This recipe shows you how to encrypt data using the pgcrypto contrib package.
Make sure you (and/or your database server) are in a country where encryption is not illegal-it still is in some countries.
In order to create and manage PGP keys, you also need the well-known GnuPG command-line utility, which is available on practically all distributions.
pgcrypto is part of the contrib collection. Starting from version 10, on Debian and Ubuntu is part of the main postgresql-10 server package, while in previous versions there was a separate package, for example, postgresql-contrib-9.6.
Install it on the database in which you want to use it, following the Adding an external module to PostgreSQL...