P.bhw98
{
PADDING-RIGHT: 0px;
PADDING-LEFT: 0px;
FONT-SIZE: 9pt;
PADDING-BOTTOM: 0px;
MARGIN: 10px 0px 5px;
LINE-HEIGHT: normal;
PADDING-TOP: 0px;
FONT-FAMILY: Verdana, Arial
}
PRE.bhw98
{
FONT-SIZE: 9pt;
PADDING-RIGHT: 5px;
PADDING-LEFT: 5px;
PADDING-BOTTOM: 5px;
MARGIN: 5px 0px;
LINE-HEIGHT: normal;
PADDING-TOP: 5px;
BACKGROUND-COLOR: #f0f0f0
}
PRE.diag
{
FONT-SIZE: 9pt;
PADDING-RIGHT: 5px;
PADDING-LEFT: 5px;
PADDING-BOTTOM: 5px;
MARGIN: 5px 0px;
LINE-HEIGHT: normal;
PADDING-TOP: 5px;
}
CODE.bhw98
{
FONT-SIZE: 9pt;
COLOR: #000000
}
TABLE.bhw98
{
BORDER-RIGHT: #808080 1px solid;
BORDER-TOP: #808080 1px solid;
FONT-SIZE: 9pt;
MARGIN: 3px 0px 10px;
BORDER-LEFT: #808080 1px solid;
LINE-HEIGHT: normal;
BORDER-BOTTOM: #808080 1px solid;
FONT-FAMILY: Verdana, Arial
}
TD.bhw98
{
BORDER-RIGHT: darkgray 1px solid;
PADDING-RIGHT: 10px;
BORDER-TOP: darkgray 1px solid;
PADDING-LEFT: 5px;
FONT-SIZE: 9pt;
PADDING-BOTTOM: 0px;
MARGIN: 0px;
BORDER-LEFT: darkgray 1px solid;
LINE-HEIGHT: normal;
PADDING-TOP: 3px;
BORDER-BOTTOM: darkgray 1px solid;
FONT-FAMILY: Verdana, Arial;
BACKGROUND-COLOR: #f0f0f0
}
STRONG.bhw98
{
FONT-WEIGHT: bolder;
FONT-SIZE: 20pt;
COLOR: #228b22;
FONT-STYLE: italic;
FONT-FAMILY: Verdana, Arial
}
LI.bhw98
{
FONT-SIZE: 9pt;
MARGIN: 3px 0px 0px 3px;
LINE-HEIGHT: normal;
FONT-FAMILY: Verdana, Arial
}
H1.bhw98
{
MARGIN-TOP: 25px;
FONT-WEIGHT: bolder;
FONT-SIZE: 12pt;
MARGIN-BOTTOM: 5px;
LINE-HEIGHT: normal;
FONT-FAMILY: Verdana, Arial
}
H2.bhw98
{
MARGIN-TOP: 20px;
FONT-WEIGHT: bolder;
FONT-SIZE: 10.5pt;
MARGIN-BOTTOM: 5px;
LINE-HEIGHT: normal;
FONT-FAMILY: Verdana, Arial
}
H3.bhw98
{
MARGIN-TOP: 15px;
FONT-WEIGHT: bolder;
FONT-SIZE: 9pt;
MARGIN-BOTTOM: 5px;
LINE-HEIGHT: normal;
FONT-FAMILY: Verdana, Arial
}
SPAN.key
{
COLOR: #0000ff
}
SPAN.num
{
COLOR: #800000
}
SPAN.str
{
COLOR: #8b008b
}
SPAN.rem
{
COLOR: #008000
}
Base64是MIME邮件中常用的编码方式之一。它的主要思想是将输入的字符串或数据编码成只含有{'A'-'Z', 'a'-'z', '0'-'9', '+', '/'}这64个可打印字符的串，故称为“Base64”。
Base64编码的方法是，将输入数据流每次取6 bit，用此6 bit的值(0-63)作为索引去查表，输出相应字符。这样，每3个字节将编码为4个字符(3×8 → 4×6)；不满4个字符的以'='填充。
const char EnBase64Tab[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
int EncodeBase64(const unsigned char* pSrc, char* pDst, int nSrcLen, int nMaxLineLen)
{
