# apt-get update

OR

# atitude update

RESULT:

GPG error: http://security.debian.org etch/updates Release: The following signatures were invalid: BADSIG A70DAF536070D3A1

FIXED

# apt-get install install debian-keyring

OR

# aptitude install debian-keyring

AND

# apt-key add /usr/share/keyrings/debian-role-keys.gpg

Ref: http://www.ecualug.org/?q=2007/01/08/blog/antares/claves_gpg_debian