Fixing PKCE Issues in Android Applications with Expo and Epic Integration

Facing PKCE Errors with Expo? Here's What You Need to Know to Connect with Epic

When building an that requires secure authentication, like those that connect with healthcare systems such as Epic, developers often run into unique challenges. One of the common issues is configuring the PKCE (Proof Key for Code Exchange) correctly. This error can be frustrating, especially when every configuration appears correct but you still receive error messages regarding invalid or missing parameters.

In this case, developers working with in Expo may experience an error stating “PKCE required for unsecured redirects,” which could stem from how the redirect URI is configured locally. Even after setting the and accurately, this error can persist if certain elements are misconfigured.

Resolving these errors requires a deep dive into how PKCE works and ensuring your app's security parameters align with the Epic platform's requirements. This article will help break down potential solutions to make sure the authentication process flows smoothly.

If you're stuck on this problem and wondering what might be missing, you're not alone! We'll go through common causes of the PKCE error and provide tips to help you fix it quickly and continue building your app with confidence 🚀.

Command	Example of use
useAuthRequest	Initializes the authentication request with specific parameters for PKCE, including response type, client ID, and endpoints. This command directly helps manage the OAuth flow for secure authorization by setting up request parameters to be sent to the Epic authorization server.
CodeChallengeMethod.S256	Defines the hashing method for the PKCE challenge. "S256" is the SHA-256 hashing standard, which is required for security-sensitive applications such as Epic integrations and ensures that the code verifier is encrypted correctly during authorization.
pkceChallenge()	Generates the PKCE codeChallenge and codeVerifier pair. This command is essential for setting up secure PKCE flow, as it provides the unique codes needed for the client to be authenticated securely by the server.
makeRedirectUri	Generates a redirect URI specific to the Expo environment, which helps localize and route the authentication flow back to the app. This command is crucial for Expo-based apps to handle authentication redirects effectively.
authorizationEndpoint	Specifies the URL for the authorization server where the user is directed to authenticate. This command sets up the endpoint in the useAuthRequest function to ensure that authorization requests are sent to the correct location for Epic’s OAuth server.
tokenEndpoint	Defines the endpoint for exchanging the authorization code for an access token. This command is critical in the OAuth flow as it directs the request to obtain access tokens, which are used for API access.
promptAsync	Triggers the authentication prompt asynchronously. This command initiates the actual authorization process, making it essential for handling user interaction with the Epic authentication server.
useEffect	Used to handle side effects and check the authentication result after the authorization flow completes. This command is important for tracking the result status (success or error) and handling it accordingly in the app.
responseType	Defines the type of response expected from the authorization server, set to “code” for the PKCE OAuth flow. This command ensures the client receives an authorization code, which is then exchanged for an access token.
scopes	Lists the specific permissions or resources the app requests from the authorization server, e.g., fhirUser for accessing user-specific healthcare data. This command helps limit access to only the necessary resources.

Using Expo-Auth-Session for PKCE Authentication in Epic API Integration

The scripts above are designed to handle PKCE (Proof Key for Code Exchange) authentication within an Expo app that connects to Epic’s secure healthcare APIs. By using the expo-auth-session library, developers can set up the OAuth process in a secure, flexible way, with parameters specific to Epic’s requirements. PKCE is essential here because it adds an extra layer of security to the authorization process, especially important when dealing with sensitive healthcare data. For example, when a healthcare provider needs to authorize access to their medical records, using PKCE helps ensure that this request cannot be tampered with. With the function, this script sets up the request parameters that the app needs to send to Epic's authorization server, including a (to identify the app), a , and the PKCE code challenge.

Another crucial part of this script is the function, which generates the code challenge and code verifier values needed for the PKCE flow. This function ensures that each session is uniquely secured, a must-have when using open internet connections, such as in public settings where data is more vulnerable. The makeRedirectUri command is then used to configure the app's redirect URI, which essentially tells Epic’s server where to redirect users after they authenticate. Here, we see the redirect URI formatted to work specifically within an Expo app environment, which is unique as it allows for handling authentication both locally and in production. This format is especially useful for developers testing apps on localhost or simulators, ensuring a smooth and secure experience for users signing in. 🛡️

The script’s other parameters, such as and , specify the specific endpoints needed for Epic’s authorization process. The authorizationEndpoint is where users are sent to login, and tokenEndpoint is where the authorization code is exchanged for an access token. This setup is crucial for a seamless user experience; without it, users might encounter issues with misconfigured endpoints, resulting in broken or insecure authentication flows. A practical scenario of this would be a clinician accessing Epic’s FHIR API to review patient information on their app. If these endpoints are configured correctly, they’re redirected seamlessly back to the app with authorized access to the data.

