Cloaker: Easy File Encryption With Windows, macOS And Linux Support

Cloaker is one of the easiest tools to encrypt and decrypt single files with cross-platform support (runs on Linux, Windows and macOS). 

The free and open source tool has a very basic Qt5 user interface on top of which you drag and drop a file you want to encrypt or decrypt, enter the password (with a minimum length of 10 characters), choose the location where to save the file, and you're done. What's more, Cloaker is portable / requires no installation.