Rating:
# UMDCTF Starbucks Write Up
## Details:
Jeopardy style CTF
Category: Reverse Engineering
Comments:
Unfortunately, you have been forced to use Java, but you are only given a single class file which doesn't seem to work.
## Write up:
I decompiled the .class file and got:
```java
public class Challenge {
public static String f1(String s) {
StringBuilder b = new StringBuilder();
char[] arr = s.toCharArray();
for(int i = 0; i < arr.length; ++i) {
b.append((char)(arr[i] + i));
}
return b.toString();
}
public static String f1_rev(String s) {
StringBuilder b = new StringBuilder();
char[] arr = s.toCharArray();
for(int i = 0; i < arr.length; ++i) {
b.append((char)(arr[i] - i));
}
return b.toString();
}
public static String f2(String s) {
int half = s.length() / 2;
return s.substring(half + 1) + s.substring(0, half + 1);
}
public static String f3() {
return f1(f2("$aQ\"cNP `_\u001d[eULB@PA'thpj]"));
}
public static void main(String[] args) {
System.out.println("You really thought finding the flag would be so easy?");
}
}
```
I then created a java file and simply changed the call to f3() in main. Compiled and ran and got:
```
UMDCTF-{pyth0n_1s_b3tt3r}
```
