Database level encryption

Hello

Please how could I encrypt my database so data can be retrieved only
with my password, when my client application starts, even if the
database is captured ?

I would like the encryption to be transparent at the application
level, which needs only provide a password and then all the database
can be accessed, and I would like to store my database on any
filesystem on my client's computers. so I do not want filesystem
ecryption either.

My client has a proximity advertising application, running with a
local PostgreSQL database, that they install on many computers from
several client companies, to cover many spots on a given geographic
area. The application always starts with my password, and my client
will be the only one that can start or stop it, and I would like the
application data in the database to be protected by that same
password.

I can only see how PostgreSQL encrypts the password or the connection
in the documentation, and for the database I can see application-level
encryption with pgcrypto (and filesystem level encryption), How could
I get database level encryption in PostgreSQL ?