Privacy Policy

UNDERTOW

Last updated 25 August 2026

Who we are

Undertow is made by Studio Cobalt, based in the Netherlands. If you want to reach a person about anything on this page, write to undertow@studiocobalt.nl.

What the app stores, and where

Undertow keeps a record of what you log: the dates of your periods, your cycle and period length, the tags you apply to days, any notes you write, and your settings. All of it is stored in a single file inside the app's own storage on your iPhone.

That file is written with complete file protection, which means iOS keeps it encrypted and unreadable while your phone is locked.

We have no access to it. Not encrypted, not anonymised, not aggregated, not in any form. There is no copy on any server we control, because we do not operate a server for this app.

What the app sends

Nothing. Undertow makes no network requests. There is no analytics, no crash reporting service, no advertising, no attribution, and no third-party software development kit of any kind inside the app.

Three things can move data off your phone, and you start all three of them:

• Export. Tapping Export writes a backup file and hands it to the iOS share sheet. Where it goes next is entirely your choice. We never see it.
• Your iPhone backup. If you back up your iPhone to iCloud or a computer, Undertow's file is included, the same as any other app's. That backup is between you and Apple, governed by Apple's terms, not ours.
• Writing to us. Tapping the support link opens your own mail app with a draft. The draft contains the app version and your iOS version, and nothing else. No cycle data, no logs, no identifiers. You can read and edit it before you send it.

If you write to us

When you email undertow@studiocobalt.nl, we receive your email address and whatever you choose to put in the message. We use it to answer you and nothing else. We do not add you to a mailing list, do not pass it to anyone, and do not use it for advertising.

Under the GDPR our lawful basis for handling that correspondence is legitimate interest: answering someone who has asked us a question. We delete support correspondence once the matter is closed and no longer useful, and at the latest after two years.

You have the right to ask what we hold about you, to have it corrected or deleted, and to complain to a supervisory authority. In the Netherlands that is the Autoriteit Persoonsgegevens. In practice, for almost everyone, what we hold is either nothing at all or one email thread.

Your purchase

You buy Undertow from Apple, not from us. Apple handles the payment and tells us nothing about you as an individual. We see anonymous sales totals by country, and that is all. Apple's handling of your purchase is covered by Apple's own privacy policy.

Children

Undertow is not directed at children and we do not knowingly hold information about anyone. Because the app collects nothing, a young person using it is in exactly the same position as anyone else: their data stays on their own phone.

Requests from authorities

If a court, a police force or any other authority asked us to hand over a user's cycle data, we would have nothing to give them. This is not a promise about how hard we would fight. It is a description of what exists. We built it this way on purpose.

Changes to this policy

If Undertow ever changes in a way that affects this page (a sync feature, an optional backup service, anything at all), we will update this policy and say so clearly in the app's release notes before the change ships. We will not quietly start collecting things.