Merge pull request #108 from julianhal/random

fessemisk add  random chall

writeups/reversing/Random Flag Generator/FesseMisk/README.md

@@ -0,0 +1,79 @@
+# Random Flag Generator
+
+## Description
+This challenge is intened to be an introduction to java.
+
+`java -jar` to run the program, see if you can find a flag in there somewhere.
+
+
+## Solution
+I use [JADX](https://github.com/skylot/jadx) to decompile the java code to see what it does. The new ctf meta is of course to use chatgpt to solve the challenge.
+
+**ME>**  
+How can i reverse the flag?
+```java
+import java.util.Base64;
+import java.util.Random;
+import java.util.Base64.Decoder;
+import java.util.Base64.Encoder;
+
+public class RandomFlagGenerator {
+   public static void main(String[] var0) {
+      String var1 = "ATU6Wy9BFBMqCCsXMVQNRT0lRABlGA==";
+      String var2 = "Den e brun";
+      String var3 = xorString(base64Decode(var1), var2);
+      String var4 = generateRandomString(20, var3);
+      System.out.println("Random Flag: " + var4);
+   }
+
+   public static String generateRandomString(int var0, String var1) {
+      String var2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz" + var1;
+      Random var3 = new Random();
+      StringBuilder var4 = new StringBuilder(var0);
+
+      for(int var5 = 0; var5 < var0; ++var5) {
+         var4.append(var2.charAt(var3.nextInt(var2.length())));
+      }
+
+      return var4.toString();
+   }
+
+   public static String xorString(String var0, String var1) {
+      StringBuilder var2 = new StringBuilder();
+
+      for(int var3 = 0; var3 < var0.length(); ++var3) {
+         var2.append((char)(var0.charAt(var3) ^ var1.charAt(var3 % var1.length())));
+      }
+
+      return var2.toString();
+   }
+
+   public static String base64Encode(String var0) {
+      Encoder var1 = Base64.getEncoder();
+      return var1.encodeToString(var0.getBytes());
+   }
+
+   public static String base64Decode(String var0) {
+      Decoder var1 = Base64.getDecoder();
+      return new String(var1.decode(var0));
+   }
+}
+```
+
+**CHATGPT>**  
+Here’s a Python script that performs the equivalent operations to reveal the flag:  
+```python
+import base64
+
+# Step 1: Decode the Base64 string
+encoded_string = "ATU6Wy9BFBMqCCsXMVQNRT0lRABlGA=="
+decoded_string = base64.b64decode(encoded_string).decode('utf-8')
+
+# Step 2: XOR the decoded string with "Den e brun" to retrieve the flag
+xor_key = "Den e brun"
+flag = ''.join(chr(ord(decoded_string[i]) ^ ord(xor_key[i % len(xor_key)])) for i in range(len(decoded_string)))
+
+print("Original Flag:", flag)
+```
+Which when run gives the flag:  
+`Original Flag: EPT{Java_for_the_W1n!}`