Yes you need the official (or modded) FPKG tool set, specifically pub-chk tool.
If you have a 5.05 or below FPKG the pass should be 32x 0s, but sometimes they aren’t, either way (as mentioned) install it and dump it to get the contents.
if you have a legit pkg you need the legit password from the developers, or the default Unreal, Unity or GameMaker passwords will work on most (passwords wont be shared here).
If you open a legit pkg with a legit password the binary contents will always be encrypted. Just remember that.