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Custom Challenges

caution

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 > Server will serve the majority of developers' use cases.

Introduction

Some Relying Parties have highly specialized requirements that require greater control over how options challenges are generated, and how challenges in authenticator responses are verified. SimpleWebAuthn supports some of these use cases with the following capabilities:

Use a custom string for challenge

It is possible to specify custom strings for the challenge option when calling generateRegistrationOptions() and generateAuthenticationOptions(). These strings will be treated as UTF-8 bytes when preparing them for use as challenges:

import { generateRegistrationOptions } from '@simplewebauthn/server';

const options = await generateRegistrationOptions({
// ...
challenge: 'simplewebauthn',
});

Authenticators will sign over the base64url-encoded challenge as per the WebAuthn spec. This must be accounted for when specifying the same custom string as the expectedChallenge option when calling verifyRegistrationResponse() and verifyAuthenticationResponse():

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

const verification = await verifyRegistrationResponse({
// ...
expectedChallenge: isoBase64URL.fromString('simplewebauthn'),
});

Customize challenge verification logic with expectedChallenge

(Pulling from https://github.com/MasterKale/SimpleWebAuthn/pull/172)

A small amount of arbitrary information can be signed over during registration and authentication by overloading the challenge value with a stringified complex value. Call generateRegistrationOptions() or generateAuthenticationOptions() like usual, then override the challenge value to add in additional data in a way that is suitable for the use case:

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

const options = await generateAuthenticationOptions({ ... });

// Remember the plain challenge
inMemoryUserDeviceDB[loggedInUserId].currentChallenge = options.challenge;

// Add a simple amount of data to be signed using whatever
// data structure is most appropriate
options.challenge = isoBase64URL.fromString(JSON.stringify({
actualChallenge: options.challenge,
arbitraryData: 'arbitraryDataForSigning',
}));

After the WebAuthn ceremony is completed, call verifyRegistrationResponse() or verifyAuthenticationResponse() and pass in a function for expectedChallenge that accepts a challenge string and returns a boolean. This will need to perform the reverse of the logic used above to ensure that "actualChallenge" is the expected challenge:

const expectedChallenge = inMemoryUserDeviceDB[loggedInUserId].currentChallenge;

const verification = await verifyAuthenticationResponse({
// ...
expectedChallenge: (challenge: string) => {
const parsedChallenge = JSON.parse(base64url.decode(challenge));
return parsedChallenge.actualChallenge === expectedChallenge;
},
});
info

expectedChallenge can also be an asynchronous function to support e.g. making a network request to retrieve data needed to complete challenge verification.

Once the response is successfully verified then use the decodeClientDataJSON() helper to retrieve the arbitrary data:

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

const { challenge } = decodeClientDataJSON(body.response.clientDataJSON);
const parsedChallenge = JSON.parse(base64url.decode(challenge));
console.log(parsedChallenge.arbitraryData); // 'arbitraryDataForSigning'