OAuth

OAuth is an open standard for access delegation. OAuth can be used to authorize users of your application by using external providers, such as Google, Facebook, Twitter, and so on.

The oauth provider supports the authorization code flow. You can configure OAuth parameters in one place, and Ktor will automatically make a request to a specified authorization server with the necessary parameters.

Add dependencies

To use OAuth, you need to include the ktor-server-auth artifact in the build script:

implementation("io.ktor:ktor-server-auth:$ktor_version")

implementation "io.ktor:ktor-server-auth:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-server-auth-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Install the Sessions plugin

In order to not ask for authorization each time a client tries to access a protected resource, the access token can be saved into the session on a successful authorization. Later, in the handler of the protected route, the access token can be retrieved from the current session and used to request the resource.

import io.ktor.server.sessions.*

install(Sessions) {
    cookie<UserSession>("user_session")
}

// ... 
data class UserSession(val state: String, val token: String)

OAuth authorization flow

The OAuth authorization flow in a Ktor application might look as follows:

A user opens a login page in a Ktor application.
Ktor makes an automatic redirect to the authorization page for a specific provider and passes the necessary parameters:
- a client ID used to access APIs of the selected provider;
- a callback or redirect URL specifying a Ktor application page that will be opened after authorization is completed;
- scopes of third-party resources required for a Ktor application;
- a grant type used to get an access token (Authorization Code in our case);
- a state parameter used to mitigate CSRF attacks and redirect users;
- optional parameters specific for a certain provider.
The authorization page shows a consent screen with the level of permissions required for a Ktor application. These permissions depend on scopes passed in step 2.
If a user approves the requested permissions, the authorization server redirects back to a redirect URL and sends the authorization code.
Ktor makes one more automatic request to the specified access token URL and passes the following parameters:
- an authorization code;
- a client ID and client secret.
The authorization server returns an access token.
A client can now use a token to make a request to the required service of the selected provider. In most cases, a token will be sent in the Authorization header using the Bearer schema.
A service validates a token, uses its scope for authorization, and returns the requested data.

Install OAuth

To install the oauth authentication provider, call the oauth function inside the install block:

import io.ktor.server.application.*
import io.ktor.server.auth.*
//...
install(Authentication) {
   oauth {
        // Configure oauth authentication
    }
}

You can optionally specify a provider name.

Configure OAuth

This section demonstrates how to configure the oauth provider for authorizing users of your application using Google. You can find the complete runnable example here: auth-oauth-google.

Prerequisites: Create authorization credentials

To access Google APIs, you need to create authorization credentials in the Google Cloud Console.

Open the Credentials page in the Google Cloud Console.
Click CREATE CREDENTIALS and choose OAuth client ID.
Choose Web application from the dropdown.
Specify the following settings:
- Authorised JavaScript origins: http://localhost:8080.
- Authorised redirect URIs: http://localhost:8080/callback. In Ktor, the urlProvider property is used to specify a redirect route that will be opened when authorization is completed.
Click CREATE. In the invoked dialog, copy the created client ID and client secret that will be used to configure the oauth provider.

Step 1: Create the HTTP client

Before configuring the oauth provider, you need to create the HttpClient that will be used by the server to make requests to the OAuth server. The ContentNegotiation client plugin with the JSON serializer is required to deserialize received JSON data after a request to the API.

val applicationHttpClient = HttpClient(CIO) {
    install(ContentNegotiation) {
        json()
    }
}

The client instance is passed to the main module function to have the capability to create a separate client instance in a server test.

fun Application.main(httpClient: HttpClient = applicationHttpClient) {

Step 2: Configure the OAuth provider

The code snippet below shows how to create and configure the oauth provider with the auth-oauth-google name.

    val redirects = mutableMapOf<String, String>()
    install(Authentication) {
        oauth("auth-oauth-google") {
            urlProvider = { "http://localhost:8080/callback" }
            providerLookup = {
                OAuthServerSettings.OAuth2ServerSettings(
                    name = "google",
                    authorizeUrl = "https://accounts.google.com/o/oauth2/auth",
                    accessTokenUrl = "https://accounts.google.com/o/oauth2/token",
                    requestMethod = HttpMethod.Post,
                    clientId = System.getenv("GOOGLE_CLIENT_ID"),
                    clientSecret = System.getenv("GOOGLE_CLIENT_SECRET"),
                    defaultScopes = listOf("https://www.googleapis.com/auth/userinfo.profile"),
                    extraAuthParameters = listOf("access_type" to "offline"),
                    onStateCreated = { call, state ->
                        redirects[state] = call.request.queryParameters["redirectUrl"]!!
                    }
                )
            }
            client = httpClient
        }
    }

The urlProvider specifies a redirect route that will be opened when authorization is completed.
Make sure that this route is added to a list of Authorised redirect URIs.
providerLookup allows you to specify OAuth settings for a required provider. These settings are represented by the OAuthServerSettings class and allow Ktor to make automatic requests to the OAuth server.
The client property specifies the HttpClient used by Ktor to make requests to the OAuth server.

After configuring the oauth provider, you need to create a protected login route inside the authenticate function that accepts the name of the oauth provider. When Ktor receives a request to this route, it will be automatically redirected to authorizeUrl defined in providerLookup.

    routing {
        authenticate("auth-oauth-google") {
            get("/login") {
                // Redirects to 'authorizeUrl' automatically
            }
        }
    }

A user will see the authorization page with the level of permissions required for a Ktor application. These permissions depend on defaultScopes specified in providerLookup.

Step 4: Add a redirect route

Apart from the login route, you need to create a redirect route that will be invoked after authorization is completed. The address of this route was specified using the urlProvider property.

Inside this route you can retrieve the OAuthAccessTokenResponse object using the call.principal function. OAuthAccessTokenResponse allows you to access a token and other parameters returned by the OAuth server.

    routing {
        authenticate("auth-oauth-google") {
            get("/login") {
                // Redirects to 'authorizeUrl' automatically
            }

            get("/callback") {
                val principal: OAuthAccessTokenResponse.OAuth2? = call.principal()
                call.sessions.set(UserSession(principal!!.state!!, principal.accessToken))
                val redirect = redirects[principal.state!!]
                call.respondRedirect(redirect!!)
            }
        }
    }

In this example, the following actions are performed after receiving a token:

a token is saved to a cookie session, whose content can be accessed inside other routes;
a user is redirected to the next route where a request to Google API is made.

Step 5: Make a request to API

After receiving a token inside the redirect route and saving it to a session, you can make the request to external APIs using this token. The code snippet below shows how to use the HttpClient to make such a request and get a user's information by sending this token in the Authorization header.

        get("/{path}") {
            val userSession: UserSession? = call.sessions.get()
            if (userSession != null) {
                val userInfo: UserInfo = httpClient.get("https://www.googleapis.com/oauth2/v2/userinfo") {
                    headers {
                        append(HttpHeaders.Authorization, "Bearer ${userSession.token}")
                    }
                }.body()
                call.respondText("Hello, ${userInfo.name}!")
            } else {
                val redirectUrl = URLBuilder("http://0.0.0.0:8080/login").run {
                    parameters.append("redirectUrl", call.request.uri)
                    build()
                }
                call.respondRedirect(redirectUrl)
            }
        }

You can find the complete runnable example here: auth-oauth-google.

Last modified: 27 June 2023