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Overview

While leveraging an official Box SDK removes common hurdles of authentication, it is possible to use the Box APIs without one. This guide reviews the steps to manually complete the OAuth 2.0 flow.
  1. Build the authorization URL
  2. Redirect the user to the authorization URL
  3. The user grants the application access to take actions on their behalf, which, if successful, provides an authorization code
  4. Redirect the user back to the application
  5. Exchange the authorization code for an Access Token
At the end of this flow, the application has an Access Token, which can be used to make API calls on behalf of the user.
The Access Token acquired through the OAuth 2.0 flow is inherently tied to the user who authorized the application.It is possible to act as another user using the as-user header.

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Prerequisites

Before continuing you will need to complete the following steps:
  • Create a Platform App within the Box Developer Console, which leverages the OAuth 2.0 authentication method.
  • Navigate to the configuration tab for the application to copy the client_id and client_secret values.
  • Ensure at least one redirect URI is configured in the configuration tab for the application.

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1. Build authorization URL

An authorization URL is comprised of the following parameters:
ParameterStatusDescription
CLIENT_IDRequiredObtained from the configuration tab of the Developer Console
REDIRECT_URIOptionalConfigured in the Developer Console and where the user is sent once granting access to the application
RESPONSE_TYPERequiredAlways set to code
STATERecommendedProtects against cross-site request forgery
If you configured multiple redirect URIs for the application, the authorization URL must include the redirect_uri parameter matching one of the URIs configured in the developer console. If the parameter is not specified, the user will see a redirect_uri_missing error and will not be redirected back to the app.
At the minimum this URL will always use the format: https://account.box.com/api/oauth2/authorize?client_id=CLIENTIDHERE&response_type=code 
var baseUrl = "https://account.box.com/api/oauth2/authorize";
var clientId = "[CLIENT_ID]";
var authorizationUrl = $"{baseUrl}?client_id={clientId}&response_type=code";

Learn more about the authorization URL
If you have Box Verified Enterprise for your Box instance turned on, you may encounter an issue using the standard account.box.com base URL. Instead, use ent.box.com in place of account.box.com.

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2. Redirect the user

Next, redirect the user to the authorization URL. The way this is done depends on the application framework. Most framework documentation provides extensive guidance on this topic. If the authorization URL is not valid for the app specified, the user will see an error page rather than a grant access screen. For example, if the redirect_uri parameter in the authorization URL does not match one of the URIs configured for your app, the user will see a redirect_uri_mismatch error. 
var authorizationUrl = $"{baseUrl}?client_id={clientId}&response_type=code";
// redirectTo(authorizationUrl);
Additional query parameters can be passed along when redirecting the user to limit down the scope, or pass along some extra state. See the authorization reference documentation for more information.

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3. User grants application access

The user is redirected to their browser to log in to their account using the Box UI. They are then presented with the list of requested scopes and the option to approve the application to take actions on their behalf.
Example OAuth 2.0 approval screen
When the user accepts this request by clicking Grant access to Box, the browser will redirect to the configured redirect URL with a query parameter containing a short-lived authorization code.
If you configured multiple redirect URIs for the application, the authorization URL must include the redirect_uri parameter matching one of the URIs configured in the developer console. If the parameter is not specified, the user will see a redirect_uri_missing error and will not be redirected back to the app.
https://your.domain.com/path?code=1234567

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4. Exchange code

The provided authorization code is valid for 30 seconds and must be exchanged for an Access Token before expiration. 
using System.Net;
using System.Net.Http;
using Newtonsoft.Json;

String authenticationUrl = "https://api.box.com/oauth2/token";
var client = new HttpClient();

var content = new FormUrlEncodedContent(new[]
{
    new KeyValuePair<string, string>("grant_type", "authorization_code"),
    new KeyValuePair<string, string>("code", "[CODE]"),
    new KeyValuePair<string, string>("client_id", "[CLIENT_ID]"),
    new KeyValuePair<string, string>("client_secret", "[CLIENT_SECRET]")
});

var response = client.PostAsync(authenticationUrl, content).Result;

class Token
{
    public string access_token { get; set; }
}

var data = response.Content.ReadAsStringAsync().Result;
var token = JsonConvert.DeserializeObject<Token>(data);
var accessToken = token.access_token;
To learn how to use an Access Token visit our guide on Making API calls.