Security concern about Knoppix 3.9 and 4.0

Hello everybody,

I have downloaded KNOPPIX_V3.9-2005-05-27-EN via Bittorrents.
Checking to the isofile, I noticed that the file is signed using a gnupgp key DSA ID 57E37087, and not one of M. Knopper key (BA8F038D) as it supposed to.
Moreover, the key indicated is not referenced on http://keyserver.veridis.com
So I am wondering, if this file is safe,

By the way I looked at the dvd Knoppix 4.0, to cross check and I could not find any signature :? .

It is true that it is not GPG signed by Klaus. This is because Mr. Knopper created Knoppix 4.0 DVD & then had professional DVD discs mass produced. They sold for 5 euros at LinuxTag 2005 about 1 week ago. Some kind person bought the DVD & then ripped the DVD to ISO. That ISO is the one that is being distributed around the world.