1. #include <signal.h>
2. #include <stdio.h>
3. #include <stdlib.h>
4. #include <string.h>
5. #include <unistd.h>
6.  
7. char * read_file(char *);
8. void give_flag();
9. void give_error();
10. void timeout();
11.  
12. int i, input_char;
13.  
14. int main(int argc, char * argv[]) {
15.   setvbuf(stdin,  NULL, _IONBF, 0);   // Switch off I/O buffering
16.   setvbuf(stdout, NULL, _IONBF, 0);   // Switch off I/O buffering
17.   setvbuf(stderr, NULL, _IONBF, 0);   // Switch off I/O buffering
18.  
19.   signal(SIGALRM, timeout);           // Set program to exit on timeout
20.   alarm(10);                          // Exit program after 10s
21.  
22.   void (*call_function)() = &give_error;  // function pointer to invoke function later on
23.   char input[128];
24.  
25.   // Check program usage
26.   if (argc < 2) {
27.     printf("Secret value needs to be supplied as an argument to this program!\n");
28.     printf("Example: ./unlockme secret\n");
29.     return 1;
30.   }
31.   else if (strlen(argv[1]) != 128) {
32.     printf("Secret value must be exactly 128 characters long!\n");
33.     return 1;
34.   }
35.  
36.   printf("Enter the secret value to unlock the flag => ");
37.   for (i = 0; i <= 128; i++) {
38.     input_char = getchar();
39.     // Exit loop if end of user input is reached
40.     if (feof(stdin) || ferror(stdin) || input_char == '\n' || input_char == '\x00') {
41.       input[i] = '\x00';
42.       break;
43.     }
44.     else {
45.       input[i] = input_char;
46.     }
47.   }
48.  
49.   // Check if user input equals to secret value
50.   if (strcmp(input, argv[1]) == 0) {
51.     call_function = &give_flag;  // give user flag!
52.   }
53.  
54.   // If user input is incorrect, print error
55.   if (call_function != &give_flag) {
56.     call_function = &give_error;
57.   }
58.  
59.   call_function();
60.  
61.   return 0;
62. }
63.  
64. // Read contents of file and return a string containing the contents
65. char * read_file(char * filename) {
66.   char * file_contents = malloc(4096 * sizeof(char));
67.  
68.   FILE * file;
69.   file = fopen(filename, "r");
70.  
71.   if (file == NULL) {
72.     printf("\nUnable to open file!\n");
73.     _exit(1);
74.   }
75.  
76.   fread(file_contents, 4096, sizeof(char), file);
77.   fclose(file);
78.  
79.   return file_contents;
80. }
81.  
82. // Print flag
83. void give_flag() {
84.   printf("\nOkay here you go: %s", read_file("flag.txt"));
85.   _exit(0);
86. }
87.  
88. // Print error message
89. void give_error() {
90.   printf("\nNah the secret value you've provided is incorrect!\n");
91.   _exit(1);
92. }
93.  
94. // Exit program after 10s
95. void timeout() {
96.   printf("\nToo slow!\n");
97.   _exit(1);
98. }