Base64编解码

编码函数

 

const BYTE Base64ValTab[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

#define AVal(x) Base64ValTab[x]

int EncodeBase64(char * pInput, char * pOutput)
{
    int i = 0;
    int loop = 0;
    int remain = 0;
    int iDstLen = 0;
    int iSrcLen = (int)strlen(pInput);

    loop = iSrcLen/3;
    remain = iSrcLen%3;

    // also can encode native char one by one as decode method
    // but because all of char in native string  is to be encoded so encode 3-chars one time is easier.

    for (i=0; i<loop; i++)
    {
        BYTE a1 = (pInput[i*3] >> 2);
        BYTE a2 = ( ((pInput[i*3] & 0x03) << 4) | (pInput[i*3+1] >> 4) );
        BYTE a3 = ( ((pInput[i*3+1] & 0x0F) << 2) | ((pInput[i*3+2] & 0xC0) >> 6) );
        BYTE a4 = (pInput[i*3+2] & 0x3F);

        pOutput[i*4] = AVal(a1);
        pOutput[i*4+1] = AVal(a2);
        pOutput[i*4+2] = AVal(a3);
        pOutput[i*4+3] = AVal(a4);
    }

    iDstLen = i*4;

    if (remain == 1)
    {
        // should pad two equal sign
        i = iSrcLen-1;
        BYTE a1 = (pInput[i] >> 2);
        BYTE a2 = ((pInput[i] & 0x03) << 4);
        
        pOutput[iDstLen++] = AVal(a1);
        pOutput[iDstLen++] = AVal(a2);
        pOutput[iDstLen++] = '=';
        pOutput[iDstLen++] = '=';
        pOutput[iDstLen] = 0x00;
    }
    else if (remain == 2)
    {
        // should pad one equal sign
        i = iSrcLen-2;
        BYTE a1 = (pInput[i] >> 2);
        BYTE a2 = ( ((pInput[i] & 0x03) << 4) | (pInput[i+1] >> 4));
        BYTE a3 = ( (pInput[i+1] & 0x0F) << 2);

        pOutput[iDstLen++] = AVal(a1);
        pOutput[iDstLen++] = AVal(a2);
        pOutput[iDstLen++] = AVal(a3);
        pOutput[iDstLen++] = '=';
        pOutput[iDstLen] = 0x00;
    }
    else
    {
        // just division by 3
        pOutput[iDstLen] = 0x00;
    }
    return iDstLen;
}

 

 

解码函数

 

const BYTE Base64IdxTab[128] =
{
    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,    255,255,255,255,
    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
};

#define BVal(x) Base64IdxTab[x]

int DecodeBase64(char * pInput, char * pOutput)
{
    int i = 0;
    int iCnt = 0;
    int iSrcLen = (int)strlen(pInput);

    char * p = pOutput;

    for (i=0; i<iSrcLen; i++)
    {
        if (pInput[i] > 127) continue;
        if (pInput[i] == '=') return p-pOutput+1;

        BYTE a = BVal(pInput[i]);
        if (a == 255) continue;
        
        switch (iCnt)
        {
        case 0:
            {
                *p = a << 2;
                iCnt++;
            }
            break;

        case 1:
            {
                *p++ |= a >> 4;
                *p = a << 4;
                iCnt++;
            }
            break;

        case 2:
            {
                *p++ |= a >> 2;
                *p = a << 6;
                iCnt++;
            }
            break;

        case 3:
            {
                *p++ |= a;
                iCnt = 0;
            }
            break;
        } 
    }

    *p = 0x00;
    return p-pOutput;
}