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);
};
})();