摘要：
Base64编码的思想是用64个基本ASCII字符重新编码数据。按顺序排列24位数据，然后将24位数据分成4组，即每组6位。在每个组的最高位之前添加两个零以组成一个字节。这样，一组三个字节的数据被重新编码为四个字节。当要编码的数据的字节数不是3的整数倍时，即分组时最后一组小于3字节。在最终编码后的末尾添加1到2“=”。
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：
1: //编码
2: value = base64encode(utf16to8(src)) 3: 4: //解码
5: value = utf8to16(base64decode(src))