[xorcist]

Files have all sorts of protection mechanisms available, both built in to the file system and on top of it, is traditionally used to store secrets, and doesn't generally leak. So that should be the starting point.

Command line argument would be visible in the process table, hopefully no one would suggest that. It also is not persistent, so it generally needs to be fed from someplace anyway. Generally a file. This is something it shares with environment variables.