picoCTF2019 vault-door-1 [Reverse Engineering]

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This vault uses some complicated arrays! I hope you can make sense of it, special agent. The source code for this vault is here: VaultDoor1.java
（適当な訳）この金庫は複雑な配列を使用しています。スペシャルエージェントのあなたが理解できると願っています。この金庫のソースコードはここです。

Hints: Look up the charAt() method online.

vault-door-trainingに続く問題です。Javaのソースコードが与えられているので、見てみます。

import java.util.*;

class VaultDoor1 {
   public static void main(String args[]) {
       VaultDoor1 vaultDoor = new VaultDoor1();
       Scanner scanner = new Scanner(System.in);
       System.out.print("Enter vault password: ");
	String userInput = scanner.next();
	String input = userInput.substring("picoCTF{".length(),userInput.length()-1);
	if (vaultDoor.checkPassword(input)) {
	    System.out.println("Access granted.");
	} else {
	    System.out.println("Access denied!");
	}
   }

   // I came up with a more secure way to check the password without putting
   // the password itself in the source code. I think this is going to be
   // UNHACKABLE!! I hope Dr. Evil agrees...
   //
   // -Minion #8728
   public boolean checkPassword(String password) {
       return password.length() == 32 &&
              password.charAt(0)  == 'd' &&
              password.charAt(29) == '7' &&
              password.charAt(4)  == 'r' &&
              password.charAt(2)  == '5' &&
              password.charAt(23) == 'r' &&
              password.charAt(3)  == 'c' &&
              password.charAt(17) == '4' &&
              password.charAt(1)  == '3' &&
              password.charAt(7)  == 'b' &&
              password.charAt(10) == '_' &&
              password.charAt(5)  == '4' &&
              password.charAt(9)  == '3' &&
              password.charAt(11) == 't' &&
              password.charAt(15) == 'c' &&
              password.charAt(8)  == 'l' &&
              password.charAt(12) == 'H' &&
              password.charAt(20) == 'c' &&
              password.charAt(14) == '_' &&
              password.charAt(6)  == 'm' &&
              password.charAt(24) == '5' &&
              password.charAt(18) == 'r' &&
              password.charAt(13) == '3' &&
              password.charAt(19) == '4' &&
              password.charAt(21) == 'T' &&
              password.charAt(16) == 'H' &&
              password.charAt(27) == '1' &&
              password.charAt(30) == 'f' &&
              password.charAt(25) == '_' &&
              password.charAt(22) == '3' &&
              password.charAt(28) == 'e' &&
              password.charAt(26) == '5' &&
              password.charAt(31) == 'd';
   }
}

charAtは文字列からn番目の文字列を抜き出すメソッドです（参考）。多少面倒ですが、順番通りに並べ替えればフラグです。

答え：picoCTF{d35cr4mbl3_tH3_cH4r4cT3r5_8b748e}

picoCTF2019 write-upまとめへ