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FIDO2

This solution includes components that implement FIDO2 authentication, i.e. sign with Face, Touch, YubiKey, etc.:

  • FIDO2 credentials are stored in an Amazon DynamoDB table, so that a virtual unlimited amount of credentials can be stored, with details such as: friendly name ("My iPhone"), last used date, how many times used, etc.
  • Amazon REST API supports creating, updating and deleting FIDO2 credentials. This REST API is protected by a Cognito User Pools authorizer, meaning you must already be signed-in via different means (e.g, using Magic Link) to register a FIDO2 credential. Only the sign-in-challenge resource is public, to enable "usernameless" authentication, see below.
  • AWS Lambda functions that implement the Amazon Cognito Custom Authentication flow, reading FIDO2 credential public keys from the DynamoDB table.
  • Front End library functions, to work with this Custom Auth flow––can be used in Web, React, React Native.
  • Pre-built sample React component to add/update/delete authenticators.

FIDO2 credentials

Customizing Auth - e.g. to use custom FIDO2 challenges

If you want to do customization of this solution that goes beyond the parameters of the Passwordless construct, e.g. to use custom FIDO2 challenges, see CUSTOMIZE-AUTH.md

FIDO2 Browser Support

FIDO2 (/WebAuthn) is still a relatively new standard and not all browsers support it to the fullest yet. Currently, we recommend to use Chrome, as it best supports FIDO2 in our experience.

Here are some issues in other browsers that we know of today:

  • Mobile Safari on older iOS versions (seen on 16.0, no longer on 16.3) allows users to create multiple passkeys for the same username-website combination, whereby the older passkey is implicitly replaced by the newest. This can lead to confusion, as it's not possible for the FIDO2 backend of the solution here, to detect that the newer credential should replace the old one (as iOS did itself). Thus it simply stores the new credential without removing older ones. The user will then still see the older credentials in the credential list.
  • Firefox support for WebAuthn is limited (seen on 102.8, said to be fixed in 109), e.g. doesn't support security keys with PIN, nor MacOS Touch.

AWS Architecture

FIDO2 AWS Architecture

Registering new FIDO2 Authenticators

First you must sign-in with a magic link (or any other means). After that you can register a FIDO2 capable authenticator to log in with next time.

sequenceDiagram
    autonumber
    actor User
    participant BJS as Browser JavaScript
    participant BLS as Browser Storage
    participant BC as Browser WebAuthn core
    participant API as REST API+Lambda
    participant DB as DynamoDB
    BJS->>BLS: Query user FIDO2 enabled
    Activate BJS
    Activate BLS
    BLS->>BJS: null
    Deactivate BLS
    BJS->>User: Show enable-face-or-touch-login dialog
    Deactivate BJS
    Activate User
    User->>BJS: Click "Enable Face or Touch sign-in"
    Activate BJS
    BJS->>API: Start create-credential, include JWT (ID token)
    Activate API
    API->>API: Verify JWT
    API->>API: Generate random challenge
    API->>DB: Store challenge
    Activate DB
    DB->>API: OK
    Deactivate DB
    API->>BJS: Challenge and other FIDO2 options
    Deactivate API
    BJS->>BC: navigator.credentials.create()
    Activate BC
    BC->>User: Show register-authenticator native dialog
    User->>BC: Execute gesture (e.g. touch, face)
    BC->>BJS: FIDO2 public key response
    Deactivate BC
    BJS->>User: Show input-friendly-authenticator-name custom dialog
    User->>BJS: Friendly name
    BJS->>API: Complete create-credential
    Activate API
    API->>DB: Lookup challenge
    Activate DB
    DB->>API: Challenge
    Deactivate DB
    API->>API: Verify authenticator response
    API->>DB: Store credential
    Activate DB
    DB->>API: OK
    Deactivate DB
    API->>BJS: Credential metadata
    Deactivate API
    BJS->>BLS: Store FIDO2 enabled user
    Activate BLS
    BLS->>BJS: OK
    Deactivate BLS
    BJS->>User: "Authenticator activated successfully"
    Deactivate BJS
    Deactivate User
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Sign-in with username

