I've asked a similar question here earlier iOS: Is it ok to store a RSA private key (use to decrypt text) in your application document directory?
However, the person recommend me to use keychain access. This will not work for me because after further research only mac users can use keychains in the first place. Which limits the people who can do this.
I was wondering if it is secure to use Apple's file sharing https://support.apple.com/en-us/HT201301 to allow the user to use his/her private key with the iOS App? According to the documents regarding file sharing only my App should be able to access the private key because it is store in the document directory. There is some text that needs to be decrypted in my App.