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Custom User IDs


The following functionality is opt-in and is not required for typical use! SimpleWebAuthn remains focused on simplifying working with the WebAuthn API, and the functionality covered in Packages > @simplewebauthn/server will serve the majority of developers' use cases.


Some Relying Parties may wish to use their own format of user IDs instead of allowing generateRegistrationOptions() to generate a unique, random, WebAuthn-specific identifier to maximize user privacy.

In SimpleWebAuthn v9 it was possible to use such string values directly by simply assigning them to userID when calling generateRegistrationOptions(). As of SimpleWebAuthn v10, though, RP's now must account for the library-orchestrated use of base64url string encoding to get bytes to the browser during registration, and userHandle bytes to the server during authentication.


This is not a guide on how to use e.g. a user's email as their WebAuthn ID. Values used for userID MUST NOT contain personally-identifying information (PII)! A value like "WA1234567890ABC" is okay, but a value like "[email protected]" is not!

If you don't want to worry about any of this then consider skipping the rest of this guide and revisiting the general guidance offered in Packages > @simplewebauthn/server.


Use the isoUint8Array helper to convert the custom user identifier UTF-8 string into a Uint8Array:

import { isoUint8Array } from '@simplewebauthn/server/helpers';

const options = generateRegistrationOptions({
// ...
userID: isoUint8Array.fromUTF8String('customUserIDHere'),

The value will be the base64url-encoded UTF-8 bytes. When passed into @simplewebauthn/browser's startRegistration() the bytes will be decoded back to the raw UTF-8 bytes and passed to the authenticator.


@simplewebauthn/browser's startAuthentication() method will encode the raw credential.response.userHandle bytes out of the WebAuthn response to make it easy to send them to the back end.

Back on the server, the isoBase64URL helper can be used to convert userHandle back into a recognizable UTF-8 string:

import { isoBase64URL } from '@simplewebauthn/server/helpers';

const credential = await receiveFromBrowser();
isoBase64URL.toUTF8String(credential.response.userhandle), // 'customUserIDHere'

Error: String values for `userID` are no longer supported

@simplewebauthn/server's generateRegistrationOptions() will throw this error when a string value is passed in for the userID argument. To fix the problem, review the Registration section above for guidance on refactoring your code to massage your string identifier into a Uint8Array.