BASE64编码和解码

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：

13

44

51

26

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;
}