用js编解码base64

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：

   1:  //下面是64个基本的编码

   2:  var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

   3:  var base64DecodeChars = new Array(

   4:      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,

   5:      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,

   6:      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,

   7:      52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,

   8:      -1,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,

   9:      15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,

  10:      -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,

  11:      41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1);

  12:  //编码的方法

  13:  function base64encode(str) {

  14:      var out, i, len;

  15:      var c1, c2, c3;

  16:      len = str.length;

  17:      i = 0;

  18:      out = "";

  19:      while(i < len) {

  20:      c1 = str.charCodeAt(i++) & 0xff;

  21:      if(i == len)

  22:      {

  23:          out += base64EncodeChars.charAt(c1 >> 2);

  24:          out += base64EncodeChars.charAt((c1 & 0x3) << 4);

  25:          out += "==";

  26:          break;

  27:      }

  28:      c2 = str.charCodeAt(i++);

  29:      if(i == len)

  30:      {

  31:          out += base64EncodeChars.charAt(c1 >> 2);

  32:          out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));

  33:          out += base64EncodeChars.charAt((c2 & 0xF) << 2);

  34:          out += "=";

  35:          break;

  36:      }

  37:      c3 = str.charCodeAt(i++);

  38:      out += base64EncodeChars.charAt(c1 >> 2);

  39:      out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));

  40:      out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));

  41:      out += base64EncodeChars.charAt(c3 & 0x3F);

  42:      }

  43:      return out;

  44:  }

  45:  //解码的方法

  46:  function base64decode(str) {

  47:      var c1, c2, c3, c4;

  48:      var i, len, out;

  49:      len = str.length;

  50:      i = 0;

  51:      out = "";

  52:      while(i < len) {

  53:      

  54:      do {

  55:          c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];

  56:      } while(i < len && c1 == -1);

  57:      if(c1 == -1)

  58:          break;

  59:      

  60:      do {

  61:          c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];

  62:      } while(i < len && c2 == -1);

  63:      if(c2 == -1)

  64:          break;

  65:      out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));

  66:      

  67:      do {

  68:          c3 = str.charCodeAt(i++) & 0xff;

  69:          if(c3 == 61)

  70:          return out;

  71:          c3 = base64DecodeChars[c3];

  72:      } while(i < len && c3 == -1);

  73:      if(c3 == -1)

  74:          break;

  75:      out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));

  76:      

  77:      do {

  78:          c4 = str.charCodeAt(i++) & 0xff;

  79:          if(c4 == 61)

  80:          return out;

  81:          c4 = base64DecodeChars[c4];

  82:      } while(i < len && c4 == -1);

  83:      if(c4 == -1)

  84:          break;

  85:      out += String.fromCharCode(((c3 & 0x03) << 6) | c4);

  86:      }

  87:      return out;

  88:  }

  89:  function utf16to8(str) {

  90:      var out, i, len, c;

  91:      out = "";

  92:      len = str.length;

  93:      for(i = 0; i < len; i++) {

  94:      c = str.charCodeAt(i);

  95:      if ((c >= 0x0001) && (c <= 0x007F)) {

  96:          out += str.charAt(i);

  97:      } else if (c > 0x07FF) {

  98:          out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));

  99:          out += String.fromCharCode(0x80 | ((c >>  6) & 0x3F));

 100:          out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));

 101:      } else {

 102:          out += String.fromCharCode(0xC0 | ((c >>  6) & 0x1F));

 103:          out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));

 104:      }

 105:      }

 106:      return out;

 107:  }

 108:  function utf8to16(str) {

 109:      var out, i, len, c;

 110:      var char2, char3;

 111:      out = "";

 112:      len = str.length;

 113:      i = 0;

 114:      while(i < len) {

 115:      c = str.charCodeAt(i++);

 116:      switch(c >> 4)

 117:      { 

 118:        case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:

 119:          // 0xxxxxxx

 120:          out += str.charAt(i-1);

 121:          break;

 122:        case 12: case 13:

 123:          // 110x xxxx   10xx xxxx

 124:          char2 = str.charCodeAt(i++);

 125:          out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));

 126:          break;

 127:        case 14:

 128:          // 1110 xxxx  10xx xxxx  10xx xxxx

 129:          char2 = str.charCodeAt(i++);

 130:          char3 = str.charCodeAt(i++);

 131:          out += String.fromCharCode(((c & 0x0F) << 12) |

 132:                         ((char2 & 0x3F) << 6) |

 133:                         ((char3 & 0x3F) << 0));

 134:          break;

 135:      }

 136:      }

 137:      return out;

 138:  }

调用：

//编码
value = base64encode(utf16to8(src))

//解码
value = utf8to16(base64decode(src))