Eureka is a simple tool to encrypt and decrypt files.
If you have Go installed and
/usr/local/go/bin is in your PATH, you should be able to simply get the binary by doing
go get github.com/mimoo/eureka
./eureka -encrypt -file [your-file] will encrypt a file AND give you a one-time 256-bit AES key.
You're supposed to upload that file somewhere, and send the key to your recipient in a separate channel.
The recipient can use the key and the file like that:
./eureka -decrypt -file [encrypted-file] -key [hex-key]