Define Yubikey and how does Yubikey work?

YubiKey is a security token that allows users to add a second authentication factor to online services from tier.The YubiKey comes in different variants, for example the YubiKey 4 and the YubiKey U2F. All YubiKeys are hardware tokens and are connected to a USB port. Most feature an inductive button and one model also has NFC (the YubiKey Neo). The variants differ regarding form factor and the number of supported features.

There are alternative solutions available that provide similar or a subset of the multi-feature YubiKey 4. For example, classic hardware chipcard readers (perhaps even featuring a keypad) in combination with an OpenPGP compatible chipcard. The YubiKey U2F is only a U2F device, i.e. a device that is able to generate a origin specific public/private key pair and returns a key handle and a public key to the caller. Like other inexpensive U2F devices, the private keys are not stored, instead they are symmetrically encrypted (with an internal key) and returned as the key handle. Using the key-handle, the U2f device is then able to sign a challenge, thus creating a response as part of a multi factor authentication. Since U2F is an open standard (that is also pushed by corporations like Google), there are several alternative inexpensive U2F hardware tokens available (search for 'FIDO U2F key').

How does YubiKey work on iPhone ? The YubiKey 5Ci allows for direct connection to iOS/iPadOS devices with a Lightning port. Some models that use this port include (but are not limited to) iPhone SE, iPhone 7, iPhone 8, iPhone X, and most modern iPads (not including the newest iPad Pro, which uses a USB-C port). The functionality of the 5Ci is limited to Yubico OTP and WebAuthn without an app that specifically supports the YubiKey 5Ci over Lightning, such as 1Password and LastPass.