C program for steganography - c

Question

Relatively new to programming, been assigned to a task to create a C program to encode and decode 2 ppm images to receive a secret message. I've came up with code but its not getting past the image format if statement in the getPPM function. Any tips on what changes I could make to my code? Also in my main(), am I passing in the correct parameters?
typedef struct NODE {
char *val;
struct NODE *next;
} NODE;
/*
Stores the RGB values of a pixel
*/
typedef struct Pixel {
int red;
int green;
int blue;
} Pixel;
/*
Stores the contents of a PPM file
*/
typedef struct PPM {
char *format;
NODE *comments;
int width, height;
int max;
Pixel *arr;
} PPM;
NODE *copy(NODE *first)
{
NODE *second = NULL, *previous = NULL;
while (first != NULL) {
NODE *temp = (NODE *)malloc(sizeof(NODE));
temp->val = first->val;
temp->next = NULL;
if (second == NULL) {
second = temp;
previous = temp;
} else {
previous->next = temp;
previous = temp;
}
first = first->next;
}
return second;
}
//Copy the PPM File Returning Pointer
struct PPM* createPPM(PPM *old)
{
PPM* new = (PPM *)malloc(sizeof(PPM));
strcpy(new->format, old->format);
new->comments = copy(old->comments);
new->height = old->height;
new->width = old->width;
new->max = old->max;
new->arr = (Pixel *)malloc(old->height * old->width * sizeof(Pixel));
memcpy(new->arr, old->arr, old->height * old->width * sizeof(Pixel));
return new;
}
/*
Returns the PPM struct from the file fd
*/
PPM *getPPM(FILE *fd) {
PPM *image = (PPM *)malloc(sizeof(PPM));
image->format = (char *)malloc(MAX_LEN);
fgets(fd, MAX_LEN, image->format);
sscanf(fd, "%s", image->format);
if (strcmp(image->format, "P3") != 0) {
printf("Invalid Image Type");
exit(0);
}
char c = getc(fd);
image->comments = (NODE*)malloc(sizeof(NODE));
NODE *temp = image->comments;
while ((c = getc(fd)) == '#') {
fseek(fd, -1, SEEK_CUR);
char str[50];
fgets(str, 50, fd);
temp->val = (char*)malloc(strnlen(str, 50));
strcpy(temp->val, str);
temp->val[strlen(temp->val) - 1] = 0;
temp->next = (NODE*)malloc(sizeof(NODE));
temp = temp->next;
temp->next = NULL;
}
fseek(fd, -1, SEEK_CUR);
fscanf(fd, "%d", &image->width);
fscanf(fd, "%d", &image->height);
fscanf(fd, "%d", &image->max);
image->arr = (Pixel*)malloc(image->height * image->width * sizeof(Pixel));
int t = 0;
int j = 0;
while (j < image->height * image->width) {
t = fscanf(fd, "%d", &image->arr[j].red);
t = fscanf(fd, "%d", &image->arr[j].green);
t = fscanf(fd, "%d", &image->arr[j].blue);
j = j + 1;
}
return image;
}
PPM *loadPPMFromFile(char *filename) {
FILE *file;
/* TODO: step 1, open the file */
file = fopen("C:\\Users\\Olivia\\source\\repos\\f28hs-2020-21-cwk1-c\\PPMfiles", "r");
/* step 2: Check that file has been loaded correctly; show error otherwise*/
if (file != NULL) {
PPM *ppm;
/* ODO: step 3, get the PPM data from the file with the getPPM function */
getPPM(file);
if (ppm == NULL) {
/* TODO: step 4, display error if file cannot been parsed into a PPM struct*/
printf("File cannot be parsed.\n");
}
/* TODO: step 5, close the file */
fclose(file);
/* step 6: return the PPM */
return ppm;
} else {
fclose(file);
return NULL;
}
}
/*
Prints a PPM struct in the format of a PPM file
*/
void showPPM(PPM *ppm) {
printf("%s\n", ppm->format); //print format
//print comments
NODE *n = ppm->comments;
while (n->next != NULL) {
printf("%s\n", n->val);
n = n->next;
}
//print width, height and max
printf("%d %d\n%d\n", ppm->width, ppm->height, ppm->max);
//print the array containing the pixels
int j;
for (j = 0; j < ppm->height * ppm->width; ++j) {
printf("%d %d %d\n", ppm->arr[j].red, ppm->arr[j].green, ppm->arr[j].blue);
}
return;
}
/*
Encodes text into red field of PPM
Returns the encoded PPM
*/
PPM *encode(char *text, PPM *i) {
PPM *str = createPPM(i);
int random;
srand((unsigned)time(NULL));
int randMax = (i->height * i->width) / (strlen(text) + 1);
random = rand() % randMax;
if (random < 1) {
random = 1;
}
int k = 0;
int j = random;
//Red fields swapped with ASCII int
while (k < strlen(text)) {
if (str->arr[j].red == text[k]) {
j = j + 1; // if the values are the same we encode in the next pixel.
} else {
str->arr[j].red = text[k];
k = k + 1;
j = j + random;
}
}
return str;
}
/*
Compares 2 PPM structs and decodes the message hidden within
Returns the decoded message if the images have the same dimensions
Returns NULL otherwise
*/
char *decode(PPM *i1, PPM *i2) {
int i = 0;
int j = 0;
char *str = (char*)malloc(sizeof(char));
while (i < i1->height * i1->width) {
if (i1->arr[i].red != i2->arr[i].red) {
str[j] = i2->arr[i].red;
j = j + 1;
}
i = i + 1;
}
str = realloc(str, i);
return str;
printf("%s", str);
}
/* TODO: Question 3 */
int main(int argc, char *argv[]) {
/* check arguments */
switch (argc) {
case 2:
/* TODO: not enough arguments, print an error message */
printf("Not enough arguments. \n");
break;
case 3:
if (strcmp(argv[1], "e") == 0) { //Argument "e" - encode PPM
PPM *ppm = loadPPMFromFile(argv[2]);
createPPM(ppm);
/*Check that PPM is valid; continue if it is, exit otherwise */
if (ppm != NULL) {
PPM *encodedPPM;
/* TODO: encode the ppm with the text and assign to encodedPPM */
encodedPPM = encode(argv[2], encodedPPM);
/*Check that no error occured*/
if (encodedPPM == NULL) {
return EXIT_FAILURE;
} else {
/* TODO: print a confirmation that encoding was successful */
printf("Encoding successful. \n");
/* TODO: print the PPM to standard output with showPPM */
showPPM(encodedPPM);
return EXIT_SUCCESS;
}
} else {
return EXIT_FAILURE;
}
} else {
printf("Unrecognised or incomplete command line.\n");
return EXIT_FAILURE;
}
break;
case 4:
if (strcmp(argv[1], "d") == 0) { //Argument "d" - decode PPM
PPM *comparisonPPM;
PPM *encodedPPM;
/* TODO: get comparison file filename from argv, load it with
loadPPMFromFile then assign to comparisonPPM */
comparisonPPM = loadPPMFromFile(argv[2]);
/* TODO: get encoded file filename from argv, load it with
loadPPMFromFile then assign to encodedPPM */
encodedPPM = loadPPMFromFile(argv[2]);
/*Check both PPMs are valid; continue if so, exit otherwise */
if (comparisonPPM != NULL && encodedPPM != NULL) {
char *decodedMsg;
/* TODO: decode the encodedPPM with the comparisonPPM and assign to decodedMsg */
decodedMsg = decode(encodedPPM, comparisonPPM);
/*Check that the message has been decoded*/
if (decodedMsg != NULL) {
/* TODO: print a confirmation message that the message was decoded */
printf("Decoded message:\n");
/* TODO: print the decoded message */
printf("%p",decodedMsg);
return EXIT_SUCCESS;
} else {
return EXIT_FAILURE;
}
} else {
return EXIT_FAILURE;
}
} else {
fprintf(stderr, "Unrecognised or incomplete command line.\n\n");
return EXIT_FAILURE;
}
break;
default:
fprintf(stderr, "Unrecognised or incomplete command line.\n\n");
return EXIT_FAILURE;
}
}

