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<p><FONT class=normalfont><B><font color=blue>MIME之Base64编解码</font></B></FONT><BR><FONT class=smallfont color=#ff9900>2004-04-23 15:18 pm</FONT><BR><FONT class=normalfont>作者：bhw98<br>来自：Linux知识宝库<br>联系方式：无名<br><br>关键字 Base64,MIME<br>
<br>
<br>
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”。<br>
<br>
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。<br>
<br>
const char EnBase64Tab[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";<br>
<br>
int EncodeBase64(const unsigned char* pSrc, char* pDst, int nSrcLen, int nMaxLineLen)<br>
{<br>
unsigned char c1, c2, c3; // 输入缓冲区读出3个字节<br>
int nDstLen = 0; // 输出的字符计数<br>
int nLineLen = 0; // 输出的行长度计数<br>
int nDiv = nSrcLen / 3; // 输入数据长度除以3得到的倍数<br>
int nMod = nSrcLen % 3; // 输入数据长度除以3得到的余数<br>
<br>
// 每次取3个字节，编码成4个字符<br>
for (int i = 0; i < nDiv; i ++)<br>
{<br>
// 取3个字节<br>
c1 = *pSrc++;<br>
c2 = *pSrc++;<br>
c3 = *pSrc++;<br>
<br>
// 编码成4个字符<br>
*pDst++ = EnBase64Tab[c1 >> 2];<br>
*pDst++ = EnBase64Tab[((c1 << 4) | (c2 >> 4)) & 0x3f];<br>
*pDst++ = EnBase64Tab[((c2 << 2) | (c3 >> 6)) & 0x3f];<br>
*pDst++ = EnBase64Tab[c3 & 0x3f];<br>
nLineLen += 4;<br>
nDstLen += 4;<br>
<br>
// 输出换行？<br>
if (nLineLen > nMaxLineLen - 4)<br>
{<br>
*pDst++ = '
';<br>
*pDst++ = '';<br>
nLineLen = 0;<br>
nDstLen += 2;<br>
}<br>
}<br>
<br>
// 编码余下的字节<br>
if (nMod == 1)<br>
{<br>
c1 = *pSrc++;<br>
*pDst++ = EnBase64Tab[(c1 & 0xfc) >> 2];<br>
*pDst++ = EnBase64Tab[((c1 & 0x03) << 4)];<br>
*pDst++ = '=';<br>
*pDst++ = '=';<br>
nLineLen += 4;<br>
nDstLen += 4;<br>
}<br>
else if (nMod == 2)<br>
{<br>
c1 = *pSrc++;<br>
c2 = *pSrc++;<br>
*pDst++ = EnBase64Tab[(c1 & 0xfc) >> 2];<br>
*pDst++ = EnBase64Tab[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];<br>
*pDst++ = EnBase64Tab[((c2 & 0x0f) << 2)];<br>
*pDst++ = '=';<br>
nDstLen += 4;<br>
}<br>
<br>
// 输出加个结束符<br>
*pDst = '</FONT><br>
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