picoCTF 2024 | Binary Search WriteUp

Bu yazıda, picoCTF platformunda yer alan “Binary Search” isimli meydan okumayı çözeceğiz. Meydan okuma sıcak soğuk oyunu oynayarak bayrağı bulmamızı istiyor. Bunun haricinde kaynak kodları verilmiş ama işimize yaramayacak gibi görünüyor.

#!/bin/bash

target=$(( (RANDOM % 1000) + 1 ))

echo "Welcome to the Binary Search Game!"
echo "I'm thinking of a number between 1 and 1000."

trap 'echo "Exiting is not allowed."' INT
trap '' SIGQUIT
trap '' SIGTSTP

MAX_GUESSES=10
guess_count=0

while (( guess_count < MAX_GUESSES )); do
      read -p "Enter your guess: " guess

      if ! [[ "$guess" =~ ^[0-9]+$ ]]; then
          echo "Please enter a valid number."
          continue
      fi

      (( guess_count++ ))

      if (( guess < target )); then
          echo "Higher! Try again."
      elif (( guess > target )); then
          echo "Lower! Try again."
       else
          echo "Congratulations! You guessed the correct number: $target"

          flag=$(cat /challenge/metadata.json | jq -r '.flag')
          echo "Here's your flag: $flag"
          exit 0  # Exit with success code
       fi
done

echo "Sorry, you've exceeded the maximum number of guesses."
exit 1  # Exit with error code to close the connection

SSH servisine bağlandıktan sonra oyun başlıyor. Rastgele bir sayı seçiyor ve sıcak soğuk oyununu oynatarak bu sayıyı bulmamızı istiyor. Sayıyı bulduğumuzda ise bize bayrağı veriyor.

C:\Users\Appdone>ssh -p 57893 ctf-player@atlas.picoctf.net
ctf-player@atlas.picoctf.net's password:
Welcome to the Binary Search Game!
I'm thinking of a number between 1 and 1000.
Enter your guess: 800
Higher! Try again.
Enter your guess: 900
Lower! Try again.
Enter your guess: 860
Higher! Try again.
Enter your guess: 880
Higher! Try again.
Enter your guess: 890
Lower! Try again.
Enter your guess: 885
Higher! Try again.
Enter your guess: 886
Higher! Try again.
Enter your guess: 889
Congratulations! You guessed the correct number: 889
Here's your flag: picoCTF{***}
Connection to atlas.picoctf.net closed.