Please update your PGDG apt-key

Install .. without verification [y/N]? N

2013-10-16 — @jaredowenbeck

The key that the PostgreSQL Global Development Group (PGDG) uses to sign its APT packages expired recently.

sudo apt-key list | grep expired
pub 4096R/ACCC4CF8 2011-10-13 [expired: 2013-10-13]

If you didn’t know it expired, you might get warnings from apt like:

W: GPG error: http://apt.postgresql.org precise-pgdg Release: The following signatures were invalid: KEYEXPIRED 1381654177

or

WARNING: The following packages cannot be authenticated! libpq5 pgdg-keyring postgresql-contrib-9.3 postgresql-plperl-9.3 postgresql-client-9.3 postgresql-9.3
Install these packages without verification [y/N]? N

A new key has been issued. Simply delete the expired key and install the new key from a trusted source.