PicoCTF - RPS
Background
In C++, std::strstr() is a predefined function used for string handling. string.h is the header file required for string functions. This function takes two strings s1 and s2 as an argument and finds the first occurrence of the sub-string s2 in the string s1. The process of matching does not include the terminating null-characters(‘\0’), but function stops there. Syntax:
2
3
4
5char *strstr (const char *s1, const char *s2); Parameters: s1: This is the main string to be examined. s2: This is the sub-string to be searched in s1 string.Return Value: This function returns a pointer points to the first character of the found s2 in s1 otherwise a null pointer if s2 is not present in s1. If s2 points to an empty string, s1 is returned.
Source code
:::spoiler Source Code
#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;
}
:::
Recon
這一題有reverse的感覺，主要是利用strstr()這個function拿到win++，在第100行的地方是利用strstr()搜索字串達到判斷勝利的功能，但是如果我們把三種結果結合在一起，則這一段結果就一定會試true
Exploit
Payload: paperscissorsrock
:::spoiler Whole Progress
1 |
|
:::
Flag: picoCTF{50M3_3X7R3M3_1UCK_58F0F41B}