CyberQuest 2025 - RPS

Description

RPS

Ever played a game of rock, paper, scissors where couldn’t read you opponent? What if he/she always chooses based on a set pattern, you could always know how to beat your opponent. It would be pretty useful in my opinion.

Creators comments:

• No brute forcing is required
• Use the given file
• VPN connection is required

Flag format: CQ25{flag}

By incarrnati0n

cq25-challenge0[1-9]-a.cq.honeylab:41111

Metadata

• Filename: game_redacted.cpp
• Tags: brute-force, rand, srand, time(0)

Solution

In this challenge we have to win against the server in rock-paper-scissors 5 times in a row.

The srand function is seeded with time(0), which should make the rand function predicable and it might be possible the predict the hands of the server (even with mod 3), however, there are some problems, which make this impossible:

• It is unknown when the services are started (or restarted) on the server
• All the players are using the same shared instances

Because of this, as far as I know all solutions from the players were based on brute-forcing.

I also tried brute-forcing and after a short time I was lucky:

nc 10.10.100.111 41111
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: paper
You lose!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: paper
You lose!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 1
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 2
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: paper
You lose!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 1
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 2
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: rock
It's a draw!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: rock
It's a draw!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: paper
You lose!
Current number of wins: 0
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 1
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 2
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 3
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Current number of wins: 4
Fancy a game of rock, paper, scissors?
Press 1 to play a game
Press 2 to exit the game
1
Enter your choice (1-3)
1:Rock, 2:Paper, 3:Scissor
1
Your hand: rock
Computer's hand: scissor
Congrats you win!
Congrats! Here is your flag: CQ25{Gr34t_gU3sS1nG}

Flag: CQ25{Gr34t_gU3sS1nG}