Cloud file storage services are aplenty and many of them offer multi-gigabytes of free storage without any hassles. But when you are storing sensitive data on the cloud, then it is not very easy to trust the third party service providers due to many reasons like hacking of the cloud accounts. To ensure that nobody else can access your files even if they can access the cloud storage accounts, you have to encrypt the files before storing them on the cloud. To make the encryption and decryption of files stored on the cloud automated, you can use the open-source software Cryptomator.
Cryptomator offers heavy encryption using the AES cipher and can encrypt the data automatically as you update it on the cloud. It works by creating a vault inside which you can store your files. The vault always stays encrypted and can be accessed only through the Cryptomator program.
The first and obvious step is to add a new vault. If you have any cloud syncing programs (Google drive, OneDrive, Dropbox etc) installed, then you can choose the vault location inside the synced folders so that the vault can be automatically synced.
After you have created a new vault, you can choose a name and drive letter through which it can be accessed in Windows. Once this optional step is taken care of, you can unlock the vault by supplying the password that was used during the vault creation. It opens up in Windows File Explorer where you can perform various file operations as usual. Once you are done working with the files, you can lock the vault again.
Conclusion: Cryptomator is basically a file encryption program that works by creating an encrypted vault and keeping all the encrypted files inside it. But for transferring the files to or from the cloud storage accounts you have to depend on the cloud service specific clients.
You can get the Cryptomator application from https://cryptomator.org/.