BASE64编码和解码

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本文介绍了Base64编码的基本原理，包括其转换规则和码表，并提供了使用VC实现Base64编码和解码的具体示例代码。

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Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一，Base64要求把每三个8Bit的字节转换为四个6Bit的字节（3*8 = 4*6 = 24），然后把6Bit再添两位高位0，组成四个8Bit的字节，也就是说，转换后的字符串理论上将要比原来的长1/3，转换之后通过一个BASE64的码表就可以得到BASE64编码。如果字符串不是3的倍数的话，转换时字节不足的地方用‘=’代替。

BASE64码表：

索引	对应字符	索引	对应字符	索引	对应字符	索引	对应字符
0	A	17	R	34	i	51	z
1	B	18	S	35	j	52	0
2	C	19	T	36	k	53	1
3	D	20	U	37	l	54	2
4	E	21	V	38	m	55	3
5	F	22	W	39	n	56	4
6	G	23	X	40	o	57	5
7	H	24	Y	41	p	58	6
8	I	25	Z	42	q	59	7
9	J	26	a	43	r	60	8
10	K	27	b	44	s	61	9
11	L	28	c	45	t	62	+
12	M	29	d	46	u	63	/
13	N	30	e	47	v
14	O	31	f	48	w
15	P	32	g	49	x
16	Q	33	h	50	y

举个例子：假如源字符串是

00110110

11001100

11011010,则按照每6位划分成4个字节,并在高2为补零（6位不足，所以在高2位补零，组成一字节），结果如下:

00001101

00101100

00110011

00011010

对应的十进制分别是：

根据码表，可以BASE64编码为：Nsza。解码只是编码的逆过程。

vc实现BASE64编码如下：

//定义码表

const char* JBase64::pszEncodeTable = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

//解码函数

bool JBase64::Encode( const BYTE* pByteIn, int nInLength, BYTE* pByteOut, int& nOutLength ) { bool bReturn = false; assert(pByteIn != NULL); assert(pByteOut != NULL); assert(nInLength > 0); if (pByteIn == NULL || pByteOut == NULL || nInLength <= 0) { goto exit0; } int nCount = nInLength / 3; int nMode = nInLength % 3; BYTE* pOut = pByteOut; for (int i = 0; i < nCount; i++) { *pOut++ = pszEncodeTable[(pByteIn[0] >> 2)]; *pOut++ = pszEncodeTable[((pByteIn[0] & 3) << 4) | (pByteIn[1] >> 4)]; *pOut++ = pszEncodeTable[(pByteIn[1] & 0xf) << 2 | (pByteIn[2] >> 6)]; *pOut++ = pszEncodeTable[(pByteIn[2] & 0x3f)]; pByteIn += 3; } if (nMode != 0) { *pOut++ = pszEncodeTable[(pByteIn[0] >> 2)]; *pOut++ = pszEncodeTable[((pByteIn[0] & 3) << 4) | ((nMode == 1) ? 0 : (pByteIn[1] >> 4))]; *pOut++ = (nMode == 1) ? '=':pszEncodeTable[((pByteIn[1] & 0xf) << 2)]; *pOut++ = '='; } *pOut = '/0'; nOutLength = static_cast<int>(pOut - pByteOut); bReturn = true; exit0: return bReturn; }

//定义解码表

     const unsigned char JBase64::pszDecodeTable[256] =
    {
   255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
   255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
         255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
         255, 255, 255, 255, 255, 255, 255, 62, 255, 255, 255, 63,
         52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 255, 255,
         255, 254, 255, 255, 255,   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, 255, 255, 255, 255, 255,
         255, 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, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255
};

//解码函数

bool JBase64::Decode( const BYTE* pByteIn, int nInLength, BYTE* pByteOut, int& nOutLength ) { bool bReturn = false; assert(pByteIn != NULL); assert(pByteOut != NULL); assert(nInLength > 0); assert(nInLength % 4 == 0); if (pByteIn == NULL || pByteOut == NULL || nInLength % 4 != 0) { goto exit0; } BYTE tmp[4]; BYTE* pOut = pByteOut; int nLength = nInLength / 4; for (int i = 0; i < nLength; i++) { tmp[0] = pszDecodeTable[pByteIn[0]]; tmp[1] = pszDecodeTable[pByteIn[1]]; tmp[2] = pszDecodeTable[pByteIn[2]]; tmp[3] = pszDecodeTable[pByteIn[3]]; *pOut++ = ((tmp[0] << 2) | (tmp[1] >> 4)); *pOut++ = ((tmp[1] << 4) | ((tmp[2] == 254 ? 0 : tmp[2] >> 2))); *pOut++ = (((tmp[2] == 254 ? 0 : tmp[2]<< 6)) | ((tmp[3] == 254 ? 0 : tmp[3]))); pByteIn += 4; } *pOut = '/0'; nOutLength = static_cast<int>(pOut - pByteOut); bReturn = true; exit0: return bReturn; }