This key must be set to enable the Active Record Encryption feature within Rails that Mastodon uses to encrypt and decrypt some database attributes. A new Active Record keys can be generated by running: nix build -f '<nixpkgs>' mastodon; cd result; RAILS_ENV=production ./bin/rails db:encryption:init If this file does not exist, it will be created with a new Active Record keys.

"/var/lib/mastodon/secrets/active-record-encryption-deterministic-key"