W: Não existem chaves públicas para os seguintes IDs de chaves:

#aptitude update
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy Release.gpg
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy Release
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy/contrib amd64 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy/main amd64 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy/contrib Translation-pt_BR
Ign cdrom://[Debian GNU/Linux 7.3.0 _Wheezy_ - Official amd64 DVD Binary-1 20131215-04:56] wheezy/contrib Translation-pt
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release
Hit http://security.debian.org wheezy/updates/main Sources
Hit http://security.debian.org wheezy/updates/contrib Sources
Hit http://security.debian.org wheezy/updates/main amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
W: Não existem chaves públicas para os seguintes IDs de chaves:
9D8D8F5BC758C503

Tente assim para corrigir:

# aptitude install debian-keyring debian-archive-keyring

Pegue a chave ID e substitua pela sua.

#gpg --keyserver subkeys.pgp.net  --recv-keys 9D8D8F5BC758C503
#gpg --armor --export 9D8D8F5BC758C503 | apt-key add -