FYI, there’s a thread with more replies, including links to an initial decryption tool:
There is a trade-off between the convenience of cloud backups, the user protection of encryption, the protection to Nabu Casa of encryption, and the ability to take a backup and expand the individual .tar.gz parts for a whole raft of reasons.
Sadly, the “plausible deniability” protection of a hosting provider from user content (“no access, no subpoenas”) is going to beat the needs of a few technical users.
Thankfully, a few hardy community members have looked at GitHub and worked out the crypto…
I also don’t want to be the first to reply to a restoration community support question to say “No key; No backup; No buts” 
If this helps, 
this post!