If we know the username (e.g. because the user typed it in, or we have it in app storage), we can immediately initiate sign in (via Custom Auth flow) with Amazon Cognito:

sequenceDiagram
    autonumber
    actor User
    participant BJS as Browser JavaScript
    participant BLS as Browser Storage
    participant BC as Browser WebAuthn core
    participant C as Cognito
    participant DA as DefineAuth
    participant CA as CreateAuthChallenge
    participant VA as VerifyAnswer
    participant DB as DynamoDB
    User->>BJS: Open web app
    Activate User
    Activate BJS
    BJS->>BLS: Query FIDO2 enabled users
    Activate BLS
    BLS->>BJS: FIDO2 Users
    Deactivate BLS
    BJS->>User: Show username sign-in buttons
    Deactivate BJS
    User->>BJS: Click "Sign-in as <username> with face or touch"
    Activate BJS
    BJS->>C: InitiateAuth
    Activate C
    C->>DA: Invoke
    Activate DA
    DA->>C: Custom challenge
    Deactivate DA
    C->>CA: Invoke
    Activate CA
    CA->>CA: Generate challenge
    CA->>DB: Query credential IDs
    Activate DB
    DB->>CA: Credential IDs
    Deactivate DB
    CA->>C: FIDO2 challenge, credential IDs, options
    Deactivate CA
    C->>BJS: FIDO2 challenge, credential IDs, options
    Deactivate C
    BJS->>BC: navigator.credentials.get()
    Activate BC
    BC->>User: Show sign-use-authenticator native dialog
    User->>BC: Execute gesture (e.g. touch, face)
    BC->>BJS: FIDO2 signature response
    Deactivate BC
    BJS->>C: RespondToAuthChallenge
    Activate C
    C->>VA: Invoke
    Activate VA
    VA->>VA: Verify client data
    VA->>VA: Verify challenge
    VA->>DB: Get credential public key
    Activate DB
    DB->>VA: Credential public key
    Deactivate DB
    VA->>VA: Verify signature
    VA->>C: Answer correct: true
    Deactivate VA
    C->>DA: Invoke
    Activate DA
    DA->>C: Succeed Auth
    Deactivate DA
    C->> BJS: JWTs
    Deactivate C
    BJS->>BLS: Store JWTs
    Activate BLS
    BLS->>BJS: OK
    Deactivate BLS
    BJS->>User: "You are signed in"
    Deactivate BJS
    Deactivate User
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Sign-in with passkey––without username (usernameless)

To initiate sign-in (via Custom Auth flow) with Amazon Cognito, we first need to establish the username. This works as follows:

  1. We call the (public) API /sign-in-challenge
  2. This returns a challenge, that we'll use with WebAuthn authentication (in navigator.credentials.get()). Note that in this case we don't pass an array of credential IDs to navigator.credentials.get(), because we wouldn't know them yet.
  3. If the user has a Passkey (that is, a discoverable credential), the navigator.credentials.get() call will succeed. In its return value will be a userHandle, this will equal the username of the user in Amazon Cognito (in specific circumstances only––explanation below), so now we can initiate sign-in with Amazon Cognito similar as before.

Note on userHandle and username

This solution supports usernameless FIDO2 authentication only if you use opaque usernames for your users in Amazon Cognito, i.e. UUIDs. This is because only in that case will we use the username as the WebAuthn credential userHandle. Reason: the userHandle must be opaque, and must not give away who the owning human being is. So, if you use readable usernames (e.g. johndoe), this solution won't use that as userHandle but uses the user's sub (a UUID generated by Amazon Cognito) instead––but you cannot initiate sign-in with the sub (if different from username).

