> For the complete documentation index, see [llms.txt](https://docs.cotter.app/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.cotter.app/features/passwordless-login/how-it-works.md).

# How it works

There are several different cases for user registration and login, and we're going to cover them all:

### Registration

![Registration with Cotter's Trusted Device](/files/-MEjHpcQLCKgGtxbn6XJ)

When a user registers to your app, you can immediately call Cotter's SDK to register the mobile device as a Trusted Device. This way, the user's account can now be accessed from the trusted device without needing a password.

### Login from a Trusted Device

![Logging-in from a Trusted Device](/files/-MEjHwvrecyViDUP48sd)

Logging in from a trusted device is instant. Your users can immediately access their account without doing anything. Your app can optionally require Biometric/PIN to make it more secure.

### Login from a Non-Trusted Device

![Login from a Non-Trusted Device](/files/-MEjI28l_lHOpx2OlPl6)

If your user try to login from a different phone, or from a web browser, then they will receive a prompt in their Trusted Device, asking them to approve the login request. Once approved, the user can access their account from Non-Trusted Device. **But this doesn't mean that the new device is now a trusted device.** Subsequent logins from a Non-Trusted Device will **always** require approval, even when approved.

### Registering a New Device as a Trusted Device

![Adding a New Trusted Device](/files/-MEjInS_0JWSfygUjI2v)

The user can register another device as an additional Trusted Device. The Trusted Device will need to scan a QR Code shown in the other device to explicitly register that other device as a Trusted Device.

Once the new device is registered as a Trusted Device, the user can now access their account from both devices with ease.

## Quickstarts

{% content-ref url="/pages/-M6Eb1vLZRs1\_0ZAMKrR" %}
[React Native – Sign in with Device](/quickstart-guides/react-native-quickstart.md)
{% endcontent-ref %}

{% content-ref url="/pages/-M9aaGUmTZYju\_sjd17x" %}
[Flutter – Sign in with Device](/quickstart-guides/flutter-sign-in-with-device.md)
{% endcontent-ref %}

{% content-ref url="/pages/-M6EaO2M9ot00du8W\_bR" %}
[iOS – Sign in with Device](/quickstart-guides/ios-quickstart.md)
{% endcontent-ref %}

## SDK Reference

#### React Native

{% content-ref url="/pages/-M2kTuM6W90CT9IpIxHq" %}
[Sign In with Device](/sdk-reference/react-native/react-native-sdk-passwordless-login.md)
{% endcontent-ref %}

#### Flutter

{% content-ref url="/pages/-M9angupK73fWUZjM7fe" %}
[Sign In with Device](/sdk-reference/flutter/sign-in-with-device.md)
{% endcontent-ref %}

**iOS**

{% content-ref url="/pages/-M4q-LjGBIcIq3PgnYt6" %}
[Sign In with Device](/sdk-reference/ios/ios-sdk-passwordless-login.md)
{% endcontent-ref %}

#### Android

{% content-ref url="/pages/-MCxyo3ix-jqWDQysF0g" %}
[Sign In with Device](/sdk-reference/android/sign-in-with-device.md)
{% endcontent-ref %}