PSET4 Recover Problem to repair .jpg's signature in hexadecimal

Question

I'm not sure if I use correctly the hexadecimal notation to verify if there is a .jpeg signature in the beginning of my BLOCK of 512 bytes. My program compile well but no photograph is revealed. May be someone can help me with that? Here a part of my code. Great thanks for your time.

int main(int argc, char* argv[])
{    
    typedef uint8_t BYTE;
    typedef uint32_t SIGN;
    SIGN buffer[BLOCK];
    int count = 0;
    FILE* pic;
    char picname[8];


    // open file
    FILE* inptr = fopen("card.raw", "r");
    char* name = "card.raw";

    if (inptr == NULL)
    {
        printf("Could not open %s.\n", name);
        return 1;
    }

    // until end of data is reached    
    while (fread(&buffer, BLOCK, 1, inptr) == 1)
    {
        // read 512 bytes
        // fread(&buffer, BLOCK, 1, inptr);

        printf("%0x\n", buffer[0]);
        printf("%0x\n", 0xffd8ffe0);

        if (buffer[0] == 0xffd8ffe0 || buffer[0] == 0xffd8ffe1)
        {
            if (count > 0)
        {

Answer 1

1) What's the second argument of fread supposed to be? Are you sure passing BLOCK is correct?

2) Why are you reading only 4 bytes (I guess that's your intention) at a time? You know the file is formatted in chunks of 512 bytes, take advantage of that so that you don't loop so many times.

3) The problem with what you're doing (once you fix what i mentioned in the first point) is one of endianness. You can read about it here: http://betterexplained.com/articles/understanding-big-and-little-endian-byte-order/

The safest and easiest way would be to store each byte separately and evaluate them in the same way.

Answer 2

To answer your specific question, no, this isn't the way to check for a signature in the beginning of the buffer. The following line,

        if (buffer[0] == 0xffd8ffe0 || buffer[0] == 0xffd8ffe1)

doesn't work because you can't compare strings of bytes like this. You need to compare single bytes to single bytes, similar to this:

    if ( buffer[0] == 0xff && buffer[1] == 0xd8 ....)

I'll let you figure out how the rest of it should look, but that's the general idea. There are other techniques, but that becomes a larger conversation.

If this answers your question, please click the check mark and accept. Let's keep up on forum maintenance. ;-)