The Quick Read
Your calendar knows where you sleep, who your doctor is, where your kids go to school, and when your house is empty. Two of the four major calendar apps can read every event you create. The other two cannot.
Proton Calendar and Tuta Calendar both publish end-to-end encryption claims for event details, including titles, locations, descriptions, and attendees. [1] [2] Apple explicitly excludes iCloud Calendar from Advanced Data Protection because of the CalDAV standard. [3] Google Calendar is encrypted in transit and at rest, but Google retains access keys and processes calendar content. [4]
The trade-off is real. The two end-to-end encrypted options are smaller ecosystems with fewer integrations. Google and Apple give you calendar on every device, sharing with everyone, and rich features. They also can read what you put in.
Why Your Calendar Is One of the Most Sensitive Files You Own
A calendar is a complete map of your life. It usually contains:
- Home and work addresses (recurring events labeled with locations)
- Daily routines (gym at 6am, school pickup at 3pm)
- Travel dates (when your house is empty)
- Medical appointments (oncology, psychiatry, fertility, addiction treatment)
- Relationship details (lunch with a lawyer, message from a divorce attorney)
- Political and religious events (planned meetings, services)
- Financial activity (meetings with accountants, brokers, attorneys)
Apple's own iCloud documentation lists 25 categories of data that are end-to-end encrypted with Advanced Data Protection enabled. Calendar is not one of them. [3] Apple's reasoning is that calendar data uses the CalDAV standard, and "industry standards (CalDAV and CardDAV) that do not provide built-in support for end-to-end encryption." [3]
Google's privacy notice for Calendar explicitly lists what it processes: calendar content for "spam filtering, virus detection, malware protection and the ability to search for calendar entries," performance data, name and email, contacts, location, and search queries. [4]
How Calendar Encryption Actually Works
"Encrypted" is doing a lot of work in marketing copy. Three different models matter:
In transit: Your data is encrypted while it travels over the network. Anyone watching your Wi-Fi cannot read it. The provider can.
At rest: Your data is encrypted on the provider's servers. A thief who steals the hard drive cannot read it. The provider can, because the provider holds the decryption key.
End-to-end encrypted (E2EE): Your data is encrypted on your device before it leaves it. The provider never holds the key. A subpoena to the provider produces nothing useful, because the provider cannot read your data.
Apple and Google Calendar are encrypted in transit and at rest. They are not end-to-end encrypted. Proton Calendar and Tuta Calendar claim end-to-end encryption for the actual event contents.
The Four Calendar Apps Compared
Google Calendar
Google Calendar is the default calendar on Android and on Chrome browsers. It is also the calendar that backs most calendar integrations in third-party apps.
Google's own privacy help page for Calendar states the data is "encrypted in-transit and at-rest" and that entries are also stored locally on the device. [4] The page does not mention end-to-end encryption.
What Google processes is itemized in the same documentation: calendar content, performance data, name and email, contacts, location, and search queries. [4] Google says it does not use Calendar data "for advertising purposes, period," because Calendar primarily stores personal content. [4]
What Google will hand over: Google's general access policy for personal content is "Google respects your privacy. We access your private content only when we have your permission or are required to by law." [4] For work and school accounts, Google states plainly that "your administrator may be able to view your calendar." [4]
What a Google Calendar subpoena would reveal: the content of every event, the location of every event, every attendee on every invite, search history within Calendar, and who you share calendars with.
Apple Calendar (iCloud)
Apple Calendar is the default calendar on macOS and iOS. With an iCloud account, it syncs across every Apple device and shares with anyone via CalDAV.
Apple's Advanced Data Protection (ADP) is the highest level of iCloud encryption Apple offers. With ADP enabled, Apple states the number of end-to-end encrypted categories rises to 25. [3] Calendar is not in that list. Apple explicitly leaves Calendar, Contacts, and Reminders out of end-to-end encryption. [3]
Apple's published reason is the CalDAV standard. Apple's documentation states: "Contacts uses 'industry standards (CalDAV and CardDAV) that do not provide built-in support for end-to-end encryption.'" [3] Calendars use the same CalDAV standard, with the same limitation. [3]
Apple's standard data protection (without ADP) uses Apple-held keys. Apple states that with ADP enabled, "No one else can access your end-to-end encrypted data, not even Apple," but Calendar is not in the protected category either way. [3]
What an Apple Calendar subpoena would reveal: with or without ADP, the content of every event, the location of every event, every attendee, and every invite. Apple uses the same CalDAV-based infrastructure regardless of the ADP setting.
