To solve the challenges, use gpg to decrypt the .zip using the passphrase given in club. Then, unzip challenges.zip and try out challenge1 and then challenge 2. I've included a compiled version of the fernflower decompiler as fernflower.jar. You can run this tool by using java -jar fernflower.jar [jar to decompile here] [temporary folder for the new jar]. This will place the decompiled jar inside the new temporary folder. You will need to unzip the .jar to access the decompiled source files (a .jar is really just a .zip in disguise).