Description
The compiled KeygenMe.class contains the flag in reverse order. Decompile (or strings/grep) and reverse the characters to reveal it.
Use jd-gui (or another Java decompiler) to view KeygenMe.class and export it as KeygenMe.java.
Grep for `str.charAt` to extract the characters, then reverse the string.
cat KeygenMe.java | grep "str.char" | cut -d "'" -f2 | tr -d '\n' | rev
Solution
- Step 1Decompile or stringsjd-gui quickly shows the obfuscated code: the flag is built via repeated `str.charAt` calls, but in reverse order.
- Step 2Reverse the charactersExtract the characters (via grep/cut) and pipe them through `rev` to recover picoCTF{...}.
Flag
picoCTF{700l1ng_r3qu1r3d_2bf...}
The challenge name is a hint: you need fresh Java tooling to read the class file.