MIME之Base64编解码

    Base64,MIME

    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 = '';

        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'
    };