Budget Zen is a simple and encrypted expense management app. You can learn more about it here, as this is the app.

Currently it's available on every device via web browser, and you can browse its source code.

You have a 30-day free trial (no credit card required), and at the end, you can pay €18 / year, or €2 / month, no limits.

Note that logging in will take up a few seconds. This is intentional, in order to generate a safer assymetric encryption key. After logging in, the app should be blazing fast in any device.