GoKawiilCryptomator offers open-source client-side encryption for cloud storage files
Cryptomator is an open-source tool that creates encrypted vaults on a user's device, encrypting individual files and their filenames with AES-256 before they sync to services such as Dropbox, Google Drive, OneDrive, pCloud, Nextcloud and ownCloud. It also works offline for local drives, USB storage and network folders, and has apps for Android and iOS. Users can create a recovery key to reset a lost vault password, since Cryptomator itself cannot decrypt files or recover passwords.