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Company B is launching a new game app for mobile devices. Users will log into the game using their existing
social media account to streamline data capture. Company B would like to directly save player data and scoring
information from the mobile app to a DynamoDS table named Score Data When a user saves their game the
progress data will be stored to the Game state S3 bucket.
What is the best approach for storing data to DynamoDB and S3?

A.
Use an EC2 Instance that is launched with an EC2 role providing access to the Score Data DynamoDB
table and the GameState S3 bucket that communicates with the mobile app via web services.

B.
Use temporary security credentials that assume a role providing access to the Score Data DynamoDB table
and the Game State S3 bucket using web identity federation.

C.
Use Login with Amazon allowing users to sign in with an Amazon account providing the mobile app with
access to the Score Data DynamoDB table and the Game State S3 bucket.

D.
Use an IAM user with access credentials assigned a role providing access to the Score Data DynamoDB
table and the Game State S3 bucket for distribution with the mobile app.

Explanation:
Web Identity Federation
Imagine that you are creating a mobile app that accesses AWS resources, such as a game that runs on a
mobile device and stores player and score information using Amazon S3 and DynamoDB.
When you write such an app, you’ll make requests to AWS services that must be signed with an AWS access
key. However, we strongly recommend that you do not embed or distribute long-term AWS credentials with
apps that a user downloads to a device, even in an encrypted store. Instead, build your app so that it requests
temporary AWS security credentials dynamically when needed using web identity federation. The supplied
temporary credentials map to an AWS role that has only the permissions needed to perform the tasks required
by the mobile app.
With web identity federation, you don’t need to create custom sign-in code or manage your own user identities.
Instead, users of your app can sign in using a well-known identity provider (IdP) —such as Login with Amazon,
Facebook, Google, or any other OpenID Connect (OIDC)-compatible IdP, receive an authentication token, and
then exchange that token for temporary security credentials in AWS that map to an IAM role with permissions
to use the resources in your AWS account. Using an IdP helps you keep your AWS account secure, becauseyou don’t have to embed and distribute long-term security credentials with your application.
For most scenarios, we recommend that you use Amazon Cognito because it acts as an identity broker and
does much of the federation work for you. For details, see the following section, Using Amazon Cognito for
Mobile Apps.
If you don’t use Amazon Cognito, then you must write code that interacts with a web IdP (Login with Amazon,
Facebook, Google, or any other OIDC-compatible IdP) and then calls the AssumeRoleWithWebIdentity API to
trade the authentication token you get from those IdPs for AWS temporary security credentials. If you have
already used this approach for existing apps, you can continue to use it.
Using Amazon Cognito for Mobile Apps
The preferred way to use web identity federation is to use Amazon Cognito. For example, Adele the developer
is building a game for a mobile device where user data such as scores and profiles is stored in Amazon S3 and
Amazon DynamoDB. Adele could also store this data locally on the device and use Amazon Cognito to keep it
synchronized across devices. She knows that for security and maintenance reasons, long-term AWS security
credentials should not be distributed with the game. She also knows that the game might have a large number
of users. For all of these reasons, she does not want to create new user identities in IAM for each player.
Instead, she builds the game so that users can sign in using an identity that they’ve already established with a
well-known identity provider, such as Login with Amazon, Facebook, Google, or any OpenID Connect (OIDC)-
compatible identity provider. Her game can take advantage of the authentication mechanism from one of these
providers to validate the user’s identity.
To enable the mobile app to access her AWS resources, Adele first registers for a developer ID with her
chosen IdPs. She also configures the application with each of these providers. In her AWS account that
contains the Amazon S3 bucket and DynamoDB table for the game, Adele uses Amazon Cognito to create IAM
roles that precisely define permissions that the game needs. If she is using an OIDC IdP, she also creates an
IAM OIDC identity provider entity to establish trust between her AWS account and the IdP.
In the app’s code, Adele calls the sign-in interface for the IdP that she configured previously. The IdP handles
all the details of letting the user sign in, and the app gets an OAuth access token or OIDC ID token from the
provider. Adele’s app can trade this authentication information for a set of temporary security credentials that
consist of an AWS access key ID, a secret access key, and a session token. The app can then use these
credentials to access web services offered by AWS. The app is limited to the permissions that are defined in
the role that it assumes.
The following figure shows a simplified flow for how this might work, using Login with Amazon as the IdP. For
Step 2, the app can also use Facebook, Google, or any OIDC-compatible identity provider, but that’s not shown
here.
Sample workflow using Amazon Cognito to federate users for a mobile application

A customer starts your app on a mobile device. The app asks the user to sign in.
The app uses Login with Amazon resources to accept the user’s credentials.The app uses Cognito APIs to exchange the Login with Amazon ID token for a Cognito token.
The app requests temporary security credentials from AWS STS, passing the Cognito token.
The temporary security credentials can be used by the app to access any AWS resources required by the app
to operate. The role associated with the temporary security credentials and its assigned policies determines
what can be accessed.
Use the following process to configure your app to use Amazon Cognito to authenticate users and give your
app access to AWS resources. For specific steps to accomplish this scenario, consult the documentation for
Amazon Cognito.
(Optional) Sign up as a developer with Login with Amazon, Facebook, Google, or any other OpenID Connect
(OIDC)–compatible identity provider and configure one or more apps with the provider. This step is optional
because Amazon Cognito also supports unauthenticated (guest) access for your users.
Go to Amazon Cognito in the AWS Management Console. Use the Amazon Cognito wizard to create an identity
pool, which is a container that Amazon Cognito uses to keep end user identities organized for your apps. You
can share identity pools between apps. When you set up an identity pool, Amazon Cognito creates one or two
IAM roles (one for authenticated identities, and one for unauthenticated “guest” identities) that define
permissions for Amazon Cognito users.
Download and integrate the AWS SDK for iOS or the AWS SDK for Android with your app, and import the files
required to use Amazon Cognito.
Create an instance of the Amazon Cognito credentials provider, passing the identity pool ID, your AWS account
number, and the Amazon Resource Name (ARN) of the roles
that you associated with the identity pool. The
Amazon Cognito wizard in the AWS Management Console provides sample code to help you get started.
When your app accesses an AWS resource, pass the credentials provider instance to the client object, which
passes temporary security credentials to the client. The permissions for the credentials are based on the role or
roles that you defined earlier.

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