How do I let users create an account with Google Sign-In and then switch to passkeys?

A good pattern is:

1. Use Google Sign-In for first-time registration
2. Create a local account in your system tied to the Google identity
3. After the user is in, prompt them to add a passkey
4. Let the passkey become their preferred or primary sign-in method
5. Keep Google as a recovery / fallback option if you want

Recommended flow

1) User signs up with Google

• User clicks “Continue with Google”
• You verify the Google identity token on your backend
• Create or link a user record using the Google account’s stable identifier
  • Store something like:
    • user_id
    • google_sub / Google subject ID
    • email
    • verified email flag
• Log the user into your app

2) After sign-in, enroll a passkey

Once the user is authenticated with Google, show a prompt like:

• “Set up a passkey for faster sign-in next time”
• “Use Face ID, Touch ID, or device PIN to sign in”

Then:

• Generate a WebAuthn registration challenge on your server
• Send it to the client
• Call navigator.credentials.create(...)
• Verify the returned attestation on the server
• Store the credential public key and credential ID in your database

3) Use passkey for future sign-ins

When the user returns:

• Offer “Sign in with passkey”
• Generate a WebAuthn authentication challenge
• Use navigator.credentials.get(...)
• Verify the assertion on the server
• Log them in

4) Handle account linking carefully

If a user signs in with Google first, then later creates a passkey, the passkey should be attached to the same user account.

Important:

• Don’t create a separate account for the passkey
• Link the credential to the existing authenticated user
• Allow multiple passkeys per account if possible

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UX options

Option A: Google first, passkey later

Best for:

• Fast onboarding
• Easy conversion to passkeys after trust is established

Flow:

• Google Sign-In
• App session established
• Offer passkey enrollment immediately or after successful first use

Option B: Google as account creation, passkey as “upgrade”

Best for:

• Apps that want passwordless but need a reliable bootstrap method

Flow:

• Google creates the account
• User adds passkey
• Later they can sign in with passkey directly

Option C: Google + passkey side-by-side

Best for:

• Maximum convenience
• Better recovery and cross-device support

Flow:

• Both are valid login methods
• User can choose either at sign-in

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Things to watch out for

1) Email is not enough to link accounts

Google email can change or be unverified in some edge cases. Use the Google subject ID (sub) as the real unique identifier.

2) Passkeys need a stable relying party ID

Your passkey should be registered for your site’s domain, e.g.:

• example.com

The relying party ID must match your domain setup.

3) Cross-device passkeys

Modern passkeys can sync through iCloud Keychain / Google Password Manager, so users may be able to use them on other devices depending on their ecosystem.

4) Recovery

If passkey is the main sign-in method, consider recovery options:

• Google Sign-In fallback
• Email fallback
• Backup codes
• Support flow for lost device

5) Avoid account fragmentation

Make sure the user’s Google-based account and passkey-based login map to the same backend user record.

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Simple backend data model

You’ll usually want tables like:

Users

• id
• email
• google_sub
• created_at

WebAuthn credentials

• id
• user_id
• credential_id
• public_key
• sign_count
• transports
• created_at

A user can have:

• 1 Google identity
• 0 or more passkeys

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Best practice recommendation

If you want the smoothest transition:

• Use Google Sign-In to bootstrap trust
• Immediately ask to enroll a passkey after first successful login
• Make passkey the default sign-in option afterward
• Keep Google as backup unless you have a reason not to

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Example flow in one sentence

“Sign up with Google, verify the account on the backend, create the user record, then prompt the user to register a passkey and store that credential on the same account for future sign-in.”

If you want, I can also give you:

• a WebAuthn + Google Sign-In architecture diagram
• a backend implementation outline
• or sample code for a specific stack like Node.js, Python, Go, or Firebase.