How to achieve authorization code flow with PKCE with mobile app (React Native)?

To achieve Authorization Code Flow with Proof Key for Code Exchange (PKCE) in a React Native mobile app, you'll need to follow these steps:

1. Register your App with the Authorization Server: First, you need to register your mobile app with the authorization server (e.g., OAuth 2.0 server) to obtain client credentials (client ID and client secret). This registration process may vary depending on the authorization server you are using.

2. Implement PKCE in your React Native App: PKCE is a security enhancement for OAuth 2.0 Authorization Code Flow that is specifically designed for public clients (such as mobile apps). It prevents authorization code interception attacks.

   In your React Native app, you need to generate a random "code verifier" and its "code challenge." The code challenge is a SHA-256 hash of the code verifier. These values will be used during the authorization process.

3. Open the Authorization URL in a Webview: To start the authorization process, open a webview (WebView or InAppBrowser) in your React Native app and direct the user to the authorization endpoint of the authorization server. Include the necessary parameters, such as client ID, response type (code), redirect URI, and the generated code challenge.

4. User Grants Authorization: The user will be prompted to log in and grant the requested permissions (scopes) to your app. After granting authorization, the authorization server will redirect the user back to the specified redirect URI with an authorization code.

5. Exchange Authorization Code for an Access Token: Once you receive the authorization code in the redirect URI, exchange it for an access token by making a POST request to the token endpoint of the authorization server. Include the code verifier in this request to verify the code.

6. Use the Access Token for API Calls: Use the obtained access token to authenticate API calls to protected resources on the resource server.

Here is a basic example of implementing PKCE in React Native using the react-native-app-auth library:

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import { authorize, refresh, revoke } from 'react-native-app-auth';

const config = {
  clientId: 'YOUR_CLIENT_ID',
  redirectUrl: 'YOUR_REDIRECT_URI',
  scopes: ['openid', 'profile', 'email'], // Adjust scopes based on your requirements
  serviceConfiguration: {
    authorizationEndpoint: 'AUTHORIZATION_ENDPOINT_URL',
    tokenEndpoint: 'TOKEN_ENDPOINT_URL',
  },
};

const generateRandomString = (length) => {
  // Implement a function to generate a random string (code verifier)
};

const calculateCodeChallenge = (codeVerifier) => {
  // Implement a function to calculate the code challenge (SHA-256 hash of code verifier)
};

const login = async () => {
  const codeVerifier = generateRandomString(64);
  const codeChallenge = calculateCodeChallenge(codeVerifier);

  const authConfig = {
    ...config,
    codeChallenge,
    codeChallengeMethod: 'S256',
  };

  try {
    const result = await authorize(authConfig);
    console.log('Authorization result:', result);
    // Use the result.authorizationCode to exchange for an access token
  } catch (error) {
    console.error('Authorization error:', error);
  }
};

Remember to adjust the configuration and URLs with your specific authorization server details.

For a more complete implementation, you may also need to handle token expiration, token refreshing, and token revocation.

Note: Make sure to store sensitive data like the client secret securely, and follow best practices for securing your React Native app and handling OAuth tokens.