apt-update signature invalid

[vnevoa]

Hi.
I stumbled upon this error too, after installing Hardy fresh in a Mac G3.
The Mac was connected to the Internet via a defective Ethernet cable, and lost most of the IP packets, but some got through with corrupted data.
As a result, my "apt-get updates" corrupted the apt system, and this thread's error started happening. Only after that I noticed the cable problem.

I did all that is described in this thread, as well as many others, and even some other stuff I desperately invented.
In the end, what solved the problem was:
1 - remove the offending repository from the "sources.list";
2 - do "apt-get update";
3 - insert the offending repository back into the "sources.list";
4 - do "apt-get update" again; it should be working now.

Other stuff I did before:
- sudo apt-get --reinstall install apt; # had the cd-rom in the sources.list;
- sudo apt-key remove 437D05B5; # to remove the offending key;
- sudo apt-key update; # to reinstall the offending key;

But as I said, it was only after tricking apt-get's cache system that things started working again.

Hope this helps someone.