unsigned char c1, c2, c3; // 输入缓冲区读出3个字节
int nDstLen = 0; // 输出的字符计数
int nLineLen = 0; // 输出的行长度计数
int nDiv = nSrcLen / 3; // 输入数据长度除以3得到的倍数
int nMod = nSrcLen % 3; // 输入数据长度除以3得到的余数
// 每次取3个字节，编码成4个字符
for (int i = 0; i < nDiv; i ++)
{
// 取3个字节
c1 = *pSrc++;
c2 = *pSrc++;
c3 = *pSrc++;
// 编码成4个字符
*pDst++ = EnBase64Tab[c1 2];
*pDst++ = EnBase64Tab[((c1 4) | (c2 4)) & 0x3f];
*pDst++ = EnBase64Tab[((c2 2) | (c3 6)) & 0x3f];
*pDst++ = EnBase64Tab[c3 & 0x3f];
nLineLen += 4;
nDstLen += 4;
// 输出换行？
if (nLineLen nMaxLineLen - 4)
{
*pDst++ = '\r';
*pDst++ = '\n';
nLineLen = 0;
nDstLen += 2;
}
}
// 编码余下的字节
if (nMod == 1)
{
c1 = *pSrc++;
*pDst++ = EnBase64Tab[(c1 & 0xfc) 2];
*pDst++ = EnBase64Tab[((c1 & 0x03) 4)];
*pDst++ = '=';
*pDst++ = '=';
nLineLen += 4;
nDstLen += 4;
}
else if (nMod == 2)
{
c1 = *pSrc++;
c2 = *pSrc++;
*pDst++ = EnBase64Tab[(c1 & 0xfc) 2];
*pDst++ = EnBase64Tab[((c1 & 0x03) 4) | ((c2 & 0xf0) 4)];
*pDst++ = EnBase64Tab[((c2 & 0x0f) 2)];
*pDst++ = '=';
nDstLen += 4;
}
// 输出加个结束符
*pDst = '\0';
return nDstLen;
}
Base64解码方法中，最简单的也是查表法：将64个可打印字符的值作为索引，查表得到的值（范围为0-63）依次连起来，拼凑成字节形式输出，就得到解码结果。
const char DeBase64Tab[] =
{
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
62, // '+'
0, 0, 0,
63, // '/'
52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // '0'-'9'
0, 0, 0, 0, 0, 0, 0,
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, // 'A'-'Z'
0, 0, 0, 0, 0, 0,
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, // 'a'-'z'
};
int DecodeBase64(const char* pSrc, unsigned char* pDst, int nSrcLen)
{
int nDstLen; // 输出的字符计数
int nValue; // 解码用到的长整数
int i;
i = 0;
nDstLen = 0;
// 取4个字符，解码到一个长整数，再经过移位得到3个字节
while (i < nSrcLen)
{
if (*pSrc != '\r' && *pSrc!='\n')
{
nValue = DeBase64Tab[*pSrc++] 18;
nValue += DeBase64Tab[*pSrc++] 12;
*pDst++ = (nValue & 0x00ff0000) 16;
nDstLen++;
if (*pSrc != '=')
{
nValue += DeBase64Tab[*pSrc++] 6;
*pDst++ = (nValue & 0x0000ff00) 8;
nDstLen++;
if (*pSrc != '=')
{
nValue += DeBase64Tab[*pSrc++];
*pDst++ =nValue & 0x000000ff;
nDstLen++;
}
}
i += 4;
}
else // 回车换行，跳过
{
pSrc++;
i++;
}
}
// 输出加个结束符
*pDst = '\0';
return nDstLen;
}
[相关资源]
RFC/STD文档：Internet FAQ Archives
bhw98的专栏：http://www.csdn.net/develop/author/netauthor/bhw98/
首次发布: 2003-06-23
最后修订: 2003-07-21
