• 编码

    1、取待编码字符串前3位（8bit/位），转换为二进制的ascii编码（共24bit）。

    2、将转换后的二进制编码按每6位分为4块，将每块二进制转换成ascii码值

    3、对照base64解码表即可得到对应的base64编码

    4、若取不到3位待编码字符串则用0补充，划分区间后全0的块用‘=’代替

    nw4fN6.png

  • 解码

    1、取待解码字符串前4位（8bit/位），对照base64编码表，转换为映射的数字，并将每一个数转成6位二进制数（共24bit），

    2、将24位二进制分为3份8bit数据

    3、对照ascii码表转换对应字符，解码完成

    4、在第一步中若有‘=’字符则将其转为000000

    Q2PWVJ.png

附base64编码对照表

nw4Hud.png

base64_decode

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const char * base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";//可以自己定制，比如说把+和/改成-和_以满足url编码的要求。
int base64_decode( const char * base64, unsigned char * bindata )
{
    int i, j;
    unsigned char k;
    unsigned char t;
    unsigned char temp[4];
    for ( i = 0, j = 0; base64[i] != '\0' ; i += 4 )//每四个字节为一组
    {
        memset( temp, 0xFF, sizeof(temp) );
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i] )
                temp[0]= k;//获取一组中每个字节对应的编号
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i+1] )
                temp[1]= k;
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i+2] )
                temp[2]= k;
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i+3] )
                temp[3]= k;
        }

        bindata[j++] = ((unsigned char)(((unsigned char)(temp[0] << 2))&0xFC)) |((unsigned char)((unsigned char)(temp[1]>>4)&0x03));//这些就是补0补位的操作了

        if ( base64[i+2] == '=' )
            break;

        bindata[j++] = ((unsigned char)(((unsigned char)(temp[1] << 4))&0xF0)) | ((unsigned char)((unsigned char)(temp[2]>>2)&0x0F));

        if ( base64[i+3] == '=' )
            break;

        bindata[j++] = ((unsigned char)(((unsigned char)(temp[2] << 6))&0xF0)) |((unsigned char)(temp[3]&0x3F));

    }
    return j;
}

base64_encode

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char * base64_encode( const unsigned char * bindata, char * base64, int binlength )
{
    int i, j;
    unsigned char current;

    for ( i = 0, j = 0 ; i < binlength ; i += 3 )
    {
        current = (bindata[i] >> 2) ;
        current &= (unsigned char)0x3F;
        base64[j++] = base64char[(int)current];

        current = ( (unsigned char)(bindata[i] << 4 ) ) & ( (unsigned char)0x30 ) ;
        if ( i + 1 >= binlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(bindata[i+1] >> 4) ) & ( (unsigned char) 0x0F );
        base64[j++] = base64char[(int)current];

        current = ( (unsigned char)(bindata[i+1] << 2) ) & ( (unsigned char)0x3C ) ;
        if ( i + 2 >= binlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(bindata[i+2] >> 6) ) & ( (unsigned char) 0x03 );
        base64[j++] = base64char[(int)current];

        current = ( (unsigned char)bindata[i+2] ) & ( (unsigned char)0x3F ) ;
        base64[j++] = base64char[(int)current];
    }
    base64[j] = '\0';
    return base64;
}