C語言base64編碼轉碼

 

 

一、最近，在做一個智能鎖項目，鎖上具有一個貓眼攝像頭，當有客人來訪時，需要通過貓眼抓拍圖片上傳到手機app端，讓主人知道來訪者是誰，項目中使用到了Base64編解碼技術，所謂Base64編解碼就是把每三個8bit的字節轉換爲四個6bit的字節（3*8 = 4*6 = 24），然後把6bit的字節再添兩位高位0，組成四個8bit的字節，也就是說，轉換後的字符串理論上將要比原來的長1/3。例如：

轉換前 11111111, 11111111, 11111111 （二進制）

轉換後 00111111, 00111111, 00111111, 00111111 （二進制）

上面的三個字節是原文，下面的四個字節是轉換後的Base64編碼，其前兩位均爲0。Base64編碼是使用64個可打印ASCII字符（A-Z、a-z、0-9、+、/）將任意字節序列數據編碼成ASCII字符串，另有“=”符號用作後綴用途。 其中索引表如下圖所示：

                                           

二、代碼部分

Base64編碼

const char * base64char  = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
char * base64_encode( const unsigned char * originChar, char * base64, int originlength )//base64編碼
{
    int i, j;
    unsigned char current;
 
    for ( i = 0, j = 0 ; i < originlength ; i += 3 )
    {
        current = (originChar[i] >> 2) ;
        current &= (unsigned char)0x3F;
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)(originChar[i] << 4 ) ) & ( (unsigned char)0x30 ) ;
        if ( i + 1 >= originlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(originChar[i+1] >> 4) ) & ( (unsigned char) 0x0F );
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)(originChar[i+1] << 2) ) & ( (unsigned char)0x3C ) ;
        if ( i + 2 >= originlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(originChar[i+2] >> 6) ) & ( (unsigned char) 0x03 );
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)originChar[i+2] ) & ( (unsigned char)0x3F ) ;
        base64[j++] = base64char[(int)current];
    }
    base64[j] = '\0';
    return base64;
}

 Base64解碼部分

int base64_decode( const char * base64, unsigned char * originChar )//base64解碼
{
    int i, j;
    unsigned char k;
    unsigned char temp[4];
    for ( i = 0, j = 0; base64[i] != '\0' ; i += 4 )
    {
        memset( temp, 0xFF, sizeof(temp) );
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i] )
                temp[0]= k;
        }
        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;
        }
 
        originChar[j++] = ((unsigned char)(((unsigned char)(temp[0] << 2))&0xFC)) |
                ((unsigned char)((unsigned char)(temp[1]>>4)&0x03));
        if ( base64[i+2] == '=' )
            break;
 
        originChar[j++] = ((unsigned char)(((unsigned char)(temp[1] << 4))&0xF0)) |
                ((unsigned char)((unsigned char)(temp[2]>>2)&0x0F));
        if ( base64[i+3] == '=' )
            break;
 
        originChar[j++] = ((unsigned char)(((unsigned char)(temp[2] << 6))&0xF0)) |
                ((unsigned char)(temp[3]&0x3F));
    }
    return j;
}

 三、總結

Base64編碼其實就是將二進制轉成字符串，Base64解碼其實就是將字符串轉成二進制，文中順便也提供瞭解碼代碼方便大家調用與交流，好了，最後提供兩個編解碼在線工具鏈接好方便大家論證：

https://base64.us/ 

http://tool.chinaz.com/tools/imgtobase