App Flip for Android

OAuth-based App Flip linking (App Flip) inserts your Android app into the Google Account Linking flow. A traditional account linking flow requires the user to enter their credentials in the browser. The use of App Flip defers user sign-in to your Android app, which allows you to leverage existing authorizations. If the user is signed in to your app, they don't need to re-enter their credentials to link their account. A minimal amount of code changes are required to implement App Flip on your Android app.

In this document, you learn how to modify your Android app to support App Flip.

Try the sample

The App Flip linking sample app demonstrates an App Flip-compatible account linking integration on Android. You can use this app to verify how to respond to an incoming App Flip intent from Google mobile apps.

The sample app is preconfigured to integrate with the App Flip Test Tool for Android, which you can use to verify your Android app's integration with App Flip before you configure account linking with Google. This app simulates the intent triggered by Google mobile apps when App Flip is enabled.

How it works

The following steps are required to carry out an App Flip integration:

The Google app checks if your app is installed on the device using its package name.
The Google app uses a package signature check to validate that the installed app is the correct app.
The Google app builds an intent to start a designated activity in your app. This intent includes additional data required for linking. It also checks to see if your app supports App Flip by resolving this intent through the Android framework.
Your app validates that the request is coming from the Google app. To do so, your app checks the package signature and the provided client ID.
Your app requests an authorization code from your OAuth 2.0 server. At the end of this flow, it returns either an authorization code or an error to the Google app.
The Google app retrieves the result and continues with account linking. If an authorization code is provided, the token exchange happens server-to-server, the same way it does in the browser-based OAuth linking flow.

Modify your Android app to support App Flip

To support App Flip, make the following code changes to your Android app:

Add an <intent-filter> to your AndroidManifest.xml file with an action string that matches the value you entered in the App Flip Intent field.

<activity android:name="AuthActivity">
  <!-- Handle the app flip intent -->
  <intent-filter>
    <action android:name="INTENT_ACTION_FROM_CONSOLE"/>
    <category android:name="android.intent.category.DEFAULT"/>
  </intent-filter>
</activity>

Validate the calling app's signature.

private fun verifyFingerprint(
        expectedPackage: String,
        expectedFingerprint: String,
        algorithm: String
): Boolean {

    callingActivity?.packageName?.let {
        if (expectedPackage == it) {
            val packageInfo =
                packageManager.getPackageInfo(it, PackageManager.GET_SIGNATURES)
            val signatures = packageInfo.signatures
            val input = ByteArrayInputStream(signatures[0].toByteArray())

            val certificateFactory = CertificateFactory.getInstance("X509")
            val certificate =
                certificateFactory.generateCertificate(input) as X509Certificate
            val md = MessageDigest.getInstance(algorithm)
            val publicKey = md.digest(certificate.encoded)
            val fingerprint = publicKey.joinToString(":") { "%02X".format(it) }

            return (expectedFingerprint == fingerprint)
        }
    }
    return false
}

Extract the client ID from the intent parameters and verify that the client ID matches the expected value.

private const val EXPECTED_CLIENT = "<client-id-from-actions-console>"
private const val EXPECTED_PACKAGE = "<google-app-package-name>"
private const val EXPECTED_FINGERPRINT = "<google-app-signature>"
private const val ALGORITHM = "SHA-256"
...

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    val clientId = intent.getStringExtra("CLIENT_ID")

    if (clientId == EXPECTED_CLIENT &&
        verifyFingerprint(EXPECTED_PACKAGE, EXPECTED_FINGERPRINT, ALGORITHM)) {

        // ...authorize the user...
    }
}

Upon successful authorization, return the resulting authorization code back to Google.

// Successful result
val data = Intent().apply {
    putExtra("AUTHORIZATION_CODE", authCode)
}
setResult(Activity.RESULT_OK, data)
finish()

If an error occurred, return an error result instead.

Note: Do not handle the error directly within your app, instead return the error result via setResult. This ensures that the appropriate fallback method is trigerred if the app linking flow fails.
```
// Error result
val error = Intent().apply {
    putExtra("ERROR_TYPE", 1)
    putExtra("ERROR_CODE", 1)
    putExtra("ERROR_DESCRIPTION", "Invalid Request")
}
setResult(-2, error)
finish()
```

Content of the launch intent

The Android intent that launches your app includes the following fields:

CLIENT_ID (String): Google client_id registered under your app.
SCOPE (String[]): A list of scopes requested.
REDIRECT_URI (String): The redirect URL.

Content of the response data

The data returned to the Google app is set in your app by calling setResult(). This data includes the following:

AUTHORIZATION_CODE (String): The authorization code value.
resultCode (int): Communicates the success or failure of the process and takes one of the following values:
- Activity.RESULT_OK: Indicates success; an authorization code is returned.
- Activity.RESULT_CANCELLED: Signals that the user has cancelled the process. In this case, the Google app will attempt account linking using your authorization URL.
- -2: Indicates that an error has occurred. Different types of errors are described below.
ERROR_TYPE (int): The type of error, which takes one of the following values:
- 1: Recoverable error: The Google app will attempt account linking using the authorization URL.
- 2: Unrecoverable error: The Google app aborts account linking.
- 3: Invalid or missing request parameters.
ERROR_CODE (int): An integer representing the nature of the error. To see what each error code means, refer to the table of error codes.
ERROR_DESCRIPTION (String, optional): Human-readable status message describing the error.

Note: See the OAuth 2.0 standard for more information on possible errors and the optional contents of the error_description field.

A value for the AUTHORIZATION_CODE is expected when resultCode == Activity.RESULT_OK. In all other cases, the value for AUTHORIZATION_CODE needs to be empty. If resultCode == -2, then the ERROR_TYPE value is expected to be populated.

Table of error codes

The table below shows the different error codes and whether each is a recoverable or unrecoverable error:

Error code	Meaning	Recoverable	Unrecoverable
`1`	`INVALID_REQUEST`	✔
`2`	`NO_INTERNET_CONNECTION`		✔
`3`	`OFFLINE_MODE_ACTIVE`	✔
`4`	`CONNECTION_TIMEOUT`	✔
`5`	`INTERNAL_ERROR`	✔
`6`	`AUTHENTICATION_SERVICE_UNAVAILABLE`		✔
`8`	`CLIENT_VERIFICATION_FAILED`	✔
`9`	`INVALID_CLIENT`	✔
`10`	`INVALID_APP_ID`	✔
`11`	`INVALID_REQUEST`	✔
`12`	`AUTHENTICATION_SERVICE_UNKNOWN_ERROR`		✔
`13`	`AUTHENTICATION_DENIED_BY_USER`		✔
`14`	`CANCELLED_BY_USER`		✔
`15`	`FAILURE_OTHER`		✔
`16`	`USER_AUTHENTICATION_FAILED`	✔

For all error codes, you must return the error result via setResult to ensure the appropriate fallback is trigerred.