There should be a feature that locks the app after minutes of inactivity to prevent hacking attempts by third parties. Biometrics like fingerprint and iris and voiceprint should be used to unlock the screen again like Telegram chat app. When confidential document containing personal data is sent to the recipient, the recipient has to unlock using his biometrics like fingerprint and iris and voiceprint to authenticate his or her identity and to prevent third parties from opening the document. If amendments to the signature is required, it has to ask for permission from the sender or owner to prevent forgery and illegal amendment.