The last couple of times I installed Debian Etch, this warning message appeared after doing an apt-get update:

W: There are no public key available for the following IDs:
A70DAF536070D3A1
W: You might want to run apt-get update to correct these problems

There are no public key so all your base are now belong to us!

Bad grammar aside, the recommended action of typing apt-get update does not fix the problem. So here is a command which does:

apt-get install debian-archive-keyring

This will download the GPG key which is needed to verify the package repository so you know you have a genuine copy of Debian.