There are many pieces of software compatible with GnuPG (a.k.a. It supports asymmetric and symmetric encryption. You need to scroll all the way to the bottom under “Binary Releases”. You can get the official, free, open-source download from here. It’s one of the most cross-platform pieces of software out there: There is software which offers AES-256 bit encryption, open-source AND cross-platform – it’s called GnuPG and has been out for 20 years. “As far as I know, there is no comparable software to AxCrypt available to OS X users which offers AES 256 encryption, is open-source AND cross-platform.”