Syncing and security
How to make sure you only share the things you want to
Because iCloud synchronizes data between devices and Apple’s servers, it’s moving lots of your personal documents and data, so it’s important to protect that data from being intercepted by the wrong people.
In order to do that, Apple uses extensive encryption provided you have two–factor authentication enabled (see “Protect iCloud privacy with 2FA”). Encryption encodes your data in a format that is essentially impossible for others to intercept and decode, and Apple uses encryption on all your data transfers and almost all the data stored on its servers too. The only thing it doesn’t encrypt by default is your iCloud Mail, but if you want to encrypt that too you can enable S/MIME encryption in your mail apps.