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Is I cal share safe to use?
These passages from two support documents are worth reading if you have concerns about the security of your calendar data. Both pertain to having Calendar app data in a secure setting.
Cloud secures the user’s information by encrypting it when it's in transit, storing it in an encrypted format, and securing the user's encryption keys in Apple data centres. When processing data stored in a third-party data centre, encryption keys are accessed only by Apple software in use on secure servers, and only while conducting the necessary processing. For additional privacy and security, many Apple services use end-to-end encryption, which means that only the user can access their information, and only on trusted devices where the user is signed in with their Apple ID.
Keep your calendars up to date and share them with iCloud
Share and collaborate on calendars:
You can share calendars with friends and family who use iCloud, and they can view shared calendars in the Calendar app. You decide if the people you share with can edit the calendar or just view it. When collaborators make changes to a calendar, everyone sees those changes in real time.