Proton Calendar
Proton Calendar launched as a Proton desktop beta in 2020, with Android in April 2022 and iPhone in November 2022. Proton is based in Switzerland.
Proton's product page states: "Proton Calendar automatically secures all your events with end-to-end encryption." [5] Proton also states: "Events received from people who use other calendars are secured with zero-knowledge encryption." [5]
Proton's blog post on encryption specifies which fields are protected: "All of an event's critical information, including the title, description, location, and attendees, are stored securely and privately with end-to-end encryption." [1]
What Proton explicitly does not do: "Proton does not monitor your activity or serve ads." [5] On jurisdiction: "Based in Switzerland: We're legally required to protect your data and can never share your information with foreign law enforcement without a valid Swiss court order." [5]
What Proton can be compelled to provide: account metadata (your account email, login timestamps, billing information) and IP addresses from login events. Proton cannot provide event titles, locations, descriptions, or attendees, because Proton does not hold the keys.
Tuta Calendar
Tuta (formerly Tutanota) launched Tuta Calendar on September 18, 2024. Tuta is based in Germany.
Tuta describes Tuta Calendar as "the only zero-knowledge calendar with quantum-safe encryption." [2] Tuta's documentation states: "Whether you schedule events, receive event invitations or share your calendar with others, all steps in the Tuta Calendar are automatically encrypted end-to-end." [2]
Tuta goes further on reminders than Proton. Tuta states: "These reminders are kept hidden from your phone's operating system so that not even Google or Apple can see your events." [2] Tuta also states that "the time when a notification is sent to the user is obscured from our servers so that we remain in the dark about all our users' schedule appointments." [6]
Tuta's security page is explicit that "all data is encrypted, but also the reminders are E2E encrypted" and that "our calendar is an outstanding achievement." [6] Tuta uses ECDH (x25519) plus Kyber-1024 for post-quantum protection on top of its symmetric encryption. [6]
What Tuta can be compelled to provide: the same kind of metadata Proton can, mainly account email and login timestamps. Tuta's encryption model means it cannot read event titles, locations, attendees, descriptions, or even the time an event starts.
What Stays Visible Even With End-to-End Encryption
End-to-end encryption for calendars is not absolute. Even Proton and Tuta need to know when to send a reminder, so some timing data has to leak to the server in some form.
Proton's documentation admits this: event timing metadata is needed to deliver calendar functionality. Proton's marketing claims about end-to-end encryption cover event titles, descriptions, locations, and attendees, but the practical implementation requires the server to know when an event starts so it can send the reminder. [1]
Tuta's response is to also encrypt reminder timing. Tuta states: "Even the time when a notification is sent to the user is obscured from our servers." [6] This is a more aggressive design, and it is the strongest claim of any major calendar provider.
CalDAV standards impose real limits. CalDAV is the industry standard calendar protocol, and it does not support end-to-end encryption. That is why Apple's iCloud Calendar and most corporate calendar systems (Outlook, Google Workspace) all use the same plaintext-on-the-server model. The only way to get end-to-end encryption is to use a calendar that does not speak CalDAV to the server, which is what Proton and Tuta do.
Sharing and Cross-Platform Use
Google Calendar
Share with anyone using a Gmail or Google Workspace account. Also share by email address to anyone with an internet calendar (CalDAV or ICS subscription). Free with any Google account.
Apple Calendar
Share with anyone using an iCloud account, and read-only ICS subscriptions to anyone with an internet calendar. Free with any Apple ID. Sync is built into every Mac, iPhone, and iPad.
Proton Calendar
Share with other Proton users through Proton's shared encrypted calendars feature. Import external calendars (Google, iCloud, Outlook) into Proton and Proton will encrypt them. Proton Calendar does not let you publish a publicly shareable read-only link to non-Proton users. Requires a paid Proton plan for full use; limited availability on the free tier.
Tuta Calendar
Share with other Tuta users. Import external calendars via ICS import. Calendar sharing to non-Tuta users is restricted to Tuta accounts. Free tier is available.
The Comparison At a Glance
Google Calendar
- Encryption: In transit and at rest [4]
- End-to-end: No
- Provider can read events: Yes
- Free tier: Yes, full features
- Cross-platform: Web, Android, iOS
- Sharing: Anyone with an email or CalDAV link
Best for: People whose calendar needs to interoperate with Google ecosystem, schools, and corporate accounts.
