Having just recently upgraded flash on both my systems I know the command Stevo mentioned works. I use the following variation (from another post by Stevo btw):
If that still doesn't work, my suggestion would be to disable the Debian Multimedia Repo through Synaptic, click the reload button, close Synaptic, and then try the command again. From the little information I found on the subject on Google it seems to point to a problem with the checksum on the Multimedia Repo when a newer version of Flash is released. It happened to a few people going from version 10 to 10.1, the info I found is here: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=592598
Thus, when you try installing 11.2 with the above command, it is checking the sha512sum against an older version (say 11.1) hosted on the Multimedia Repo and you are receiving that error. This should eventually resolve itself if you want to wait.
So, it is only a guess, but I would try disabling the Multimedia Repo and then trying Stevo's command again. If it doesn't work, then I'm wrong about my hypothesis.