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POSteganography in C
 

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  1. POSteganography in C
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    <p>Trying to do basic Steganography on a PPM Image.</p> <p>I have the basic algorithm completed. Read in the file, check the header starts with P6, get the image width and height, and the pixel data.</p> <p>I need to have a total of four methods: ReadPPM, WritePPM, WriteMsg and ReadMsg.</p> <p>I have the ReadImg and WriteImg methods down, but where I am stuck is with my WriteMsg method. This is basic steganography that just writes each bit of the string to the last bit in each byte. The first 8 bytes are suppose to contain the size of the string being hidden, then each byte after that starts the hidden message.</p> <p>My idea was to create a massive array that holds the binary code for the size of the string, then the binary code of the string itself. I'm just trying to figure out how I would take that array and add it to each byte in the image.</p> <p>Any help is much appreciated. Here is my current code:</p> <pre><code>#include&lt;stdio.h&gt; #include&lt;stdlib.h&gt; typedef struct { unsigned char red,green,blue; } PPMPixel; typedef struct { int x, y; PPMPixel *data; } PPMImage; #define CREATOR "RPFELGUEIRAS" #define RGB_COMPONENT_COLOR 255 static PPMImage *readPPM(const char *filename) { char buff[16]; PPMImage *img; FILE *fp; int c, rgb_comp_color; //open PPM file for reading fp = fopen(filename, "rb"); if (!fp) { fprintf(stderr, "Unable to open file '%s'\n", filename); exit(1); } //read image format if (!fgets(buff, sizeof(buff), fp)) { perror(filename); exit(1); } //check the image format if (buff[0] != 'P' || buff[1] != '6') { fprintf(stderr, "Invalid image format (must be 'P6')\n"); exit(1); } //alloc memory form image img = (PPMImage *)malloc(sizeof(PPMImage)); if (!img) { fprintf(stderr, "Unable to allocate memory\n"); exit(1); } //check for comments c = getc(fp); while (c == '#') { while (getc(fp) != '\n') ; c = getc(fp); } ungetc(c, fp); //read image size information if (fscanf(fp, "%d %d", &amp;img-&gt;x, &amp;img-&gt;y) != 2) { fprintf(stderr, "Invalid image size (error loading '%s')\n", filename); exit(1); } //read rgb component if (fscanf(fp, "%d", &amp;rgb_comp_color) != 1) { fprintf(stderr, "Invalid rgb component (error loading '%s')\n", filename); exit(1); } //check rgb component depth if (rgb_comp_color!= RGB_COMPONENT_COLOR) { fprintf(stderr, "'%s' does not have 8-bits components\n", filename); exit(1); } while (fgetc(fp) != '\n') ; //memory allocation for pixel data img-&gt;data = (PPMPixel*)malloc(img-&gt;x * img-&gt;y * sizeof(PPMPixel)); if (!img) { fprintf(stderr, "Unable to allocate memory\n"); exit(1); } //read pixel data from file if (fread(img-&gt;data, 3 * img-&gt;x, img-&gt;y, fp) != img-&gt;y) { fprintf(stderr, "Error loading image '%s'\n", filename); exit(1); } fclose(fp); return img; } void writePPM(const char *filename, PPMImage *img) { FILE *fp; //open file for output fp = fopen(filename, "wb"); if (!fp) { fprintf(stderr, "Unable to open file '%s'\n", filename); exit(1); } //write the header file //image format fprintf(fp, "P6\n"); //comments fprintf(fp, "# Created by %s\n",CREATOR); //image size fprintf(fp, "%d %d\n",img-&gt;x,img-&gt;y); // rgb component depth fprintf(fp, "%d\n",RGB_COMPONENT_COLOR); // pixel data fwrite(img-&gt;data, 3 * img-&gt;x, img-&gt;y, fp); fclose(fp); } void writeMsg(PPMImage *img, char *s) { int i; int len; len = sizeof(s); if (img) { j = 0; for (i=0; i &lt; img-&gt;x * img-&gt;y; i++) { while(j &lt; 8) { if(len &amp; 0x80) { img-&gt;data[i].red= img-&gt;data[i].red | 0x01; } else { img-&gt;data[i].red= img-&gt;data[i].red &amp; 0xFE; } len=len &lt;&lt; 1; j++; if (len &amp; 0x80) { img-&gt;data[i].green= img-&gt;data[i].green | 0x01; } else { img-&gt;data[i].green= img-&gt;data[i].green &amp; 0xFE; } len = len &lt;&lt; 1; j++; if (len &amp; 0x80) { img-&gt;data[i].blue= img-&gt;data[i].blue | 0x01; } else { img-&gt;data[i].blue= img-&gt;data[i].blue &amp; 0xFE; } j++; } } } } </code></pre>
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    1. COThe formatting of your code is mostly fine... except for the function you actually want help with! Please fix it. It's completely unreadable as it is.
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    2. COYou need two cycles: one by len and another by bits of every char, write data only inside second cycle like @Patashu said or in reverse order (`byte << i & 0x80`). Don't roll length variable!
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    3. CO`len = sizeof(s)` will return the size of a pointer. If you want the length of the string, use `strlen()`.
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