BoxCryptor is a free tool that secures your data in the cloud – no matter if you use Dropbox, SkyDrive, Google Drive or any other cloud storage provider. It does so on OSX, iOS, Linunx, Android, Windows and even Windows RT.
BoxCryptor works by on the fly encrypting all data that you place in a dedicated folder resp. network mount before it’s synced to the cloud. It uses the AES-256 standard for encryption.