OpenSK is described as 'Open-source implementation for security keys written in Rust that supports both FIDO U2F and FIDO2 standards' and is an app in the development category. There are more than 10 alternatives to OpenSK for a variety of platforms, including Android, iPhone, iPad, Linux and Android Tablet apps. The best OpenSK alternative is 2FA Authenticator (2FAS), which is both free and Open Source. Other great apps like OpenSK are Google Authenticator, andOTP, YubiKey and Nitrokey.
A simple application for multi-factor authentication, written in HTML using jQuery Mobile, jsSHA, LocalStorage and Application Cache. It implements the TOTP (Time-Based One-Time Password) algorithm according to RFC6238 and has been tested to work with Google Authenticator...
