base64是很基本的加密算法，常用于邮件和迅雷的下载地址，下面是js版的算法代码和网页示例
编码：
//分割成每6位，最大是111111，即63，所以共需要64个字符
var base64EncodeChars = [
"A", "B", "C", "D", "E", "F", "G", "H",
"I", "J", "K", "L", "M", "N", "O", "P",
"Q", "R", "S", "T", "U", "V", "W", "X",
"Y", "Z", "a", "b", "c", "d", "e", "f",
"g", "h", "i", "j", "k", "l", "m", "n",
"o", "p", "q", "r", "s", "t", "u", "v",
"w", "x", "y", "z", "0", "1", "2", "3",
"4", "5", "6", "7", "8", "9", "+", "/"
];
function base64encode(str) {
var out, i, j, len;
var c1, c2, c3;//读取待加密的三个字符
len = str.length;
i = j = 0;
out = [];
while (i < len) {
c1 = str.charCodeAt(i++) & 0xff;
if (i == len)//最后一组只有一个字符
{
out[j++] = base64EncodeChars[c1 >> 2];
out[j++] = base64EncodeChars[(c1 & 0x3) << 4];
out[j++] = "==";
break;
}
c2 = str.charCodeAt(i++) & 0xff;
if (i == len)最后一组只有俩字符
{
out[j++] = base64EncodeChars[c1 >> 2];
out[j++] = base64EncodeChars[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];
out[j++] = base64EncodeChars[(c2 & 0x0f) << 2];
out[j++] = "=";
break;
}
c3 = str.charCodeAt(i++) & 0xff;
out[j++] = base64EncodeChars[c1 >> 2];//右移2位，即取高6位，8-2=6，唔识翻去读小学
out[j++] = base64EncodeChars[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];
//c1 & 0x03是高6位置0留下低2位，再左移4位变成高5,6位，再用逻辑或与c2的高四位合并，即c2
的高4位充当第二个已加密字符的低四位
out[j++] = base64EncodeChars[((c2 & 0x0f) << 2) | ((c3 & 0xc0) >> 6)];
//c2的低4位与c3高2位合并
out[j++] = base64EncodeChars[c3 & 0x3f];
//高2位置0
//我是在解释代码还是在讲数学呢？？？？？
}
return out.join('');
}
解码：
//11111111=127，所以要128个字符，但密文是由64种字符组成的（好似咩！@￥%*^哩d咯，都唔系密文就无所谓解密啦），所以不是base64加密的记作-1，并且无法解密
var base64DecodeChars = [
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
-1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1
];
function base64decode(str) {
var c1, c2, c3, c4;//读取待解密的四个字符
var i, j, len, out;
len = str.length;
i = j = 0;
out = [];
while (i < len) {
/* c1 */
do {
c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
} while (i < len && c1 == -1);//当遇到非法密文继续读取，直到读到一个正常的密文
if (c1 == -1) break;
/* c2 */
do {
c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
} while (i < len && c2 == -1);
if (c2 == -1) break;
out[j++] = String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));//解开第一个
/* c3 */
do {
c3 = str.charCodeAt(i++) & 0xff;
if (c3 == 61) return out.join('');
c3 = base64DecodeChars[c3];
} while (i < len && c3 == -1);
if (c3 == -1) break;
out[j++] = String.fromCharCode(((c2 & 0x0f) << 4) | ((c3 & 0x3c) >> 2));//解开第二个
、
/* c4 */
do {
c4 = str.charCodeAt(i++) & 0xff;
if (c4 == 61) return out.join('');
c4 = base64DecodeChars[c4];
} while (i < len && c4 == -1);
if (c4 == -1) break;
out[j++] = String.fromCharCode(((c3 & 0x03) << 6) | c4);//解开第三个
}
return out.join('');
}