Because the cloud storage providers encrypt the data with their own keys, that means they are also in a position to decrypt the data. Now, I’m not saying that the employees of these companies would do such a thing, but the fact that they could (for example, if forced to by the government) might be concerning to some of you. This is a further concern because when you log in to your account over the Internet to access your cloud storage, it is made available to you as the cloud storage provider decrypts the data so that you can access it. What if the person logged into your account is not you?
Another issue is that most of these cloud services are a mirror of the data stored on your device. This is a very powerful paradigm because when you create a new file or update an existing file on your computer or mobile device, that file is automatically synchronized with the cloud and then synchronized with your other devices. As mentioned above, the data is encrypted by the cloud storage provider on their servers, but not on your device. So, if your device is lost or stolen, the “bad guys” can get your data that way, unless you have encrypted the data that is on your device. And therein lies the solution of how to protect your cloud storage: encrypt the data before you store it in cloud storage and do not store the decryption keys with the same company that is providing the cloud storage.
Encrypting the data on your device under your control solves all of the security concerns discussed. And the good news is that a number of companies recognizing this issue have begun to deliver products that provide client side encryption integrated with cloud storage providers. I have personally been using one called Sookasa and am very happy with it. I wanted to scan all my customer files and destroy the paper files, but since the files contain private information such as passwords and other data, I was very concerned about securing the data under my control. I wanted a product that worked on my Mac, iPhone, iPad, and interfaced with Dropbox. Sookasa provides all of that, as well as a Windows version and support for other cloud storage vendors as well.
Check out Sookasa and let me know what you think. You can learn about it here: https://www.sookasa.com