Foucault's pendulum by Umberto Eco. Just thinking about it makes me want to read this masterpiece again.
https://en.m.wikipedia.org/wiki/Foucault%27s_Pendulum
That chapter where they try to figure out the password to the computer though...
Baudolino is my favourite BTW, maybe I should re read it (struggling with time to get into Pragues Cemetery ATM).
Great book