> ## Documentation Index
> Fetch the complete documentation index at: https://stytch.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Adding PKCE to your OAuth flow

> Secure your OAuth login flow with PKCE

[Proof Key for Code Exchange (PKCE)](https://datatracker.ietf.org/doc/html/rfc7636) is a specification that builds on OAuth 2.0 to create a more secure login flow.

In the original OAuth authorization code flow, an IdP issues a one-time use code to the app, which is exchanged along with a `client_id` and `client_secret` for an auth token. For certain types of apps (i.e. SPAs, mobile apps, or native apps) there was no mechanism to hide the `client_secret`, which must be included in the app's source code and reused on every request. This meant that an attacker would be able to impersonate a user if the attacker gained access to the one-time use code. Codes in a URL can be leaked in many different ways - either through improper logging, HTTP Referrer headers, or even through another exploit like XSS.

PKCE introduces a one-time secret for each authorization flow. This secret, called a `code_verifier`, is generated and stored by the app on the user's device. Before the user is redirected to the IdP, the verifier is hashed to produce a `code_challenge`. This challenge is passed as a query parameter during the authentication flow, e.g. `/oauth/authenticate?code_challenge=E9Mel....`

After the IdP has validated the user's credentials, the user is redirected back to the app with a secure code. Now, the app must submit the code along with the original `code_verifier`. The IdP hashes the submitted `code_verifier` and validates that it matches the `code_challenge` passed as a query parameter earlier. This check ensures that the token has not been leaked, and that it is being handled by the same user that was seen at the start of the flow.

This guide will show you how to add PKCE support to your Stytch OAuth integration for added security. A PKCE integration is required for applications that use native callback URLs, for example `appname://some/callback` on mobile. Stytch's support for PKCE differs slightly from the original specification - for example we do not permit plain verifiers to be used - but the original idea of strengthening app security with a unique secret on every request remains the same.

If you get stuck while working through this guide, feel free to ask questions by [contacting us](/resources/get-support).

<Tabs>
  <Tab title="Frontend SDK">
    <Steps>
      <Step title="Enable PKCE for your app">
        Enable PKCE for all OAuth logins performed by the JavaScript SDK. By flipping this setting, you tell the Javascript SDK to use PKCE for all UI components, and to reject any OAuth logins that do not use PKCE. In order to do so, toggle the option in the [Frontend SDK page](https://stytch.com/dashboard/sdk-configuration) of the Stytch Dashboard. This makes your application resistant to attacks like [Login CSRF](https://cheatsheetseries.owasp.org/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.html#login-csrf) in which an attacker can trick a victim into logging in to the wrong account.

        <Note>The legacy `oauth.$provider.getURL()` method is not compatible with PKCE. If you are using this method already in your codebase, complete Step 2 before enabling PKCE.</Note>
      </Step>

      <Step title="Start the OAuth flow">
        Call one of the `oauth.$provider.start()` methods to start the OAuth flow. The SDK will generate a `code_verifier` and store it in local storage before redirecting the user to Stytch's OAuth start endpoint. If you use the SDK's built-in OAuth UI, this step is already handled for you.

        ```jsx theme={null}
        function OAuthStartButton() {
          const stytch = useStytch();
          const startOAuthFlow = useCallback(() => stytch.oauth.google.start({
            login_redirect_url: 'https://example.com/callback',
            signup_redirect_url: 'https://example.com/callback'
          }), [stytch])

          return (
            <button onClick={startOAuthFlow}>
              Click here to log in with Google
            </button>
          );
        }
        ```
      </Step>

      <Step title="Authenticate the OAuth token">
        At the route that the user is redirected to after the OAuth flow, pull the token out of the URL params and call `oauth.authenticate(token)` to exchange the token for a Stytch session. The SDK will automatically retrieve the `code_verifier` created earlier from local storage and pass it on to the Stytch servers.

        ```jsx theme={null}
        function OAuthCallback() {
          const stytch = useStytch();
          useEffect(() => {
            const params = new URLSearchParams(window.location.search);
            const token = params.get('token');
            if(!token) {
              // TODO: Handle redirect back to logged-out view
            }
            stytch.oauth.authenticate(token, {
              session_duration_minutes: 60,
            }).then(() => {
              // TODO: Redirect to final logged-in view
            })
          }, [stytch]);

          return (
            <div>
              Authenticating....
            </div>
          );
        }
        ```
      </Step>
    </Steps>
  </Tab>

  <Tab title="API">
    <Steps>
      <Step title="Create a Proof Key Pair">
        Create a `code_verifier` and `code_challenge` pair. To create the `code_verifier`, generate at least 256 random bits of entropy. This can be done by having a suitable random number generator create a 32-octet sequence. To create the `code_challenge`, hash the `code_verifier` using SHA256. Finally, base64url encode both to produce ASCII strings.

        ```javascript theme={null}
        const pkceChallenge = require("pkce-challenge")

        const { code_verifier, code_challenge } = pkceChallenge();
        console.log('code_verifier', code_verifier)
        console.log('code_challenge', code_challenge)
        ```
      </Step>

      <Step title="Associate the Proof Key with the User's Device">
        You'll need to associate the `code_verifier` created in the previous step with the user's device before starting the OAuth flow. One common way to do this is to set the `code_verifier` as an HTTP Only cookie. The exact mechanism to set cookies and redirect the user will depend on the web framework you are using. Be sure to include the `code_challenge` as a URL query param sent to Stytch!

        ```text theme={null}
        < HTTP/2 302
        < content-type: application/json
        < location: https://api.stytch.com/v1/public/oauth/google/start?public_token=PUBLIC_TOKEN&code_challenge=$CODE_CHALLENGE
        < set-cookie: code_verifier=$CODE_VERIFIER; Path=/; HttpOnly; Secure; SameSite=None
        ```
      </Step>

      <Step title="Authenticate the OAuth token with Stytch">
        After the user is redirected back from Stytch with a valid token, you'll need to exchange the token along with the original `code_verifier` to complete the flow and be issued a valid Stytch session. Retrieve the `code_verifier` from the cookie where it was stored in the previous step, and send it to Stytch's OAuth authenticate endpoint.

        ```javascript theme={null}
        const stytch = require("stytch")

        const client = new stytch.Client({
          project_id: "PROJECT_ID",
          secret: "SECRET",
          env: stytch.envs.test,
        });

        client.oauth.authenticate("SeiGwdj5lKkrEVgcEY3QNJXt6srxS3IK2Nwkar6mXD4=", {
          code_verifier: code_verifier
        })
          .then(resp => { console.log(resp) })
          .catch(err => { console.log(err) });
        ```
      </Step>

      <Step title="You're done!">
        You just finished all the critical components to authenticate your users via OAuth with PKCE! Have any feedback after integrating? Get in touch with us at [support@stytch.com](mailto:support@stytch.com).
      </Step>
    </Steps>
  </Tab>
</Tabs>