The **YubiKeys** are among the most popular security keys. Some YubiKey models have a wide range of features such as: [Universal 2nd Factor (U2F)](https://en.wikipedia.org/wiki/Universal_2nd_Factor), [FIDO2 and WebAuthn](basics/multi-factor-authentication.md#fido-fast-identity-online), [Yubico OTP](basics/multi-factor-authentication.md#yubico-otp), [Personal Identity Verification (PIV)](https://developers.yubico.com/PIV), [OpenPGP](https://developers.yubico.com/PGP/), [TOTP and HOTP](https://developers.yubico.com/OATH) authentication.
One of the benefits of the YubiKey is that one key can do almost everything (YubiKey 5), you could expect from a hardware security key. We do encourage you to take the [quiz](https://www.yubico.com/quiz/) before purchasing in order to make sure you make the right choice.
The [comparison table](https://www.yubico.com/store/compare/) shows the features and how the YubiKeys compare. We highly recommend that you select keys from the YubiKey 5 Series.
YubiKeys can be programmed using the [YubiKey Manager](https://www.yubico.com/support/download/yubikey-manager/) or [YubiKey Personalization Tools](https://www.yubico.com/support/download/yubikey-personalization-tools/). For managing TOTP codes, you can use the [Yubico Authenticator](https://www.yubico.com/products/yubico-authenticator/). All of Yubico's clients are open-source.
For models which support HOTP and TOTP, there are 2 slots in the OTP interface which could be used for HOTP and 32 slots to store TOTP secrets. These secrets are stored encrypted on the key and never expose them to the devices they are plugged into. Once a seed (shared secret) is given to the Yubico Authenticator, it will only give out the six-digit codes, but never the seed. This security model helps limit what an attacker can do if they compromise one of the devices running the Yubico Authenticator and make the YubiKey resistant to a physical attacker.
The firmware of YubiKey is not open-source and is not updatable. If you want features in newer firmware versions, or if there is a vulnerability in the firmware version you are using, you would need to purchase a new key.
**Nitrokey** has a security key capable of [FIDO2 and WebAuthn](basics/multi-factor-authentication.md#fido-fast-identity-online) called the **Nitrokey FIDO2**. For PGP support, you need to purchase one of their other keys such as the **Nitrokey Start**, **Nitrokey Pro 2** or the **Nitrokey Storage 2**.
The [comparison table](https://www.nitrokey.com/#comparison) shows the features and how the Nitrokey models compare. The **Nitrokey 3** listed will have a combined feature set.
Nitrokey models can be configured using the [Nitrokey app](https://www.nitrokey.com/download).
For the models which support HOTP and TOTP, there are 3 slots for HOTP and 15 for TOTP. Some Nitrokeys can act as a password manager. They can store 16 different credentials and encrypt them using the same password as the OpenPGP interface.
While Nitrokeys do not release the HOTP/TOTP secrets to the device they are plugged into, the HOTP and TOTP storage is **not** encrypted and is vulnerable to physical attacks. If you are looking to store HOTP or TOTP these secrets, we highly recommend that you use a Yubikey instead.
The Nitrokey Pro 2, Nitrokey Storage 2, and the upcoming Nitrokey 3 supports system integrity verification for laptops with the [Coreboot](https://www.coreboot.org/) + [Heads](https://osresearch.net/) firmware. Purism's [Librem Key](https://puri.sm/products/librem-key/) is a rebranded NitroKey Pro 2 with similar firmware and can also be used for the same purposes.
The Nitrokey app, while compatible with Librem Keys, requires `libnitrokey` version 3.6 or above to recognize them. Currently, the package is outdated on Windows, macOS, and most Linux distributions' repository, so you will likely have to compile the Nitrokey app yourself to get it working with the Librem Key. On Linux, you can obtain an up-to-date version from [Flathub](https://flathub.org/apps/details/com.nitrokey.nitrokey-app).
Authenticator Apps implement a security standard adopted by the Internet Engineering Task Force (IETF) called **Time-based One-time Passwords**, or **TOTP**. This is a method where websites share a secret with you which is used by your authenticator app to generate a six (usually) digit code based on the current time, which you enter while logging in for the website to check. Typically these codes are regenerated every 30 seconds, and once a new code is generated the old one becomes useless. Even if a hacker gets one six-digit code, there is no way for them to reverse that code to get the original secret or otherwise be able to predict what any future codes might be.
We highly recommend that you use mobile TOTP apps instead of desktop alternatives as Android and iOS have better security and app isolation than most desktop operating systems.
**Raivo OTP** is a native, lightweight and secure time-based (TOTP) & counter-based (HOTP) password client for iOS. Raivo OTP offers optional iCloud backup & sync. Raivo OTP is also available for macOS in the form of a status bar application, however the Mac app does not work independently of the iOS app.
