oauth.md

Required dependencies: io.ktor:%artifact_name%

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.

You can get general information about authentication and authorization in Ktor in the section.

Add dependencies {id="add_dependencies"}

OAuth authorization flow {id="flow"}

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

1. A user opens a login page in a Ktor application.

2. 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).

3. 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.

4. If a user approves the requested permissions, the authorization server redirects back to a redirect URL and sends the authorization code.

5. 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.

6. 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.

7. A service validates a token, uses its scope for authorization, and returns the requested data.

Install OAuth {id="install"}

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

install(Authentication) {
   oauth {
        // Configure oauth authentication
    }
}

You can optionally specify a provider name.

Configure OAuth {id="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.

Note that to access Google APIs you need to create authorization credentials in the Google Cloud Console. Credentials contain the client ID and client secret that will be used to configure the oauth provider.

Step 1: Create the HTTP client {id="create-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.

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="19-23"}

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

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="24"}

Step 2: Configure the OAuth provider {id="configure-oauth-provider"}

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

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="28-44"}

• The urlProvider specifies a redirect route that will be opened when authorization is completed.
• providerLookup allows you to specify OAuth settings for a required provider. These settings 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.

Step 3: Add a login route {id="login-route"}

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.

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="45-49,56,79"}

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 {id="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.

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="45-56,79"}

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 {id="request-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.

{src="snippets/auth-oauth-google/src/main/kotlin/com/example/Application.kt" lines="66-78"}

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