编码规则
Base64编码的思想是是采用64个基本的ASCII码字符对数据进行重新编码。它将需要编码的数据拆分成字节数组。以3个字节为一组。按顺序排列24 位数据，再把这24位数据分成4组，即每组6位。再在每组的的最高位前补两个0凑足一个字节。这样就把一个3字节为一组的数据重新编码成了4个字节。当所要编码的数据的字节数不是3的整倍数，也就是说在分组时最后一组不够3个字节。这时在最后一组填充1到2个0字节。并在最后编码完成后在结尾添加1到2个 “=”。

实现的代码：

//下面是64个基本的编码var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";var base64DecodeChars = new Array(    -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);//编码的方法function base64encode(str) {    var out, i, len;    var c1, c2, c3;    len = str.length;    i = 0;    out = "";    while(i < len) {    c1 = str.charCodeAt(i++) & 0xff;    if(i == len)    {        out += base64EncodeChars.charAt(c1 >> 2);        out += base64EncodeChars.charAt((c1 & 0x3) << 4);        out += "==";        break;    }    c2 = str.charCodeAt(i++);    if(i == len)    {        out += base64EncodeChars.charAt(c1 >> 2);        out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));        out += base64EncodeChars.charAt((c2 & 0xF) << 2);        out += "=";        break;    }    c3 = str.charCodeAt(i++);    out += base64EncodeChars.charAt(c1 >> 2);    out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));    out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));    out += base64EncodeChars.charAt(c3 & 0x3F);    }    return out;}//解码的方法function base64decode(str) {    var c1, c2, c3, c4;    var i, len, out;    len = str.length;    i = 0;    out = "";    while(i < len) {        do {        c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];    } while(i < len && c1 == -1);    if(c1 == -1)        break;        do {        c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];    } while(i < len && c2 == -1);    if(c2 == -1)        break;    out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));        do {        c3 = str.charCodeAt(i++) & 0xff;        if(c3 == 61)        return out;        c3 = base64DecodeChars[c3];    } while(i < len && c3 == -1);    if(c3 == -1)        break;    out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));        do {        c4 = str.charCodeAt(i++) & 0xff;        if(c4 == 61)        return out;        c4 = base64DecodeChars[c4];    } while(i < len && c4 == -1);    if(c4 == -1)        break;    out += String.fromCharCode(((c3 & 0x03) << 6) | c4);    }    return out;}function utf16to8(str) {    var out, i, len, c;    out = "";    len = str.length;    for(i = 0; i < len; i++) {    c = str.charCodeAt(i);    if ((c >= 0x0001) && (c <= 0x007F)) {        out += str.charAt(i);    } else if (c > 0x07FF) {        out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));        out += String.fromCharCode(0x80 | ((c >>  6) & 0x3F));        out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));    } else {        out += String.fromCharCode(0xC0 | ((c >>  6) & 0x1F));        out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));    }    }    return out;}function utf8to16(str) {    var out, i, len, c;    var char2, char3;    out = "";    len = str.length;    i = 0;    while(i < len) {    c = str.charCodeAt(i++);    switch(c >> 4)    {       case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:        // 0xxxxxxx        out += str.charAt(i-1);        break;      case 12: case 13:        // 110x xxxx   10xx xxxx        char2 = str.charCodeAt(i++);        out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));        break;      case 14:        // 1110 xxxx  10xx xxxx  10xx xxxx        char2 = str.charCodeAt(i++);        char3 = str.charCodeAt(i++);        out += String.fromCharCode(((c & 0x0F) << 12) |                       ((char2 & 0x3F) << 6) |                       ((char3 & 0x3F) << 0));        break;    }    }    return out;} //调用： //编码value =http://www.mamicode.com/ base64encode(utf16to8(src)) //解码value = http://www.mamicode.com/utf8to16(base64decode(src))