The document provides code for two C programs: one that hides a secret message in a ppm image file and another that retrieves the hidden message. The first program encodes the message by modifying the least significant bits of the image's pixel data, while the second program extracts and decodes the message. Key functions include managing file headers, encoding message lengths, and ensuring the message fits within the image dimensions.

1. In C program, how to write two short programs: one which hides a short, secret message in an
image file, such as ppm file, and a second program which reads the hidden message back out of
the image.
Solution
First program to write secret message
Encrypt.c
#include
#include "stego.h"
void copy_header(FILE *, int, FILE *);
int get_message_length(char[]);
int message_fits(int, int, int);
int count_new_lines(FILE *);
void encode_length(FILE *, FILE *, int);
void encode_message(FILE *, FILE *, int, char *, int, int);
int main(int argc, char **argv) {
if(argc < 3) {
printf("requires 2 parameters the message within " " and the PPM file you want to encode
the message in. Aborting .");
return 1;
}
FILE *fp;
if((fp = fopen(argv[2], "r+")) == NULL) {
printf("Couldn't open file %s. Aborting ", argv[2]);
return 1;
}
if(read_ppm_type(fp)) {
skip_comments(fp);
char *myMessage = (char *)argv[1];
int message_length = get_message_length(myMessage);
int w = get_width(fp);
int h = get_height(fp);

2. if(message_fits(message_length, w, h)) {
if(read_color_depth(fp)) {
FILE *fp_t = fopen("out.ppm","w");
int i = count_new_lines(fp);
copy_header(fp, i, fp_t);
encode_length(fp, fp_t, (message_length - 8) / 8);
encode_message(fp, fp_t, (message_length - 8), myMessage, w, h);
printf("Encoding Process Complete Take a look at out.ppm ");
fclose(fp);
fclose(fp_t);
} else {
printf(" Error: Image color depth not valid. Must be 255. ");
return 1;
}
} else {
printf(" Error: Image file is too large to print the message. Abort ");
return 1;
}
} else {
printf("Error: Wrong file format. Abort ");
return 1;
}
return 0;
}
void copy_header(FILE *fp1, int numLines, FILE *fp2) {
int i;
char temp;
rewind(fp1); //Goes back to the beginning of the file
for(i = 0; i < numLines; i++) {
while((temp = fgetc(fp1)) != EOF && temp != ' ') {
fputc(temp, fp2);
}
fputc(' ', fp2);
}
return;

3. }
int get_message_length(char my_msg[]) {
int i = 0;
while(my_msg[i] != '0') {
i++;
}
return i * 8 + 8;
}
int message_fits(int length, int w, int h) {
return length < w * h * 3;
}
int count_new_lines(FILE *fp) {
char temp; int count = 0;
rewind(fp);
while((temp = fgetc(fp)) != EOF)
if(temp == ' ')
count++;
return count;
}
void encode_length(FILE *in, FILE *out, int length) {
char temp;
int i, l;
for(i = 0; i < 8; i++) {
temp = fgetc(in);
l = length;
l >>= 7 - i;
if((l & 1) == 1) {
if((temp & 1) == 0) {
temp++;
}
} else {
if((temp & 1) == 1) {
temp--;
}
}
fputc(temp, out);

4. }
return;
}
void encode_message(FILE *in, FILE *out, int num_to_encode, char* my_message, int w, int h)
{
int encoded = 0;
unsigned char temp;
int idx = 0, num_coppied = 0;
char current;
int fileSize = (w * h * 3) - 8;
int i;
for(i = 0; i < fileSize; i++) {
temp = fgetc(in);
current = my_message[idx];
current >>= 7 - num_coppied;
num_coppied++;
if(encoded <= num_to_encode) {
encoded++;
if((current & 1) == 1) {
if((temp & 1) == 0) {
temp++;
}
} else {
if((temp & 1) == 1) {
temp--;
}
}
if(num_coppied == 8) {
idx++;
num_coppied = 0;
}
}
fputc(temp, out);
}

5. return;
}
second program which reads the hidden message
Decrypt.c
#include
#include "stego.h"
int get_msg_length(FILE *);
void decode_message(int, FILE *);
int main(int argc, char **argv) {
FILE *fp;
//Print an error if no args provided
if((fp = fopen(argv[1], "rb")) == NULL) {
printf(" Error: Please provide a file to scan ");
return 1;
}
if(read_ppm_type(fp)) {
skip_comments(fp);
get_width(fp);
get_height(fp);
if(read_color_depth(fp)) {
int length = get_msg_length(fp);
printf(" secret message ");
decode_message(length, fp);
fclose(fp);
} else {
printf("Error:Invalid Color Depth.");
return 1;
}
} else {
printf("Error:Wrong ppm File Format closing");
return 1;
}
return 0;

6. }
//Gets the length of the secret message
int get_msg_length(FILE *fp) {
char temp = 0;
int length = 0;
int i;
for(i=0; i < 8; i++) {
temp = fgetc(fp);
if(i > 0) length <<= 1;
length |= (temp & 1);
}
return length;
}
void decode_message(int length, FILE *fp) {
int readTo = length * 8, numRead = 0, i;
unsigned char charBuffer = 0;
char temp;
char secret_message[length + 1];
int idx = 0;
while(numRead <= readTo) {
temp = fgetc(fp);
if(numRead % 8 == 0) {
secret_message[idx] = charBuffer;
idx++;
charBuffer = 0;
} else {
charBuffer <<= 1;
}
charBuffer |= (temp & 1);
numRead++;
}
for(i = 1; i < idx; i++) {
printf("%c", secret_message[i]);
}
printf(" ");

7. return;
}