Nitrokey is described as 'USB key to enable highly secure encryption and signing of emails and data, as well as login to the Web, networks and computers. Other than ordinary software solutions, the secret keys are always stored securely inside the Nitrokey' and is an app. There are more than 10 alternatives to Nitrokey for a variety of platforms, including Android, iPhone, Windows, Linux and iPad. The best alternative is Authy, which is free. Other great apps like Nitrokey are Microsoft Authenticator, YubiKey, SoloKeys and OnlyKey.
Sign in is easy, convenient, and secure when you use Microsoft Authenticator. Use your phone, not your password, to log into your Microsoft account. You simply have to enter your username, then approve the notification sent to your phone.
OnlyKey is an open source alternative to YubiKey. OnlyKey natively supports: a hardware password manager, multiple two-factor methods (FIDO2, TOTP, and Yubico® OTP), passwordless SSH login, and OpenPGP.
A Password Manager for Android with Auto-Type over USB and Bluetooth, OTP and much more.
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Authorizer vs Nitrokey opinions
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LinuxDoge Its among the only open source Apps in this list. For security and Privacy, Open source is always important, as anyone can look into the code. When there is a vulnerability in the code, it doesnt depend on a company to solve the issue.