FEN user guide
FEN keeps track of shared expenses for a group of friends — a trip, a flat, a weekend — and works out who owes whom at the end.
What makes it different from most expense-splitting apps is where the data lives: there is no FEN server. Your expenses are stored on your phone and, if you want to share them, in a storage account you provide. Everything that leaves your device is encrypted, and only the people in your group can read it.
The guide has one page per area of the app:
- Groups — creating groups, adding expenses, balances, settling up, closing.
- Friends — invites, your friends list, adding people to groups quickly.
- Settings — your profile, storage backend, additional devices, local backup.
- Privacy policy — what FEN collects (and doesn't).
How FEN works
Your account is a key on your device
When you first open FEN you type a name and the app creates a cryptographic key for you. That key is your account — there is no email address, no password and no sign-in. Other people see the name you chose; the key is what actually identifies you.
This has one important consequence: nobody can reset your account for you. If you lose the device and have no backup, you lose the key, and the groups it could read. See Local store and backup for how to keep a copy.
A group is a shared, encrypted history
A group isn't a row in a database somewhere. It is a log of everything that happened — expenses added, edited, deleted, settlements recorded, members joining and leaving. Every entry is signed by whoever made it and encrypted with a key only group members hold.
Your phone replays that log to show you the current balances. Because every device replays the same log, everyone lands on the same numbers without a server deciding anything.
Your storage backend carries the log
To share a group with other people, its encrypted log has to sit somewhere both of you can reach. FEN doesn't provide that — you point it at a Nextcloud instance you control.
Whoever creates the group provides the storage for it. The operator of that storage only ever sees encrypted files. You can use FEN without a backend at all: you can join other people's groups and add expenses, and you can keep purely local groups on your own device, but you can't create a shared group of your own until you configure one.
Sync happens when you ask
FEN has no push notifications and no background server. It syncs when you open the app, and whenever you pull down on a screen to refresh. So changes made by other members appear the next time your device syncs — not instantly.
That's a deliberate trade (no server means no server watching), but it's worth knowing when you're standing at a restaurant table waiting for someone else's expense to appear. Pull down to refresh.
Setting up the first time
The first launch walks you through three steps.
- Your name. This is the name other members see. It also creates your account key. If you're moving to a replacement device, use Restore from backup file or Restore from iCloud Keychain here instead.
- Local save folder. Optional, but strongly recommended: pick a folder on the device where FEN can keep an offline copy of closed groups and a backup of your keys. You can set this later in Settings › Profile.
- Storage backend. Enter your Nextcloud details, or tap Skip for now. Without one you can still join groups and add expenses; you'll need it before you can create a group.
Nothing here is permanent — all three can be changed later in Settings.