Finally, promptAsync is used to execute the request asynchronously, which means the app doesn’t freeze while waiting for the user to authenticate. This function essentially controls the actual interaction where the app redirects the user to the Epic login and then awaits their authentication response. In practice, this prevents users from feeling like the app is unresponsive, which is especially important for maintaining a high-quality user experience. Together, these commands create a streamlined and secure PKCE authentication flow, making it easier to work within the highly regulated healthcare space while building reliable, user-friendly applications. 📲

import { useAuthRequest, CodeChallengeMethod, makeRedirectUri } from 'expo-auth-session';
import pkceChallenge from 'pkce-challenge';
const { codeChallenge, codeVerifier } = pkceChallenge();
const redirectUri = makeRedirectUri({ scheme: 'exp' });
const [request, result, promptAsync] = useAuthRequest(
    {
        usePKCE: true,
        responseType: 'code',
        clientId: 'epicClientId',
        redirectUri,
        scopes: ['fhirUser'],
        codeChallengeMethod: CodeChallengeMethod.S256,
        codeChallenge,
        extraParams: { aud: 'my FHIR R4 URL' }
    },
    {
        authorizationEndpoint: 'https://auth.epic.com/authorize',
        tokenEndpoint: 'https://auth.epic.com/token'
    }
);
const handleAuth = async () => {
    const authResult = await promptAsync();
    if (authResult.type === 'success') {
        console.log('Authentication successful:', authResult);
    } else {
        console.error('Authentication failed:', authResult.error);
    }
};

import { useAuthRequest, CodeChallengeMethod } from 'expo-auth-session';
import pkceChallenge from 'pkce-challenge';
const { codeChallenge, codeVerifier } = pkceChallenge();
const redirectUri = 'exp://localhost:8081'; // For development setup
const [request, result, promptAsync] = useAuthRequest(
    {
        usePKCE: true,
        responseType: 'code',
        clientId: process.env.EPIC_CLIENT_ID,
        redirectUri,
        scopes: ['fhirUser'],
        codeChallengeMethod: CodeChallengeMethod.S256,
        codeChallenge,
    },
    {
        authorizationEndpoint: 'https://auth.epic.com/authorize',
        tokenEndpoint: 'https://auth.epic.com/token'
    }
);
useEffect(() => {
    if (result?.type === 'error') {
        console.error('Authentication error:', result?.error);
    }
}, [result]);

import { useAuthRequest } from 'expo-auth-session';
import pkceChallenge from 'pkce-challenge';
import { renderHook } from '@testing-library/react-hooks';
test('PKCE setup test', async () => {
    const { codeChallenge, codeVerifier } = pkceChallenge();
    const [request, result, promptAsync] = useAuthRequest(
        {
            usePKCE: true,
            responseType: 'code',
            clientId: 'testClientId',
            redirectUri: 'exp://localhost:8081',
            scopes: ['fhirUser'],
            codeChallengeMethod: 'S256',
            codeChallenge,
        },
        {
            authorizationEndpoint: 'https://auth.epic.com/authorize',
            tokenEndpoint: 'https://auth.epic.com/token'
        }
    );
    expect(request).toBeTruthy();
    expect(codeChallenge).toBeTruthy();
    expect(promptAsync).toBeInstanceOf(Function);
});

Optimizing PKCE Configuration in Expo for Enhanced Security with Epic API

When building apps that need to securely connect to healthcare systems like Epic, fine-tuning the PKCE setup is crucial to avoid common authentication pitfalls. Although PKCE adds an additional layer of security, it can require meticulous configuration, especially when dealing with local testing environments. The is a common source of errors here. Epic’s OAuth server, for example, strictly requires that redirect URIs be registered and match what’s used in the application. Setting up the redirect URI in Expo can sometimes lead to issues, particularly in local development environments where Expo uses specific URLs (like exp://192.168.x.x) that may not match exactly with registered URIs.

One way to handle this is by ensuring the redirect URI generated by is precisely the URI registered in the server settings, adjusting any schemes if necessary. Another approach to solve redirect URI issues is by switching between local and production setups based on environment variables, which can help maintain flexibility without needing to re-register URIs. For instance, a developer could use a in Expo to accommodate both localhost testing and production environments seamlessly.

In addition, understanding how work with Epic’s API is vital for successful PKCE authentication. Scopes define the permissions your app requests from users. Choosing the correct scopes is essential for specific healthcare data access, such as Epic’s scope, which gives access to FHIR data for the authenticated user. Scopes can also impact the redirection process, so ensuring that they’re configured correctly reduces the chance of errors in the PKCE flow. Implementing these configurations carefully can create a more reliable, error-free connection, ensuring that your app handles secure data requests smoothly. 🚀

1. What is the purpose of in PKCE authentication?
2. is used to set up the authentication request with necessary parameters, such as client ID, redirect URI, and endpoints, which are required to initiate PKCE-based OAuth flows.
3. How can I avoid issues with local redirect URIs in Expo?
4. To avoid redirect URI issues, make sure your redirect URI in the app exactly matches what’s registered on the server. Using with the right scheme can help, or try using environment variables to switch URIs for local and production setups.
5. What does do, and why is it necessary?
6. generates a unique code challenge and code verifier, which are essential for the PKCE flow. It secures the authentication process by ensuring only authorized requests are accepted by the server.
7. Why am I receiving a PKCE error about unsecured redirects?
8. This error often occurs when the redirect URI doesn’t match the URI registered with Epic’s server. Make sure your app’s redirect URI is listed on the server, especially for local testing where URIs can vary.
9. How do I configure the correct scopes in Expo?
10. Scopes determine the level of data access granted by the API. Configure the scopes in by setting them in the scopes array, e.g., for access to FHIR data related to the user.

Getting PKCE set up correctly is essential for building a secure connection with Epic's APIs, especially in a development environment with strict URI matching. Minor adjustments, like ensuring the redirect URI exactly matches the registered one or using environment-based URIs, can prevent many PKCE errors.

By understanding the nuances of PKCE and adjusting configurations accordingly, developers can resolve these errors effectively and create a smoother authentication flow. With the right setup, app users can authenticate securely and confidently, knowing their data is protected. 👍