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 August 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.

Because caller ID is a lookup in Android’s own contacts database, and Proton’s apps never write your contacts there. The dialer has nothing to match the incoming number against, so you see a bare number. No dialer setting changes that. Once Sync Provider has written your Proton contacts into Android, the dialer shows the name and the photo like any other contact. See where it shows up above, or the full guide: Proton Contacts on Android.

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.

Android keeps a separate copy of a contact for each account, then displays them merged as a single card. That is what “view linked contacts” reveals. Sync Provider can only read and write the copy that belongs to the Proton account, because Android does not let a sync app modify records owned by a different account. So an edit made to the phone or Google copy never reaches Proton, and a forced full re-sync correctly finds nothing to send. Nothing is lost, and contacts in your other accounts are left untouched.

Most contact apps can show you which account a contact actually lives in. This is the account filter in Samsung Contacts; click it to see it full size.

The counts tell the story: 418 contacts live in Proton Contacts, 335 in Google and one on the SIM, yet “All contacts” shows 427, because Android has merged the overlapping copies into single cards.

To have your edits sync, the contacts need to live in the Proton account rather than beside it. Open a contact and look for “Move to another account” or “Change account”, and choose Proton Contacts. Many phones also offer a bulk move under Contacts settings. Alternatively, export the contacts to a .vcf file and import it choosing Proton Contacts as the destination, then delete the originals so you do not keep duplicates. It is also worth setting Proton Contacts as the default account for new contacts, so anything you add later syncs without thinking about it.

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 your contacts and calendar are encrypted with OpenPGP using your own Proton keys, in the same format Proton’s own apps use: most fields end-to-end, while a few, such as contact names and event times, stay readable to Proton, as they do in Proton’s apps. 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 most of your contact and calendar data is end-to-end encrypted with your own Proton keys before it leaves your phone.

Your session tokens and key material are encrypted at rest on your device with a hardware-backed key. The app talks to Proton’s servers, plus, on the versions from outside Google Play (GitHub/Obtainium, Galaxy Store, Uptodown), a subscription check that receives your subscription ID and no account data. The one web page the app can load, Proton’s login verification, is limited to Proton’s domains and the hCaptcha challenge Proton uses. Up to version 1.11.4 the app also downloaded time-zone definitions from tzurl.org; from 1.11.5 it does so only if you switch on “Update time zones online”. The app has no analytics, tracking or crash-reporting tools of its own; the Google Play version contains Google’s billing library, which can send Google technical logs about purchases.

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.