Postgres: Encrypted User Function. Is it possible?

is it possible to create a function on PostgreSQL server, which routines can't be seen by any Postgres-user on the server? The function gets some string as a parameter and outputs a boolean value. But nobody should know, how the calculation of the boolean value works. Why do I need it: in the function's text is a private key. Maybe there is some other possibility to "hide" the private key or salt securely.