Javascript base64加密和解密函数，增加了对中文的处理：

var BASE64={
 utf16to8: function(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;
 },

 utf8to16: function(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;
 },
 /**
 * 此变量为编码的key，每个字符的下标相对应于它所代表的编码。
 */
 enKey: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/',
 /**
 * 此变量为解码的key，是一个数组，BASE64的字符的ASCII值做下标，所对应的就是该字符所代表的编码值。
 */
 deKey: 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
 */
 encode: function(src)
 {
  src=this.utf16to8(src);
  //用一个数组来存放编码后的字符，效率比用字符串相加高很多。
  var str=new Array();
  var ch1, ch2, ch3;
  var pos=0;
  //每三个字符进行编码。
  while(pos+3<=src.length)
  {
   ch1=src.charCodeAt(pos++);
   ch2=src.charCodeAt(pos++);
   ch3=src.charCodeAt(pos++);
   str.push(this.enKey.charAt(ch1>>2), this.enKey.charAt(((ch1<<4)+(ch2>>4))&0x3f));
   str.push(this.enKey.charAt(((ch2<<2)+(ch3>>6))&0x3f), this.enKey.charAt(ch3&0x3f));
  }
  //给剩下的字符进行编码。
  if(pos<src.length)
  {
   ch1=src.charCodeAt(pos++);
   str.push(this.enKey.charAt(ch1>>2));
   if(pos<src.length)
   {
    ch2=src.charCodeAt(pos);
    str.push(this.enKey.charAt(((ch1<<4)+(ch2>>4))&0x3f));
    str.push(this.enKey.charAt(ch2<<2&0x3f), '=');
   }
   else
   {
    str.push(this.enKey.charAt(ch1<<4&0x3f), '==');
   }
  }
  //组合各编码后的字符，连成一个字符串。
  return str.join('');
 },
 /**
 * 解码。
 */
 decode: function(src)
 {
  //用一个数组来存放解码后的字符。
  var str=new Array();
  var ch1, ch2, ch3, ch4;
  var pos=0;
  //过滤非法字符，并去掉'='。
  src=src.replace(/[^A-Za-z0-9\+\/]/g, '');
  //decode the source string in partition of per four characters.
  while(pos+4<=src.length)
  {
   ch1=this.deKey[src.charCodeAt(pos++)];
   ch2=this.deKey[src.charCodeAt(pos++)];
   ch3=this.deKey[src.charCodeAt(pos++)];
   ch4=this.deKey[src.charCodeAt(pos++)];
   str.push(String.fromCharCode(
   (ch1<<2&0xff)+(ch2>>4), (ch2<<4&0xff)+(ch3>>2), (ch3<<6&0xff)+ch4));
  }
  //给剩下的字符进行解码。
  if(pos+1<src.length)
  {
   ch1=this.deKey[src.charCodeAt(pos++)];
   ch2=this.deKey[src.charCodeAt(pos++)];
   if(pos<src.length)
   {
    ch3=this.deKey[src.charCodeAt(pos)];
    str.push(String.fromCharCode((ch1<<2&0xff)+(ch2>>4), (ch2<<4&0xff)+(ch3>>2)));
   }
   else
   {
    str.push(String.fromCharCode((ch1<<2&0xff)+(ch2>>4)));
   }
  }
  //组合各解码后的字符，连成一个字符串。
  return this.utf8to16(str.join(''));
 }
};