Rating:
# RPS - picoCTF 2022 - CMU Cybersecurity Competition
Binary Exploitation, 200 Points
## Description
## RPS Solution
Let's observe the attached file [game-redacted.c](./game-redacted.c):
```c
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#define WAIT 60
static const char* flag = "[REDACTED]";
char* hands[3] = {"rock", "paper", "scissors"};
char* loses[3] = {"paper", "scissors", "rock"};
int wins = 0;
int tgetinput(char *input, unsigned int l)
{
fd_set input_set;
struct timeval timeout;
int ready_for_reading = 0;
int read_bytes = 0;
if( l <= 0 )
{
printf("'l' for tgetinput must be greater than 0\n");
return -2;
}
/* Empty the FD Set */
FD_ZERO(&input_set );
/* Listen to the input descriptor */
FD_SET(STDIN_FILENO, &input_set);
/* Waiting for some seconds */
timeout.tv_sec = WAIT; // WAIT seconds
timeout.tv_usec = 0; // 0 milliseconds
/* Listening for input stream for any activity */
ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
/* Here, first parameter is number of FDs in the set,
* second is our FD set for reading,
* third is the FD set in which any write activity needs to updated,
* which is not required in this case.
* Fourth is timeout
*/
if (ready_for_reading == -1) {
/* Some error has occured in input */
printf("Unable to read your input\n");
return -1;
}
if (ready_for_reading) {
read_bytes = read(0, input, l-1);
if(input[read_bytes-1]=='\n'){
--read_bytes;
input[read_bytes]='\0';
}
if(read_bytes==0){
printf("No data given.\n");
return -4;
} else {
return 0;
}
} else {
printf("Timed out waiting for user input. Press Ctrl-C to disconnect\n");
return -3;
}
return 0;
}
bool play () {
char player_turn[100];
srand(time(0));
int r;
printf("Please make your selection (rock/paper/scissors):\n");
r = tgetinput(player_turn, 100);
// Timeout on user input
if(r == -3)
{
printf("Goodbye!\n");
exit(0);
}
int computer_turn = rand() % 3;
printf("You played: %s\n", player_turn);
printf("The computer played: %s\n", hands[computer_turn]);
if (strstr(player_turn, loses[computer_turn])) {
puts("You win! Play again?");
return true;
} else {
puts("Seems like you didn't win this time. Play again?");
return false;
}
}
int main () {
char input[3] = {'\0'};
int command;
int r;
puts("Welcome challenger to the game of Rock, Paper, Scissors");
puts("For anyone that beats me 5 times in a row, I will offer up a flag I found");
puts("Are you ready?");
while (true) {
puts("Type '1' to play a game");
puts("Type '2' to exit the program");
r = tgetinput(input, 3);
// Timeout on user input
if(r == -3)
{
printf("Goodbye!\n");
exit(0);
}
if ((command = strtol(input, NULL, 10)) == 0) {
puts("Please put in a valid number");
} else if (command == 1) {
printf("\n\n");
if (play()) {
wins++;
} else {
wins = 0;
}
if (wins >= 5) {
puts("Congrats, here's the flag!");
puts(flag);
}
} else if (command == 2) {
return 0;
} else {
puts("Please type either 1 or 2");
}
}
return 0;
}
```
We need to win at least 5 times to get the flag.
Let's observe the winning check on ```play``` function:
```c
...
if (strstr(player_turn, loses[computer_turn])) {
puts("You win! Play again?");
return true;
}
...
```
We can see function ```strstr``` which finds the first occurrence of the substring second argument in the first argument.
```loses``` array is:
```c
char* loses[3] = {"paper", "scissors", "rock"};
```
So if our string will be ```paperscissorsrock``` we always get into the ```if``` statement because each word on ```loses``` array exists on ```paperscissorsrock```, Let's try it:
```console
┌─[evyatar@parrot]─[/pictoctf2022/binary_exploitation/rps]
└──╼ $ nc saturn.picoctf.net 52524
Welcome challenger to the game of Rock, Paper, Scissors
For anyone that beats me 5 times in a row, I will offer up a flag I found
Are you ready?
Type '1' to play a game
Type '2' to exit the program
1
1
Please make your selection (rock/paper/scissors):
paperscissorsrock
paperscissorsrock
You played: paperscissorsrock
The computer played: scissors
You win! Play again?
Type '1' to play a game
Type '2' to exit the program
1
1
Please make your selection (rock/paper/scissors):
paperscissorsrock
paperscissorsrock
You played: paperscissorsrock
The computer played: scissors
You win! Play again?
Type '1' to play a game
Type '2' to exit the program
1
1
Please make your selection (rock/paper/scissors):
paperscissorsrock
paperscissorsrock
You played: paperscissorsrock
The computer played: scissors
You win! Play again?
Type '1' to play a game
Type '2' to exit the program
1
1
Please make your selection (rock/paper/scissors):
paperscissorsrock
paperscissorsrock
You played: paperscissorsrock
The computer played: rock
You win! Play again?
Type '1' to play a game
Type '2' to exit the program
1
1
Please make your selection (rock/paper/scissors):
paperscissorsrock
paperscissorsrock
You played: paperscissorsrock
The computer played: rock
You win! Play again?
Congrats, here's the flag!
picoCTF{50M3_3X7R3M3_1UCK_32F730C2}
Type '1' to play a game
Type '2' to exit the program
Timed out waiting for user input. Press Ctrl-C to disconnect
Goodbye!
```
And we get the flag ```picoCTF{50M3_3X7R3M3_1UCK_32F730C2}```.
