Cryptomator

Cryptomator encrypts file contents and names using AES. Your passphrase is protected against bruteforcing attempts using scrypt. Directory structures get obfuscated. The only thing which cannot be encrypted without breaking your cloud synchronization is the modification date of your files.

The program creates an AES-encrypted file in your Dropbox folder, Google Drive, or whatever other location you like. Just specify the file name, location and a passphrase, and you're done.

Once the vault is set up, Cryptomator mounts it as a virtual drive on your computer.

Save any documents to that drive, and they're automatically encrypted, then uploaded to the cloud (or to wherever your storage folder is located).

When you're finished, lock the vault, the virtual drive disappears, and no-one will even know it's there.

Cryptomator is a free and open source software licensed under the GPLv3. Cryptomator’s software works on Windows, Mac, and Linux, in both 32 and 64-bit flavors. A Java version is also available, for cross-platform compatibility.

Cryptomator Tutorial: Get Started (YouTube Video)

Some people have expressed a concern that a masterkey file is stored in the Cryptomator vault. This is supposed to be there. - "Inside the storage location of a Cryptomator vault, you will find a file called masterkey.cryptomator. This file is stored in the cloud to allow convenient access to a vault on different devices. This file contains encrypted data, which is needed to derive the masterkey from your password. The file does not contain the decrypted masterkey itself. In addition, some metadata about the vault (e.g., the version of Cryptomator used to create it) is also stored in this file. The encrypted key in masterkey.cryptomator is not more sensitive than the encrypted files themselves."