JavaScript实现的encode64加密算法实例分析

//encode64编解码(function() { var codeChar = "PaAwO65goUf7IK2vi9-xq8cFTEXLCDY1Hd3tV0ryzjbpN_BlnSs4mGRkQWMZJeuh"; window.encode64 = function(str) {  var s = "";  var a = strToBytes(str);  //取得字串的字节数组, 数组长度是字串长度的2倍.  var res = a.length % 3;  //3个字节一组进行处理, 余下特殊处理  var i = 2, v;  for (; i < a.length; i += 3) {  //每3个字节用4个字符表示,   //相当于3个字符(实际上是6个字节)用8个字符编码(实际为16个字节)  //看起来容量膨胀了很多, 但是在启用压缩的情况下, 这些又被抵消掉了   v = a[i - 2] + (a[i - 1] << 8) + (a[i] << 16);   s += codeChar.charAt(v & 0x3f);   s += codeChar.charAt((v >> 6) & 0x3f);   s += codeChar.charAt((v >> 12) & 0x3f);   s += codeChar.charAt((v >> 18));  }  if (res == 1) {//字节余一位时候, 补2个字符, 64*64>256   v = a[i - 2];   s += codeChar.charAt(v & 0x3f);   s += codeChar.charAt((v >> 6) & 0x3f);  } else if (res == 2) {  //字节余2位的时候, 补3个字节, 64*64*64>256*256, 所以是可行的   v = a[i - 2] + (a[i - 1] << 8);   s += codeChar.charAt(v & 0x3f);   s += codeChar.charAt((v >> 6) & 0x3f);   s += codeChar.charAt((v >> 12) & 0x3f);  }  return s; }; window.decode64 = function(codeStr) {  var dic = [];  for (var i = 0; i < codeChar.length; i++) {   dic[codeChar.charAt(i)] = i;  }  var code = [];  var res = codeStr.length % 4;  var i = 3, v;  for (; i < codeStr.length; i += 4) {   v = dic[codeStr.charAt(i - 3)];   v += dic[codeStr.charAt(i - 2)] << 6;   v += dic[codeStr.charAt(i - 1)] << 12;   v += dic[codeStr.charAt(i)] << 18;   code.push(v & 0xff, (v >> 8) & 0xff, (v >> 16) & 0xff);  }  if (res == 2) {  //正确的字节数肯定是余2或3, 没有1的情况, 如果出现, 舍弃.   v = dic[codeStr.charAt(i - 3)];   v += dic[codeStr.charAt(i - 2)] << 6;   code.push(v & 0xff);  } else if (res == 3) {   v = dic[codeStr.charAt(i - 3)];   v += dic[codeStr.charAt(i - 2)] << 6;   v += dic[codeStr.charAt(i - 1)] << 12;   code.push(v & 0xff, (v >> 8) & 0xff);  }  return strFromBytes(code); };})();