Answer 1

"Also in my main(), am I passing in the correct parameters?"
Yes, they are correct.
"not getting past the image format if statement in the getPPM function"
There are several issues with getPPM.
Overwriting c in the statements following this one:
char c = getc(fd);
is already covered in comments.
In the following:
sscanf(fd, "%s", image->format);
if (strcmp(image->format, "P3") != 0)
{
Because sscanf() will pick up the new line if there is one, strcmp() will fail the comparison for "P3".
Suggestions are to do 1 or more of the following:
running in a debugger, place a break point at the if and view value of image->format
place if(image->format != NULL) printf("%s", image->format) after sscanf()
if you suspect \n or \r\n may be in the buffer, use image->format[strcspn(image->format, "\r\n")] = 0; to remove them.
Regarding the following, suggest you read the documentation on each C function, eg fgets():
This:
fgets(fd, MAX_LEN, image->format);
scanf(...
Should be
if(fgets(image->format, MAX_LEN, fd) != NULL)
{
scanf(...
(Argument positions changed, and test before trying to use buffer.)
Aside: Because this is C, following malloc statements
PPM* image = (PPM*)malloc(sizeof(PPM));
image->format = (char*)malloc(MAX_LEN);
are more correctly written as
PPM* image = malloc(sizeof(*image));
image->format = malloc(MAX_LEN);
(Cast is removed from malloc() and note sizeof argument in the first statement. apply as applicable throughout your code.