Sync Provider for Proton

Not for syncing. Proton doesn’t offer a CalDAV or CardDAV endpoint, so standard clients like Apple Calendar/Contacts, Thunderbird or DAVx5 can’t sync Proton Calendar or Proton Contacts over those protocols. Proton does let you share a calendar via a view-only link and export/import .ics files, and Proton Contacts can export/import vCard — but that’s read-only or a one-off snapshot, not live two-way sync. (As of June 2026.)

DAVx5 is a CalDAV/CardDAV client — it connects to a server that speaks those protocols. Proton doesn’t provide one, so there’s nothing for DAVx5 to point at, and Proton isn’t among DAVx5’s tested providers. Sync Provider takes a different route: it talks to Proton’s own API and writes contacts and events straight into Android’s system Contacts and Calendar. There’s a fuller explanation here.

Sync Provider writes your Proton contacts into Android’s system Contacts provider and your Proton events into Android’s Calendar provider — the same places Google and Samsung keep theirs. So your Proton contacts appear in Samsung Contacts, Google Contacts and your dialer, and your Proton calendar shows up in Google Calendar and other calendar apps, on your lock screen and on your watch.

Yes. It uses Android’s built-in sync framework rather than Google Play Services, so no Google account is required. On standard Android you can install from Google Play; on GrapheneOS or other de-Googled devices, install via Obtainium or the GitHub releases and activate a licence on the web.

No. Sync Provider syncs directly between Proton’s API and your phone. There’s no Nextcloud instance, no self-hosted bridge, and nothing to keep running.

Yes to both. Edits on your phone sync back to Proton and vice-versa, and all contact and calendar data is end-to-end encrypted with OpenPGP using your own Proton keys — your password never leaves your device (SRP). The Data & Encryption section above shows exactly what is stored and where.

Yes, and you don’t have to take our word for it. Any app you type a password into, including Proton’s own apps, holds it in memory for that moment, and that is unavoidable. The question that matters is whether the app sends or stores it anywhere it should not, and you can check that from outside the app. Login uses Proton’s SRP protocol, so your password is never transmitted, not even to Proton. A proof is computed on your device and only that proof is sent. Your password is used only on your device, to build that proof and to unlock your Proton keys, and it is never saved to disk. Two-factor codes go only to Proton, and your contacts and calendar are end-to-end encrypted with your own Proton keys before they leave your phone.

Your session tokens and key material are encrypted at rest on your device with a hardware-backed key. The app contacts only Proton’s servers, plus, on the GitHub and Obtainium build, a subscription check that receives only your subscription ID and no account data. There are no analytics, tracking or crash-reporting tools of any kind, and the one web page the app can load, Proton’s login verification, is locked to Proton’s own domains.

To check all of this yourself, the app uses no certificate pinning, so you can point a network proxy at it on a device you control and watch exactly what it sends, or scan the APK with a tool like Exodus Privacy for trackers. Sync for Proton is an independent app and is not affiliated with Proton. If you want to trust no third party at all, Proton’s official apps are the right choice. The source is not public, so the most reliable check is simply observing how the installed app behaves on your own device.