Google has released offical docs and a sample project

Google released a codelab

This project uses com.google.android.gms:play-services-fido's Fido2ApiClient to register a credential and also sign a challenge.

It demonstrates the Fido2 Api with hardcoded values and is NOT a full example that gets requests from a server

It also does not validate any of the responses, as that would be done server side

Just run the app and tap the Register Fido2 button. It will display the results on the app and in the logs.

Then you can tap the Sign Fido2 button, and the Fido2 api will be used to sign a challenge with the key that was just generated

As long as you do not change the Relying Party ID (in PublicKeyCredentialRpEntity), the signing key or the package name, the app should just work

According to the Web Authentication spec the relying party id is:

A valid domain string that identifies the WebAuthn Relying Party on whose behalf a given registration or authentication ceremony is being performed. A public key credential can only be used for authentication with the same entity (as identified by RP ID) it was registered with.

According to MakeCredentialOptions.Builder, Very hard to find, kind of hidden under Public Methods and then setRp:

Note: the RpId should be an effective domain (aka, without scheme or port); and it should also be in secure context (aka https connection). Apps-facing API needs to check the package signature against Digital Asset Links, whose resource is the RP ID with prepended "//". Privileged (browser) API doesn't need the check.

So for Android apps you need to host an assetlinks.json file on https://<rp_id>/.well-known/assetlinks.json to allow it to use the Fido2 apis for that domain.

For this sample app I have set the RP ID to strategics-fido2.firebaseapp.com and I am hosting this assetlinks.json:

[
  {
    "relation": ["delegate_permission/common.handle_all_urls","delegate_permission/common.get_login_creds"],
    "target": {
      "namespace": "android_app",
      "package_name": "com.entersekt.fido2",
      "sha256_cert_fingerprints": [
        "C5:8B:E3:9B:36:B3:67:12:D7:0C:DA:C5:9D:65:2A:FC:43:9B:AE:1B:76:C9:7D:A1:7E:69:2B:7A:15:AB:27:96"
      ]
    }
  }
]

on https://strategics-fido2.firebaseapp.com/.well-known/assetlinks.json where:

package_name matches the applicationId in my build.gradle and the sha256_cert_fingerprints matches the fingerprint of my signing key. This can be found by running the get_cert_fingerprint.sh script included in this project

If you want to host your own assetlinks.json for a test, there is a firebase hosting project included in the web folder

You need to have a firebase project set up that you can deploy this site to first. When that is done, go to the web folder and run:

firebase deploy --only hosting

You should now have the local assetlinks.json hosted on https://<firebase-project-id>.firebaseapp.com/.well-known/assetlinks.json and just need to set your RP ID to <firebase-project-id>.firebaseapp.com