I will try the "keyring"
----- Original Message Follows -----
Probably don't have the package installed that contains
the signing keys for the packages, do a search for
'keyring', can't tell you the exact package for Ubuntu (as
run debian) but to give you an example this is the one
required for my system:
debian-archive-keyring - GnuPG archive keys of the Debian
If you have added any repositories there is likely a
package specifically for that repository.