Ok, my problem is solved after few months. I got reply from Inversion support that I need to contact Steam support for this issue. But suddenly some Defcon (steam ed.) update was downloaded and it works fine now...

frenchfrog wrote:Contact Introversion support directly?

I did... No answer so far...

Hello, I watched many pages trying to solve the issue Linux + Steam + Defcon. My authentication failed and Im able to play just demo. I tried it on my Windows machine and it is working (full version). I tried to delete authkey (both game directory and ~/.defcon) file but new authentication failed ag...