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.
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.
OAuth, you need to include the
ktor-server-auth artifact in the build script:
Install the Sessions plugin
To avoid requesting authorization every time a client tries to access a protected resource, you can store the access token in the session upon successful authorization. You can then retrieve the access token from the current session within the handler of the protected route and use it to request the resource.
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).
stateparameter 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 the specified scopes, as configured in Step 2: Configure the OAuth provider.
If a user approves the requested permissions, the authorization server redirects back to the designated redirect URL and sends the authorization code.
Ktor makes one more automatic request to the specified access token URL, including the following parameters:
Client ID and client secret.
The authorization server responds by returning an access token.
The client can then use this token to make a request to the required service of the selected provider. In most cases, a token will be sent in the
Authorizationheader using the
The service validates the token, uses its scope for authorization, and returns the requested data.
To install the
oauth authentication provider, call the oauth function inside the
install block. Optionally, you can specify a provider name. For example, to install an
oauth provider with the name "auth-oauth-google" it would look like the following:
This section demonstrates how to configure the
oauth provider for authorizing users of your application using Google. For the complete runnable example, see 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.
Web applicationfrom the dropdown.
Specify the following settings:
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.
In the invoked dialog, copy the created client ID and client secret that will be used to configure the
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.
Step 2: Configure the OAuth provider
The code snippet below shows how to create and configure the
oauth provider with the
urlProviderspecifies a redirect route that will be invoked when authorization is completed.
providerLookupallows 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.
clientproperty specifies the HttpClient used by Ktor to make requests to the OAuth server.
Step 3: Add a 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.
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 the redirect route for the
urlProvider, as specified in Step 2: Configure the OAuth provider.
Inside this route you can retrieve the OAuthAccessTokenResponse object using the
OAuthAccessTokenResponse allows you to access a token and other parameters returned by the OAuth server.
In this example, the following actions are performed after receiving a token:
The token is saved in a Session, which content can be accessed inside other routes.
The user is redirected to the next route where a request to Google API is made.
If the requested route is not found, the user is redirected to the
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
Create a new function called
getPersonalGreeting which will make the request and return the response body:
Then, you can call the function within a
get route to retrieve a user's information:
For the complete runnable example, see auth-oauth-google.