GENEVA, March 25 — Proton, the Geneva-based privacy company that operates Proton Mail, Proton Drive, Proton VPN, and Proton Pass, on Wednesday rolled out a substantially redesigned Proton Calendar across its entire paid customer base, with end-to-end encrypted event metadata and a new shared-calendar mode that the company said preserves the encryption boundary across multiple participants.
The launch is Proton’s most significant update to its calendar product since the original Proton Calendar 1.0 release in 2019 and represents what cryptography researchers said is one of the more technically ambitious shared-calendar implementations in a consumer-grade encrypted suite.
“The technical challenge with end-to-end encrypted shared calendars has historically been that the moment you add a second participant, the simple model — encrypt to the user’s key — breaks down,” said Dr. Soren Kalitsounakis-Bauer, a cryptographer who reviewed the new design as part of an independent audit by the Berlin-based security firm Cure53. “The Proton design uses a per-calendar key share that is encrypted to each participant’s public key. It is not novel as a primitive, but executing it with the latency and the synchronization properties needed for a calendar product is genuinely difficult engineering.”
What was added
The redesign covers four areas. First, all event metadata — title, description, location, attendee list, and recurrence rules — is end-to-end encrypted at rest in Proton’s data centers, a change from the previous implementation in which only the event title and description were encrypted. Second, the new shared-calendar mode allows up to 100 participants to view, edit, and add to a shared calendar without exposing event data to Proton’s servers in plaintext. Third, the calendar now supports inbound CalDAV synchronization, allowing users to subscribe to external calendars (a Google Calendar feed, for example) without exposing the Proton calendar to the external service. Fourth, mobile clients have been substantially redesigned, with what Proton said is “feature parity” between the iOS, Android, and web versions of the product for the first time.
“We have been told for several years that the calendar was the weak point in the suite,” said an Proton spokesperson. “This release is the answer to that. The encryption model now matches what Mail and Drive have always provided, and the shared-calendar mode is something we know enterprise customers in particular have been waiting for.”
Pricing and availability
Proton Calendar is included in all paid Proton plans, beginning with Mail Plus at $4.99 per month. The free Proton Mail tier includes Calendar with a single calendar and a 100-event-per-month soft cap. Enterprise pricing for Proton Business plans, which include the new shared-calendar mode, was unchanged.
Industry context
The launch comes against a backdrop of intensifying competition for encrypted productivity-suite alternatives to the dominant Google Workspace and Microsoft 365 offerings. Tutanota, the Hannover-based encrypted-mail provider, shipped its own redesigned encrypted calendar in 2025. Skiff Calendar, which had been the most-cited encrypted-calendar alternative in 2023 and 2024, was discontinued after Notion’s 2024 acquisition of Skiff and the subsequent shutdown of the Skiff product line.
Cure53 said its full audit report on the Proton Calendar redesign will be published in mid-April.
Reginald Rosenberg-Vance is editor in chief of Consumer Tech Wire.