There is also a design choice here: we could store a separate, self defined opaque userHandle always, but then we'd always have to map that userHandle to an actual username, before we can initiate sign-in with Amazon Cognito. If username is already opaque, we can use it as userhandle and the mapping userHandle to user is done implictly by Amazon Cognito.

Potential scenario's depending on your User Pool's sign-in configuration:

  • If you have configured your Amazon Cognito User Pool to only use e-mail and/or phone number to sign in with, then under the hood Amazon Cognito will generate an opaque username for you (same as sub) and usernameless authentication can work.
  • If you have configured your Amazon Cognito User Pool to use e-mail and/or phone number to sign in with, but also allow an actual username (user or admin supplied), you must use a UUID as username––if so, usernameless authentication can work.
  • If you have configured your Amazon Cognito User Pool to only use a username to sign in with, usernameless authentication cannot work (unless, a very unlikely scenario, users use UUIDs for their username).

See below how the usernameless authentication flow works. Notes:

  • The WebAuthn interaction (navigator.credentials.get()) now takes place before invoking InitiateAuth with Amazon Cognito (in the flow with username it's after).
  • The CreateAuthChallenge trigger will return a WebAuthn challenge also (it always does this if you've enabled FIDO2), but that challenge won't be used by the client (it already signed the challenge it got earlier from the /sign-in-challenge API endpoint).
  • To add replay protection, the challenge that the API created and stored in DynamoDB, is removed from DynamoDB during verification of the challenge (atomic read+delete).
sequenceDiagram
    autonumber
    actor User
    participant BJS as Browser JavaScript
    participant BLS as Browser Storage
    participant BC as Browser WebAuthn core
    participant API as REST API+Lambda
    participant C as Cognito
    participant DA as DefineAuth
    participant CA as CreateAuthChallenge
    participant VA as VerifyAnswer
    participant DB as DynamoDB
    User->>BJS: Open web app
    Activate User
    Activate BJS
    BJS->>User: Show "Sign in with passkey" button
    Deactivate BJS
    User->>BJS: Click "Sign in with passkey"
    Activate BJS
    BJS->>API: POST /sign-in-challenge
    Activate API
    API->>API: Generate challenge
    API->>DB: Store challenge
    Activate DB
    DB->>API: Ok
    Deactivate DB
    API->>BJS: Challenge
    BJS->>BC: navigator.credentials.get()
    Activate BC
    BC->>User: Show sign-use-authenticator native dialog
    User->>BC: Execute gesture (e.g. touch, face)
    BC->>BJS: FIDO2 signature response
    Deactivate BC
    BJS->>C: InitiateAuth
    Activate C
    C->>DA: Invoke
    Activate DA
    DA->>C: Custom challenge
    Deactivate DA
    C->>CA: Invoke
    Activate CA
    CA->>CA: Generate challenge
    CA->>DB: Query credential IDs
    Activate DB
    DB->>CA: Credential IDs
    Deactivate DB
    CA->>C: FIDO2 challenge, credential IDs, options
    Deactivate CA
    C->>BJS: FIDO2 challenge, credential IDs, options
    Deactivate C
    BJS->>C: RespondToAuthChallenge
    Activate C
    C->>VA: Invoke
    Activate VA
    VA->>VA: Verify client data
    VA->>DB: Check challenge exists (and delete if so)
    Activate DB
    DB->>VA: OK
    Deactivate DB
    VA->>DB: Get credential public key
    Activate DB
    DB->>VA: Credential public key
    Deactivate DB
    VA->>VA: Verify signature
    VA->>C: Answer correct: true
    Deactivate VA
    C->>DA: Invoke
    Activate DA
    DA->>C: Succeed Auth
    Deactivate DA
    C->> BJS: JWTs
    Deactivate C
    BJS->>BLS: Store JWTs
    Activate BLS
    BLS->>BJS: OK
    Deactivate BLS
    BJS->>User: "You are signed in"
    Deactivate BJS
    Deactivate User
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