# Initiate OAuth Flow
Start the authorization process in your Canvas app by using OAuth 2.0. If you store or retrieve data, such as an authentication token, from your Canvas app’s local storage in the callback, use `window.opener.localStorage` instead of `window.localStorage`.
This code example shows how you can initiate the OAuth 2.0 authorization process in your Canvas app.
```java
access_token =
```
When you implement an OAuth 2.0 web server flow, a window pops up so the user can authenticate. If your browser partitions storage, any data stored with storage APIs such as [localStorage](https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage) and [sessionStorage](https://developer.mozilla.org/en-US/docs/Web/API/Window/sessionStorage) is accessible only in contexts with the same top-level site. Because the callback is in a different window than the Canvas app, it has a different top-level site. Therefore, if you use `window.localStorage` in the callback to store and retrieve data, such as an authentication token, then an `Unauthorized: client` or `client.oauthToken` error occurs.
Instead, to store and retrieve data from your Canvas app’s local storage in the callback, use `window.opener.localStorage`. With the [opener](https://developer.mozilla.org/en-US/docs/Web/API/Window/opener) property, you can interact with the window that opens another window if they both have the same origin. Because the callback has the same origin as the Canvas app, the local storage is accessible.
:::note
Google Chrome and other major browsers now enable storage partitioning by default. For information about their browser requirements, see [How Third-Party Cookie Restrictions Impact Salesforce Canvas Apps](https://help.salesforce.com/s/articleView?id=002192971\&type=1).
:::
**See Also**
- [OAuth 2.0 Authorization](/docs/platform/canvas-framework/guide/canvas-app-oauth-authentication.md)