PicoCTF 2019 Writeups

vault-door-8 - Points 450

Apparently Dr. Evil's minions knew that our agency was making copies of their source code, because they intentionally sabotaged this source code in order to make it harder for our agents to analyze and crack into! The result is a quite mess, but I trust that my best special agent will find a way to solve it. The source code for this vault is here:

import java.util.*;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;
import java.security.*;

class VaultDoor8 {
  
  public static void main(String args[]) {
    Scanner b = new Scanner(System.in);
    System.out.print("Enter vault password: ");
    String c = b.next();
    String f = c.substring(8, c.length() - 1);
    VaultDoor8 a = new VaultDoor8();
    if (a.checkPassword(f)) {
      System.out.println("Access granted.");
    } else {
      System.out.println("Access denied!");
    }
  }
  public char[] scramble(String password) {
    /* Scramble a password by transposing pairs of bits. */
    char[] a = password.toCharArray();
    for (int b = 0; b < a.length; b++) {
      char c = a[b];
      c = switchBits(c, 1, 2);
      c = switchBits(c, 0, 3);
      c = switchBits(c, 5, 6);      
      c = switchBits(c, 4, 7);      
      c = switchBits(c, 0, 1);      
      c = switchBits(c, 3, 4);      
      c = switchBits(c, 2, 5);      
      c = switchBits(c, 6, 7);      
      a[b] = c;
    }
    return a;
  }
  
  public char switchBits(char c, int p1, int p2) {
    /* Move the bit in position p1 to position p2, and move the bit
      that was in position p2 to position p1. Precondition: p1 < p2 */
    
    char mask1 = (char)(1 << p1);
    char mask2 = (char)(1 << p2); /* char mask3 = (char)(1<<p1<<p2); mask1++; mask1--; */
    char bit1 = (char)(c & mask1);
    char bit2 = (char)(c & mask2);
      
    char rest = (char)(c & ~(mask1 | mask2));
    char shift = (char)(p2 - p1);
    char result = (char)((bit1 << shift) | (bit2 >> shift) | rest);
    return result;
  }
 
  public boolean checkPassword(String password) {
    char[] scrambled = scramble(password);
    char[] expected = {
      0xF4, 0xC0, 0x97, 0xF0, 0x77, 0x97, 0xC0, 0xE4,
      0xF0, 0x77, 0xA4, 0xD0, 0xC5, 0x77, 0xF4, 0x86,
      0xD0, 0xA5, 0x45, 0x96, 0x27, 0xB5, 0x77, 0xF1,
      0xC1, 0xF0, 0x94, 0xC1, 0xA5, 0xC1, 0xC2, 0xA4
    };
    
    char[] chr = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',
                      'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
                      '1','2','3','4','5','6','7','8','9','0','_'};
    
    char[] xChr = new char[3];
    xChr[0] = 0xC1; xChr[1] = 0XC2; xChr[2] = 0XA4;
    int flag = 1;
    for(int i=0;i<chr.length;i++) {
      for(int j=0;j<chr.length;j++) {
        for(int k=0;k<chr.length;k++) {
          StringBuffer sb = new StringBuffer();
          sb.append(chr[i]+""+chr[j]+""+chr[k]);
          char[] tempScramble = scramble(sb.toString());
          if(tempScramble[0] == xChr[0] && tempScramble[1] == xChr[1] && tempScramble[2] == xChr[2]) {
            System.out.print(tempScramble[2]+" "+tempScramble[1]+" "+tempScramble[0]);
            scrambled[scrambled.length - 1] = tempScramble[2];
            scrambled[scrambled.length - 2] = tempScramble[1];
            scrambled[scrambled.length - 3] = tempScramble[0];
            flag = 0;
            break;
          }
        }
        if(flag == 0) {
          break;              
        }
      }
    }
    return Arrays.equals(scrambled, expected);
  }
  
}

Solution

public class Vault8BruteForce {
    
    public char switchBits(char c, int p1, int p2) {
  char mask1 = (char)(1 << p1);
  char mask2 = (char)(1 << p2); 
  char bit1 = (char)(c & mask1);
  char bit2 = (char)(c & mask2);
          
  char rest = (char)(c & ~(mask1 | mask2));
  char shift = (char)(p2 - p1);
  char result = (char)((bit1 << shift) | (bit2 >> shift) | rest);
  return result;
    }
    
    public char[] unScramble(String password) {
  char[] a = password.toCharArray();
        for (int b = 0; b < a.length; b++) {
            char c = a[b];
            c = switchBits(c, 6, 7);
            c = switchBits(c, 2, 5);
            c = switchBits(c, 3, 4);                            
            c = switchBits(c, 0, 1);
            c = switchBits(c, 4, 7);
            c = switchBits(c, 5, 6);
            c = switchBits(c, 0, 3); 
            c = switchBits(c, 1, 2);
            a[b] = c;     
        }        
        return a;
    }
    
    public static void main(String[] args) {
        Vault8BruteForce x = new Vault8BruteForce();
        String hex = "F4C097F07797C0E4F077A4D0C577F486D0A5459627B577F1C1F094C1A5C1C2A4";
        StringBuilder sb = new StringBuilder();
        for( int i=0; i< hex.length()-1; i+=2 ) {
      //grab the hex in pairs
      String output = hex.substring(i, (i + 2));
        //convert hex to decimal
      int decimal = Integer.parseInt(output, 16);
        //convert the decimal to character
      sb.append((char)decimal);
    }
        char[] temp = x.unScramble(sb.toString());
    System.out.print("picoCTF{");
        System.out.print(temp);
        System.out.println("}");
    }
}

picoCTF{s0m3_m0r3_b1t_sh1fTiNg_743a4f48b}