Determine if a message is too long to embed in an image

Question

I created a program that embeds a message in a PPM file by messing with the last bit in each byte in the file. The problem I have right now is that I don't know if I am checking if a message is too long or not correctly. Here's what I've got so far:

int hide_message(const char *input_file_name, const char *message, const char *output_file_name)
{
    unsigned char * data;
    int n;
    int width;
    int height;
    int max_color;

    //n = 3 * width * height;
    int code = load_ppm_image(input_file_name, &data, &n, &width, &height, &max_color);

    if (code)
    {
        // return the appropriate error message if the image doesn't load correctly
        return code;
    }

    int len_message;
    int count = 0;
    unsigned char letter;

    // get the length of the message to be hidden
    len_message = (int)strlen(message);


    if (len_message > n/3)
    {
        fprintf(stderr, "The message is longer than the image can support\n");
        return 4;
    }

    for(int j = 0; j < len_message; j++)
    {

        letter = message[j];
        int mask = 0x80;

        // loop through each byte
        for(int k = 0; k < 8; k++)
        {
            if((letter & mask) == 0)
            {
                //set right most bit to 0
                data[count] = 0xfe & data[count];
            }
            else
            {
                //set right most bit to 1
                data[count] = 0x01 | data[count];
            }
            // shift the mask
            mask = mask>>1 ;
            count++;
        }
    }
    // create the null character at the end of the message (00000000)
    for(int b = 0; b < 8; b++){
        data[count] = 0xfe & data[count];
        count++;
    }

    // write a new image file with the message hidden in it
     int code2 = write_ppm_image(output_file_name, data, n, width, height, max_color);

    if (code2)
    {
        // return the appropriate error message if the image doesn't load correctly
        return code2;
    }

    return 0;
}

So I'm checking to see if the length of the message (len_message) is longer that n/3, which is the same thing as width*height. Does that seem correct?

Answer 1

The check you're currently doing is checking whether the message has more bytes than the image has pixels. Because you're only using 1 bit per pixel to encode the message, you need to check if the message has more bits than the message has pixels.

So you need to do this:

if (len_message*8 > n/3)