Apple Calendar (iCloud)
- Encryption: In transit and at rest [3]
- End-to-end: No, even with Advanced Data Protection [3]
- Provider can read events: Yes
- Free tier: Yes
- Cross-platform: macOS, iOS, web (limited), Windows via Outlook add-in
- Sharing: iCloud users, ICS subscriptions
Best for: People in the Apple ecosystem who do not want to switch calendars and accept the CalDAV limit.
Proton Calendar
- Encryption: End-to-end for event contents [1]
- End-to-end: Yes
- Provider can read events: No
- Free tier: Limited
- Cross-platform: Web, Android, iOS
- Sharing: Other Proton users, imported external calendars
Best for: People who want a calendar Google and Apple cannot read, and who mostly share with other Proton users.
Tuta Calendar
- Encryption: End-to-end, including reminder timing [2] [6]
- End-to-end: Yes
- Provider can read events: No
- Free tier: Yes
- Cross-platform: Web, Android, iOS
- Sharing: Other Tuta users
Best for: People who want the strongest end-to-end encryption, including the timing of reminders, and who mostly share with other Tuta users.
What a Calendar Provider Hands Over to Law Enforcement
This is where the encryption model matters in real cases.
Google publishes a transparency report. A valid legal order (subpoena, court order, search warrant) can compel Google to disclose calendar content. Google's stated policy: "We access your private content only when we have your permission or are required to by law." [4]
Apple publishes a transparency report. A valid legal order can compel Apple to disclose iCloud data. With Advanced Data Protection enabled, Apple states it cannot access 25 categories of data. Calendar is not one of those categories, so a legal order to Apple will produce your calendar content. [3]
Proton publishes a transparency report. Proton states that under Swiss law it can be compelled to disclose metadata (account email, IP addresses from login events, billing information) and IP addresses tied to specific accounts. Proton cannot be compelled to disclose event titles, locations, attendees, or descriptions, because Proton does not hold the keys. [5]
Tuta publishes a transparency report. Tuta operates under German law and is also subject to European law-enforcement instruments. Tuta's design means it cannot disclose calendar contents even with a valid order.
How to Switch Calendar Apps Without Losing Anything
The hard part of switching is not the new app. It is migrating years of events and ongoing shared calendars.
Step 1: Export your existing calendar. Both Google Calendar and Apple Calendar can export as ICS. In Google Calendar, open the gear icon, then Settings, then "Import and export," then click Export. Google downloads a ZIP of ICS files for every calendar on the account. In Apple Calendar on Mac, select the calendar in the sidebar, then File, then Export, then Export.
Step 2: Import into Proton or Tuta. Both services accept ICS imports. Proton Calendar imports external calendars and encrypts them. Tuta Calendar accepts ICS files through its import function.
Step 3: Keep your existing calendar running in parallel. Do not delete your Google or Apple account the day you switch. Run both for at least a month, until you are sure every event and every shared calendar has migrated correctly.
Step 4: Migrate shared calendars one at a time. Ask each person you share a calendar with to move to Proton or Tuta. For calendar sharing to work end-to-end, both people have to be on the same encrypted provider.
Step 5: Delete the old calendar when you are sure. Only after you have verified every event and every shared calendar in the new app should you remove events from the old provider's servers.
What You Should Do Right Now
- If your calendar has medical appointments, legal meetings, or anything you would not want a stranger to read, switch to Proton Calendar or Tuta Calendar. Both publish end-to-end encryption claims for event contents.
- If you stay with Google or Apple, accept that Google or Apple can read your calendar if compelled, and minimize what you put there. Use plain language or codes for sensitive ones (use a personal abbreviation for a doctor, lawyer, or treatment).
- Turn off Location Services for Calendar on iOS and Android. Both operating systems let you revoke location access for the calendar app specifically.
- On Google Calendar, open Settings, then "Events from Gmail" and turn it off. Gmail-to-Calendar auto-event creation extracts booking details from your email and writes them to your calendar without explicit confirmation.
- On iPhone, open Settings, then Calendar, then confirm "Show Declined Events" is off. Otherwise your response to a declined invite exposes when you are unavailable.
- If you use Apple Calendar and care about encryption, do not rely on Advanced Data Protection to cover it. Apple does not include Calendar in ADP.
References
- Proton Blog: Is Google Calendar private? (encryption claims)
- Tuta: Encrypted Calendar (zero-knowledge and quantum-safe claims)
- Apple Support: iCloud data security and privacy overview (Advanced Data Protection categories)
- Google Calendar Help: Understand the basics of privacy in Google Calendar
- Proton: Proton Calendar product page
- Tuta: Security and encryption documentation