对于这个  
我没有接触
帮不到你什么
帮你顶起来

255表示无效数字。
base64，0~127范围内只有64个字符是有用的。

那为什么是这种排列方式呢，有什么讲究么

含义就是这个:

Base64IdxTab['+'] = 62;
Base64IdxTab['/'] = 63;
Base64IdxTab['0'] = 52;
Base64IdxTab['1'] = 53;
Base64IdxTab['2'] = 54;
Base64IdxTab['3'] = 55;
Base64IdxTab['4'] = 56;
Base64IdxTab['5'] = 57;
Base64IdxTab['6'] = 58;
Base64IdxTab['7'] = 59;
Base64IdxTab['8'] = 60;
Base64IdxTab['9'] = 61;
Base64IdxTab['A'] = 0;
Base64IdxTab['B'] = 1;
Base64IdxTab['C'] = 2;
Base64IdxTab['D'] = 3;
Base64IdxTab['E'] = 4;
Base64IdxTab['F'] = 5;
Base64IdxTab['G'] = 6;
Base64IdxTab['H'] = 7;
Base64IdxTab['I'] = 8;
Base64IdxTab['J'] = 9;
Base64IdxTab['K'] = 10;
Base64IdxTab['L'] = 11;
Base64IdxTab['M'] = 12;
Base64IdxTab['N'] = 13;
Base64IdxTab['O'] = 14;
Base64IdxTab['P'] = 15;
Base64IdxTab['Q'] = 16;
Base64IdxTab['R'] = 17;
Base64IdxTab['S'] = 18;
Base64IdxTab['T'] = 19;
Base64IdxTab['U'] = 20;
Base64IdxTab['V'] = 21;
Base64IdxTab['W'] = 22;
Base64IdxTab['X'] = 23;
Base64IdxTab['Y'] = 24;
Base64IdxTab['Z'] = 25;
Base64IdxTab['a'] = 26;
Base64IdxTab['b'] = 27;
Base64IdxTab['c'] = 28;
Base64IdxTab['d'] = 29;
Base64IdxTab['e'] = 30;
Base64IdxTab['f'] = 31;
Base64IdxTab['g'] = 32;
Base64IdxTab['h'] = 33;
Base64IdxTab['i'] = 34;
Base64IdxTab['j'] = 35;
Base64IdxTab['k'] = 36;
Base64IdxTab['l'] = 37;
Base64IdxTab['m'] = 38;
Base64IdxTab['n'] = 39;
Base64IdxTab['o'] = 40;
Base64IdxTab['p'] = 41;
Base64IdxTab['q'] = 42;
Base64IdxTab['r'] = 43;
Base64IdxTab['s'] = 44;
Base64IdxTab['t'] = 45;
Base64IdxTab['u'] = 46;
Base64IdxTab['v'] = 47;
Base64IdxTab['w'] = 48;
Base64IdxTab['x'] = 49;
Base64IdxTab['y'] = 50;
Base64IdxTab['z'] = 51;

字符 -> 数字的解码规则,
这要跟编码规则匹配，还有另一个数组(它是存在的，虽然你没贴出来)：数字->字符

哦明白了他是和assic码表相对应的，刚对应的

可以调用人\微软提供的标准的 BASE64 的加密解密算法~!