After a quite exhaustive search, it would appear that there is no real and reliable way to hide code within Oracle's database (i.e. protect the code of routines, functions, packages and triggers).
The obfuscation (WRAP), though initially promising, turned out to be useless as it is very easy to UNDO the obfuscation and get the original (readable) code back.
So, before I give up, is there any way supported by Oracle that enables real hiding of the code? (I mean, besides of blocking access using credentials).