How safe is iCloud?

“How safe is iCloud? I have a feeling that privacy is a thing of the past. How do I know that nobody’s looking at my stuff?”

Apple's iCloud service lets users sync a staggering amount of data between Macs, Windows PCs, iPhones, and iPads. Though Apple says it stores this data securely in an encrypted format, just how safe is it? 

This question is bigger than just iCloud. How safe is any online service, to be honest? 
Ultimately, with any service provider, you simply don’t know that your data isn’t being looked at unless you take steps to protect it. 

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