(psuedo) random serial for PK or at least UNIQUE NOT NULL?

Hi all.

I'm willing to change an BIGINT ID column (actually a SERIAL8) with a
BIGINT whose valules are (pseudo)random.
The main objective is to avoid guessability.
I whish I could also use it as the PK (as it's now) but that's not
really important now.
Any hint?


--
Vincenzo Romano
NotOrAnd Information Technologies
NON QVIETIS MARIBVS NAVTA PERITVS