I need to encrypt files and folders in Ubuntu. I know there are lots of programs that can do that, but I would like to be able to open the encrypted files on other platforms too. I should be possible to open the files on Linux, Windows, Android and so on. Is there a encryption program that exists on all these platforms or at least a method to open decrypt the file on other platforms?
Truecrypt is (was?) probably the most well known cross platform, open source encryption tool, but unfortunately the project was mysteriously shut down by its anonymous developers just a few months ago, when the website was replaced with a message stating that it's "no longer safe."
However, the general consensus in the security community seems to be that Truecrypt is still safe to use; it's just that it will no longer be updated by the original authors. Some Truecrypt enthusiasts are attempting to pick up on the project, and are providing downloads here. Personally I think it's still a viable encryption tool, as an ongoing, crowd-funded effort to do comprehensive audit of Truecrypt's code has not turned up any major problems after Phase 1. It is supported on all major platforms, including Windows, Mac, and Linux. Android support is provided by 3rd party apps. Of course it's up to you to decide whether it's secure enough for your needs, but I think it's worth a try.
(PS, Ed Snowden was a big fan of Truecrypt, if that means anything to you.)
GPG, the Gnu Privacy Guard, is available for Linux, Windows and Android (and others).
You'll lose some comfort though, as you would first have to decrypt manually and then open the desired file(s)...
Download veracrypt... it is a branch of the truecrypt project from other developers. It's compatible with your existing truecrypt volumes.
I would suggest Kruptos 2 Pro encryption software for cross platform (Windows/Mac OS/Android/USB) file encryption