1. [n. d.]. Apple Developer: Notifications. https://developer.apple.com/notifications/. [n. d.]. Apple Developer: Notifications. https://developer.apple.com/notifications/.
2. [n. d.]. Apple iOS Push Notifications: Security Implications Abuse Scenarios and Countermeasures. https://www.dhanjani.com/blog/2011/02/apple-ios-push-notifications-security-implications-abuse-scenarios-and-countermeasures.html. [n. d.]. Apple iOS Push Notifications: Security Implications Abuse Scenarios and Countermeasures. https://www.dhanjani.com/blog/2011/02/apple-ios-push-notifications-security-implications-abuse-scenarios-and-countermeasures.html.
3. [n. d.]. Authy | Two Factor Authentication. https://authy.com. [n. d.]. Authy | Two Factor Authentication. https://authy.com.
4. [n. d.]. dev.yubico | U2F Technical Overview. https://developers.yubico.com/U2F/Protocol_details/Overview.html. [n. d.]. dev.yubico | U2F Technical Overview. https://developers.yubico.com/U2F/Protocol_details/Overview.html.
5. [n. d.]. Duo | Duo Push. https://duo.com/product/trusted-users/two-factor-authentication/authentication-methods/duo-push. [n. d.]. Duo | Duo Push. https://duo.com/product/trusted-users/two-factor-authentication/authentication-methods/duo-push.