My understanding is that this can only be used to authenticate from a computer, How about if I want to log in to an application from a phone?

You need to look quite carefully at the Yubikey name, and remember that Amazon have a habit of putting multiple versions of products u7nder one entry, so be careful that an answer applies to the product YOU are looking at, because it might not be the same as the one the person asking the question looked at. For Yubikey, if the name mentions NFC, then it should work with any phone that supports NFC (Near Field Communi ation) PROVIDED it is turned on in the phone. NFC is the technology that lets you pay for things by tapping your phone, or holding it close to a device in a shop. if a phone does that, it will work with a Yubikey with NFC, which most do. Check the name. Then, there's USB. Many new-ish phones, like Samsung, have USB "C" connectors. This is the small one, not much more that a quarter-inch, that leads can be plugged into EITHER WAY UP. it is not the same as the similarly-sized connector that only goes in one way round. You can get adapters, however, that convert and if you phone has the older style, you'd need a suitable adapter. Next, some Yubikeys have the original larger style USB connectors. These are designed for use in older ports on PCs, laptops, etc and will not fit, as-is, with phones but again, adapters can be bought. Finally, some iPhones have Thunderbolt ports. The Yubikey 5Ci supports this. this is the weird-looking version with a connector on both ends. One end is USB C and the other is Thunderbolt. So the answer to the question is that provided the phone has NFC and it is enabled, or providing you have the correct connector (or a suitable converter) then, with the right Yubikey, it should work. But it is hard to be categoric without knowing which particular phone you have because, as the above shows, there are a variety of possible combinations. Oh, and I agree with an earlier answer - using NFC can be a bit fussy. However, overall, Yubikey is a first class product and a very good example of a major step-up in personal online security.

Does this work as a two-factor authentication method for signing into outlook.com? If so, which browsers are supported?

Sure does. In fact with Edge you can setup password-less logon to outlook.com. with chrome and Firefox you can just use it as a second factor after the username and password.

Does this have any kind of backup in case you lose it like some software based 2FA does? Or is it game over if it goes wandering?

You use this device as a 2 factor authorization which works with 3rd party software which you purchase separately .It has no software itself. It simply contains a chip which generates a Code which your software recognizes and as it needs to be Physically pressed by a human being it is not possible for hackers to program software to activate it. If you lose it you simply deactivate your device in you software of choice. For example you could use it with a password manager software which you purchase separately. To login into a software you use a password , that is the 1st factor of Authorization, then you insert your Key and press the button , that is the second factor.So if someone gets hold of your password they still can't login in without your stick. It's almost Bullet proof if you ask me. If you lose your Yubikey and wished to deactivate the Yubikey you would do so via a mobile phone which you used to setup your software in the first place. For example you would receive an SMS or Email depending on the Software producer.

Right question. If the security key is made to last 20 to 30 years. How many years do the website your registered to support your current security key

This is something I will need to look into nearer the time but also best to check it out on Youtube so that you know where you stand before you buy the item.

Flash memory type	USB
Compatible phone models	Personal Computer,Laptop,Smartphone
Product Dimensions	4.5 x 1.78 x 0.38 cm; 10 g
Item model number	Y-335
Manufacturer	YUBICO
Are Batteries Included	No
Item Weight	10 g
Guaranteed software updates until	unknown

YubiKey 5C NFC - Two Factor Authentication USB and NFC Security Key, Fits USB-C Ports and Works with Supported NFC Mobile Devices - Protect Your Online Accounts with More Than a Password

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