Small and Mighty
The YubiKey 5Ci is a two-factor hardware authenticator. Using devices like this have been shown to effectively halt account takeovers.
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The YubiKey 5Ci has a USB-C connector on one side and an Apple Lightning connector on the other. It is intended to work with computers, Android devices, iPhones, and some iPads.
Plug and Play
The YubiKey 5Ci is well built and fits easily into iOS devices. Support for it from Apple and other developers, however, remains limited.
Keychain Security
Smaller than other security keys, the YubiKey 5Ci fits easily on a keyring. It's also durable enough to take a beating in your pocket.