auth.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

typedef enum auth_level {
  ANONYMOUS = 1,
  GUEST = 2,
  USER = 3,
  ADMIN = 4,
  ROOT = 5
} auth_level_t;
  
struct user {
  char *name;
  auth_level_t level;
};

void give_flag(){
  char flag[48];
  FILE *f = fopen("flag.txt", "r");
  if (f == NULL) {
    printf("Flag File is Missing. Problem is Misconfigured, please contact an Admin if you are running this on the shell server.\n");
    exit(0);
  }

  if ((fgets(flag, 48, f)) == NULL){
    puts("Couldn't read flag file.");
    exit(1);
  };
  
  puts(flag);
  fclose(f);
}

void menu(){
  puts("Available commands:");
  puts("\tshow - show your current user and authorization level");
  puts("\tlogin [name] - log in as [name]");
  puts("\tset-auth [level] - set your authorization level (must be below 5)");
  puts("\tget-flag - print the flag (requires authorization level 5)");
  puts("\treset - log out and reset authorization level");
  puts("\tquit - exit the program");
}

int main(int argc, char **argv){
  char buf[512];
  char *arg;
  uint32_t level;
  struct user *user;

  setbuf(stdout, NULL);

  menu();

  user = NULL;
  while(1){
    puts("\nEnter your command:");
    putchar('>'); putchar(' ');

    if(fgets(buf, 512, stdin) == NULL)
      break;

    if (!strncmp(buf, "show", 4)){
      if(user == NULL){
	puts("Not logged in.");
      }else{
	printf("Logged in as %s [%u]\n", user->name, user->level);
      }

    }else if (!strncmp(buf, "login", 5)){
      if (user != NULL){
	puts("Already logged in. Reset first.");
	continue;
      }

      arg = strtok(&buf[6], "\n");
      if (arg == NULL){
	puts("Invalid command");
	continue;
      }

      user = (struct user *)malloc(sizeof(struct user));
      if (user == NULL) {
	puts("malloc() returned NULL. Out of Memory\n");
	exit(-1);
      }
      user->name = strdup(arg);
      printf("Logged in as \"%s\"\n", arg);

    }else if(!strncmp(buf, "set-auth", 8)){
      if(user == NULL){
	puts("Login first.");
	continue;
      }

      arg = strtok(&buf[9], "\n");
      if (arg == NULL){
	puts("Invalid command");
	continue;
      }

      level = strtoul(arg, NULL, 10);

      if (level >= 5){
	puts("Can only set authorization level below 5");
	continue;
      }

      user->level = level;
      printf("Set authorization level to \"%u\"\n", level);

    }else if(!strncmp(buf, "get-flag", 8)){
      if (user == NULL){
	puts("Login first!");
	continue;
      }

      if (user->level != 5){
	puts("Must have authorization level 5.");
	continue;
      }

      give_flag();
    }else if(!strncmp(buf, "reset", 5)){
      if (user == NULL){
	puts("Not logged in!");
	continue;
      }

      free(user->name);
      user = NULL;

      puts("Logged out!");
    }else if(!strncmp(buf, "quit", 4)){
      return 0;
    }else{
      puts("Invalid option");
      menu();
    }
  }
}