Binary Search

• Description: Want to play a game? As you use more of the shell, you might be interested in how they work! Binary search is a classic algorithm used to quickly find an item in a sorted list. Can you find the flag? You'll have 1000 possibilities and only 10 guesses. Cyber security often has a huge amount of data to look through - from logs, vulnerability reports, and forensics. Practicing the fundamentals manually might help you in the future when you have to write your own tools!
• Difficulty: Easy

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🔎 Solution

The challenge presents a simple yet classic game: guess the correct number between 1 and 1000. However, there's a catch - we only have 10 attempts to find the answer.

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

To efficiently solve this, we can apply the binary search algorithm, a fundamental technique for quickly narrowing down a sorted range. We start by guessing 500 - the midpoint of the initial range. If the correct number is lower than 500, we adjust our range to [1, 499] and guess the midpoint of that range next. Conversely, if the number is higher than 500, we shift our focus to [501, 1000] and continue in the same fashion.
By repeatedly halving the search space based on whether the target number is higher or lower than our current guess, we converge on the correct number within 10 steps. For example:

Enter your guess: 500
Higher! Try again.
Enter your guess: 750
Higher! Try again.
Enter your guess: 875
Higher! Try again.
Enter your guess: 937
Higher! Try again.
Enter your guess: 969
Higher! Try again.
Enter your guess: 984
Lower! Try again.
Enter your guess: 976   
Lower! Try again.
Enter your guess: 972
Higher! Try again.
Enter your guess: 974
Lower! Try again.
Enter your guess: 973
Congratulations! You guessed the correct number: 973

Once the number is correctly identified, the challenge rewards us with the flag.

Here's your flag: picoCTF{g00d_gu355_1597707f}

🚩Flag

picoCTF{g00d_gu355_1597707f}