Adding multi-factor authentication (MFA) reduces the risk of user account take-over, phishing attacks, and password theft. Adding MFA while providing a frictionless sign-in experience requires you to offer a variety of MFA options that support a wide range of users and devices. Let’s see how you can achieve that with Amazon Cognito and Duo MFA.

Amazon Cognito user pools are user directories that are used by Amazon Web Services (AWS) customers to manage the identities of their customers and to add sign-in, sign-up and user management features to their customer-facing web and mobile applications. Duo Security is an APN Partner that provides unified access security and multi-factor authentication solutions.

In this blog post, I show you how to use Amazon Cognito custom authentication flow to integrate Duo MFA into your sign-in flow and offer a wide range of MFA options to your customers. Some second factors available through Duo MFA are mobile phone SMS passcodes, approval of login via phone call, push-notification-based approval on smartphones, biometrics on devices that support it, and security keys that can be attached via USB.

How it works

Amazon Cognito user pools enable you to build a custom authentication flow that authenticates users based on one or more challenge/response cycles. You can use this flow to integrate Duo MFA into your authentication as a custom challenge.

Duo Web offers a software development kit to make it easier for you to integrate your web applications with Duo MFA. You need an account with Duo and an application to protect (which can be created from the Duo admin dashboard). When you create your application in the Duo admin dashboard, note the integration key (ikey), secret key (skey), and API hostname. These details, together with a random string (akey) that you generate, are the primary factors used to integrate your Amazon Cognito user pool with Duo MFA.

Note: ikey, skey, and akey are referred to as Duo keys.

Duo MFA will be integrated into the sign-in flow as a custom challenge. To do that, you need to generate a signed challenge request using Duo APIs and use it to load Duo MFA in an iframe and request the user’s second factor. When the challenge is answered by the user, a signed response is returned to your application and sent to Amazon Cognito for verification. If the response is valid then the MFA challenge is successful.

Let’s take a closer look at the sequence of calls and components involved in this flow.

Implementation details

In this section, I walk you through the end-to-end flow of integrating Duo MFA with Amazon Cognito using a custom authentication flow. To help you with this integration, I built a demo project that provides deployment steps and sample code to create a working demo in your environment.

Create and configure a user pool

The first step is to create the AWS resources needed for the demo. You can do that by deploying the AWS CloudFormation stack as described in the demo project.

A few implementation details to be aware of:

  • The template creates an Amazon Cognito user pool, application client, and AWS Lambda triggers that are used for the custom authentication.
  • The template also accepts ikey, skey, and akey as inputs. For security, the parameters are masked in the AWS CloudFormation console. These parameters are stored in a secret in AWS Secrets Manager with a resource policy that allows relevant Lambda functions read access to that secret.
  • Duo keys are loaded from secrets manager at the initialization of create auth challenge and verify auth challenge Lambda triggers to be used to create sign-request and verify sign-response.

Authentication flow

Figure 1: User authentication process for the custom authentication flow

Figure 1: User authentication process for the custom authentication flow

The preceding sequence diagram (Figure 1) illustrates the sequence of calls to sign in a user, which are as follows:

  1. In your application, the user is presented with a sign-in UI that captures their user name and password and starts the sign-in flow. A script—running in the browser—starts the sign-in process using the Amazon Cognito authenticateUser API with CUSTOM_AUTH set as the authentication flow. This validates the user’s credentials using Secure Remote Password (SRP) protocol and moves on to the second challenge if the credentials are valid.

    Note: The authenticateUser API automatically starts the authentication process with SRP. The first challenge that’s sent to Amazon Cognito is SRP_A. This is followed by PASSWORD_VERIFIER to verify the user’s credentials.

  2. After the SRP challenge step, the define auth challenge Lambda trigger will return CUSTOM_CHALLENGE and this will move control to the create auth challenge trigger.
  3. The create auth challenge Lambda trigger creates a Duo signed request using the Duo keys plus the username and returns the signed request as a challenge to the client. Here is a sample code of what create auth challenge should look like:
    <JavaScript> exports.handler = async (event) => { //load duo keys from secrets manager and store them in global variables if(ikey == null || skey == null || akey == null){ const promise = new Promise(function(resolve, reject) { secretsManagerClient.getSecretValue({SecretId: secretName}, function(err, data) { if (err) {throw err; } else { if ('SecretString' in data) { secret = JSON.parse(data.SecretString); ikey = secret['duo-ikey']; skey = secret['duo-skey']; akey = secret['duo-akey']; } } resolve(); }); }) await promise; } var username = event.userName; var sig_request = duo_web.sign_request(ikey, skey, akey, username); event.response.publicChallengeParameters = { sig_request: sig_request }; return event;
    };
    

  4. The client initializes the Duo Web library with the signed request and displays Duo MFA in an iframe to request a second factor from the user. To initialize the Duo library, you need the api_hostname that is generated for your application in the Duo dashboard, the sign-request that was received as a challenge, and a callback function to invoke after the MFA step is completed by the user. This is done on the client side as follows:
    <JavaScript> //render Duo MFA iframe $("#duo-mfa").html('<iframe id="duo_iframe" title="Two-Factor Authentication" </iframe>'); Duo.init({ 'host': api_hostname, 'sig_request': challengeParameters.sig_request, 'submit_callback': mfa_callback });
    

  5. Through the Duo iframe, the user can set up their MFA preferences and respond to an MFA challenge. After successful MFA setup, a signed response from the Duo Web library will be returned to the client and passed to the callback function that was provided in Duo.init call.

    Figure 2: The first time a user signs in, Duo MFA displays a Start setup screen

    Figure 2: The first time a user signs in, Duo MFA displays a Start setup screen

  6. The client sends the Duo signed response to the Amazon Cognito service as a challenge response.
  7. Amazon Cognito sends the response to the verify auth challenge Lambda trigger, which uses Duo keys and username to verify the response.
    <JavaScript>
    const duo_web = require('duo_web');
    exports.handler = async (event) => { //load duo keys from secrets manager and store them in global variables var username = event.userName; //-------get challenge response const sig_response = event.request.challengeAnswer; const verificationResult = duo_web.verify_response(ikey, skey, akey, sig_response); if (verificationResult === username) { event.response.answerCorrect = true; } else { event.response.answerCorrect = false; } return event;
    };
    

  8. Validation results and current state are passed once again to the define auth challenge Lambda trigger. If the user response is valid, then the Duo MFA challenge is successful. You can then decide to introduce additional challenges to the user or issue tokens and complete the authentication process.

Conclusion

As you build your mobile or web application, keep in mind that using multi-factor authentication is an effective and recommended approach to protect your customers from account take-over, phishing, and the risks of weak or compromised passwords. Making multi-factor authentication easy for your customers enables you to offer authentication experience that protects their accounts but doesn’t slow them down.

Visit the security pillar of AWS Well-Architected Framework to learn more about AWS security best practices and recommendations.

In this blog post, I showed you how to integrate Duo MFA with an Amazon Cognito user pool. Visit the demo application and review the code samples in it to learn how to integrate this with your application.

If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, start a new thread on the Amazon Cognito forum or contact AWS Support.

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Author

Mahmoud Matouk

Mahmoud is a Senior Solutions Architect with the Amazon Cognito team. He helps AWS customers build secure and innovative solutions for various identity and access management scenarios.