Boxcryptor secures your files with an encryption program made in Germany. The tool comes with a free level and an upgraded level that is available per user per month. Only you can access your files when you have Boxcryptor and it supports nearly every cloud provider.
Boxcryptor is an incredible free option to protect your cloud data with practically no downside. If your concern is cloud security then Boxcryptor is the best option.
You can access all your data free of charge by using BoxCryptor and allowing someone to check on it regardless of where you were.