While not an API as such both OS's provide a User defaults/App default storage area. It's not that hard to encrypt your key in an outside process and then have

While not an API as such both OS's provide a User defaults/App default storage area. It's not that hard to encrypt your key in an outside process and then have your app write it to that location in encrypted format. Then make a request to the server to supply the key to decrypt the api key. Then again you are making it a little harder to break in but not much. The better tactic is to just limit the API access to as little as possible and make the API as secure as possible.