I got licensee version 34 (encrypted payloads too) to swap finally!
Okay, so this was awesome. Rocket League’s been having a bunch of updates lately, which breaks the packages of people trying to get free stuff. So, in a Reddit thread, I ended up dishing out a bunch of swapped asset files for people in this situation. There’s a new boost which everybody wanted really bad since it was really rare (but they decided to make it unlockable at rocket pass lv99 so it doesn’t even matter anymore), and it was UNSWAPPABLE for three reasons:

1. it was encrypted specially
2. i hadnt encountered specially encrypted packages meaning their handling was wrong
3. aes keys werent out for it yet (or so I thought)

I’ll start with 3. I had written a tool which looks in the games cache for AES keys, and dumped them, and it worked great. What I didn’t expect is the filename to change between versions. So, I was super confused since yesterday since the AES key for the boost was just… missing. Thankfully, the geniuses at the RLMM discord got it and helped me figure out that the file I was reading from was just outdated. Great!
Anyways, not too big of a deal, just a big delay. Now it’s time for the real challenge: figuring out how the hell to handle this type of package.
First, I did some research. The twelve bytes of padding in files with licensee version >=33 before the summary are actually a nonce/aes iv, not just padding. Then, the header is encrypted with AES-CTR with the twelve first bytes of the iv just being the nonce. And the compressed chunks are encrypted too here: each one is also AES-CTRed with their own nonces in FCompressedChunkInfo.
Thing is, I don’t have an implementation to really go off anymore, meaning there’s a lot more trial and error. For the original implementation, there were one or two existing asset swappers (though they were vibecoded meaning more of a puzzle to figure out what they’re even doing), but this time I was mostly relying on emii’s fork of UEViewer. Anyways, after spending a while figuring out what exactly I had to masquerade (payload, key, swap nonces) I finally got it to work!