Again, it depends on what you mean by reasonably secure.
Are you worried about someone with access to your unlocked device? If so, there are few secure options (without implementing your own password, or keychain set_master_password), because anything that your app can access can also be accessed by others.
If that's not your concern, then many options are equally secure:
File local to your device
Something stored in your Keychain
(Since files are encrypted on all newer devices)
Though you could also encrypt the file with a password for storage in iCloud, if you are really paranoid.
Ssh keys shouldn't change frequently, so using ftp or samba or a temporary web page or temporary git server would all work to get files into the device one time. Also, google api keys can usually be downloaded directly from the developer dashboard or whatever it is called these days, directly to each device.