The 2.0 version of Underpass for Mac was available to download on the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. This free Mac application is an intellectual property of Jeffrey Johnson. Underpass for Mac lies within Security Tools, more precisely Personal Security.
From the developer:
Underpass is a chat and file transfer application that features end-to-end encryption for complete privacy and security. Underpass is different from every other encrypted chat app because it does not use a third-party service. It's just two devices and a password. You can chat securely over the Internet or on a LAN, such as in an office, school, coffee shop, or home.
You may want to check out more Mac applications, such as TelnetLauncher, which might be related to Underpass.