let a = 0, j = 0, i = 0; i < strbuf.length; i++) {
+        a = (a + 1) % 256;
+        j = (j + box[a]) % 256;
+        tmp = box[a];
+        box[a] = box[j];
+        box[j] = tmp;
+
+        // 从密匙簿得出密匙进行异或，再转成字符
+        // s += String.fromCharCode(string[i] ^ (box[(box[a] + box[j]) % 256]));
+        /* jshint -W016*/
+        strbuf[i] = strbuf[i] ^ (box[(box[a] + box[j]) % 256]);
+    }
+
+    if (operation === 'decode') {
+        s = strbuf.toString();
+        if ((s.substr(0, 10) === '0'.repeat(10) ||
+                s.substr(0, 10) - getTimestamp() > 0) &&
+            s.substr(10, 16) === md5(s.substr(26) + keyb).substr(0, 16)) {
+            s = s.substr(26);
+        } else {
+            s = '';
+        }
+    } else {
+        s = strbuf.toString('base64');
+        let regex = new RegExp('=', 'g');
+
+        s = s.replace(regex, '');
+        s = keyc + s;
+    }
+
+    return s;
+};
+
+// console.log(module.exports('abcdef123', 'encode', 'key123', 99))