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So there is a virtual reality device, the Nerve Gear, popularly used for MMOs.

As far as I'm concerned, we never see SAO or Alfheim using any sort of login
screen.

Now, it makes sense for SAO to lack a login screen, since you can't log out.
But what about Alfheim? How do normal, sane games authenticate their players?

  * Do MMO Nerve Gear games use username/password credentials? 
    * Is it implied **_anywhere_** in the series? If I recall correctly, Kirito would just put on his Nerve Gear and instantly enter Alfheim without any sort of password screen.
  * Maybe the device serves as the credential itself? 
    * But if I steal your device, I could impersonate you in all games...
    * Besides, using a device ID is not secure at all. If I figure out your device's ID, I could hack mine to use such ID and then login as yourself.
  * Perhaps they do some complex biological scan on your body to create an unique ID... 
    * That doesn't sound very reliable. Or healthy.


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sword-art-online
How do online Nerve Gear games handle player authentication?


Your question can pretty much be answered by watching episode 1 of _Sword Art
Online_.

The Nerve Gear uses a username and password identification system as shown at
01:37 of episode 1.

![Account and password login screen](https://i.stack.imgur.com/MZapF.png)

But as you mentioned, this does not occur in _Alfheim Online_. This is where
we can consider the Nerve Gear to be the authentication factor, being a device
that stores the save data.

> We know it can save data Yui's data was saved to his Nerve Gear and was
> transferred to _Alfheim Online_ as a result.

* * *

> But if I steal your device, I could impersonate you in all games...

This would indeed be true, but doesn't the same count for login credentials?
If you steal somebody's username and password, you can also impersonate that
person in game. And don't forget, it's harder to steal physical objects all
around the world than to steal some username and password from your home
computer.


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Extract from the light novel - Volume 3 Fairy Dance (ALfheim arc), p67-68

> With that in mind, I put the NERvGear on my head and buckled the chin strap.
> With the frame in place and the goggles down, I closed my eyes.
>
> Anxiety and excitement caused my heart to beat rapidly, as I tried to slow
> my racing heart beat, I said, “LINK START!”
>
> The light passing through my closed eyelids suddenly disappeared. The
> transmission from my optic nerves was suddenly severed, and my eyes were
> enveloped by darkness.
>
> [...]
>
> At last, the final OK appeared, and the next instant brought me falling
> through the darkness into the colors of a rainbow, an illusion of the world.
> After passing through this series of rings, I had arrived in a different
> world.
>
> \-- Actually, it's still a little early to say that. Out of the darkness
> emerged an account registration interface. The main ALfheim Online LOGO
> gradually appeared, accompanied by a soft female voice.
>
> Following the instructions given, I began creating my account and character.
> At chest height was a pale and shiny virtual keyboard and I entered a User
> ID and password as prompted. I had many years of experience before beginning
> SAO, so this process was of some familiarity to me. Since this is a
> downloadable MMO game, I would normally need to select the payment method,
> but I had bought this game and it came with a one month free trial.
>
> Next I chose the nickname for my character. I didn’t put much thought into
> it, but entered the name «Kirito».

The ALfheim Arc in the anime doesn't show all the details.

Since Kazuto used the same login in both games - as you may know, ALfheim's
database is a clone of SAO's one - he kept the same account.


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