Batwa

The problem

Every couple of days I ran the same errand on my own phone. Open JazzCash, look at the balance. Open Easypaisa, look. Open the bank app, look. Add the three in my head, then take off the rent that hadn't left the account yet, the unpaid electricity bill, the internet due next week. Whatever was left was what I could spend.

That number was always wrong. Bills already promised still counted as money, so the middle of the month felt comfortable and the end of it did not.

The apps that fix this want an account and a link to your bank, and most have never heard of Easypaisa. Handing over a full picture of a salary so an app can tell you that you spend too much on food is a poor trade, and it is the only one on offer.

Approach

Batwa is Urdu for wallet. It leads with one figure, free to spend, which is total balance minus everything already committed. A bill you have entered but not paid is not money, so it comes off on the day you log it, not the day it leaves. The arithmetic is a post of its own, including the double-count I had to take out of it.

There is no account, because the PIN is the key. Four digits go through PBKDF2 at 150k iterations into an AES-256-GCM key, and the ledger sits in IndexedDB as ciphertext. Nothing is stored that could rebuild the PIN. That cost is real. Forget it and the data is gone, which is why Settings has an export and the home screen keeps asking you to take one.

A claim like that is worth nothing if you have to take my word for it, so every line that touches the data is in the open at github.com/zubairbinshaukat/batwa. Install it from the browser at batwa.zubyr.dev.

Fingerprint unlock had to not weaken that. It uses the WebAuthn PRF extension: the authenticator hands back a secret that unwraps the PIN-derived key, and where there is no PRF, enrolment is refused rather than downgraded. A fingerprint that only gates a screen is worse than none, because it looks like protection.

Splitting money with family is where the thinking went. Shared spaces run through a relay I wrote on Cloudflare Workers, one Durable Object per space, holding an opaque ciphertext string under a random 22-character id and a hashed write token. It never learns a name, an amount, or who is in a space.

A relay that understands nothing cannot resolve conflicts either, so the merge has to be right on the phones. Last-writer-wins on the whole document throws away an edit every time two people are offline at once. So every piece of the blob carries its own revision: the space fields, each entry's shared fields, and each member's own row, which only that member ever writes. merge(a, b) and merge(b, a) come out byte-identical, so three phones converge in whatever order they sync.

No build step anywhere. Vanilla ES modules, plain CSS, zero runtime dependencies, so on a mobile-shaped product what I deploy is what I wrote.

Outcome

It is what I use. The three-app ritual is gone, and I trust the number on the home screen because the bills come off it before I see it.

Usage is me and the people I split expenses with, so there are no adoption figures here and I am not going to invent any. What I will say is that the privacy claim holds up when you go looking, and you can go looking. The relay has twenty tests, and none of them can make it hand back anything it could read.