webauthn.js

// paste in console of any https site to run (e.g. this page)
// sample arguments for registration
// https://fidoalliance.org/specs/fido-u2f-v1.1-id-20160915/fido-u2f-raw-message-formats-v1.1-id-20160915.html#authentication-response-message-success

var createCredentialDefaultArgs = {
    publicKey: {
        // Relying Party (a.k.a. - Service):
        rp: {
            name: "Acme"
        },

        // User:
        user: {
            id: new Uint8Array(16),
            name: "[email protected]",
            displayName: "John P. Smith"
        },

        pubKeyCredParams: [{
            type: "public-key",
            alg: -7
        }],

        attestation: "direct",

        timeout: 60000,

        challenge: new Uint8Array([ // must be a cryptographically random number sent from a server
            0x8C, 0x0A, 0x26, 0xFF, 0x22, 0x91, 0xC1, 0xE9, 0xB9, 0x4E, 0x2E, 0x17, 0x1A, 0x98, 0x6A, 0x73,
            0x71, 0x9D, 0x43, 0x48, 0xD5, 0xA7, 0x6A, 0x15, 0x7E, 0x38, 0x94, 0x52, 0x77, 0x97, 0x0F, 0xEF
        ]).buffer
    }
};

// sample arguments for login
var getCredentialDefaultArgs = {
    publicKey: {
        timeout: 60000,
        // allowCredentials: [newCredential] // see below
        challenge: new Uint8Array([ // must be a cryptographically random number sent from a server
            0x79, 0x50, 0x68, 0x71, 0xDA, 0xEE, 0xEE, 0xB9, 0x94, 0xC3, 0xC2, 0x15, 0x67, 0x65, 0x26, 0x22,
            0xE3, 0xF3, 0xAB, 0x3B, 0x78, 0x2E, 0xD5, 0x6F, 0x81, 0x26, 0xE2, 0xA6, 0x01, 0x7D, 0x74, 0x50
        ]).buffer
    },
};

// register / create a new credential
var cred = await navigator.credentials.create(createCredentialDefaultArgs)
console.log("NEW CREDENTIAL", cred);

// normally the credential IDs available for an account would come from a server
// but we can just copy them from above...
var idList = [{
    id: cred.rawId,
    transports: ["usb", "nfc", "ble"],
    type: "public-key"
}];
getCredentialDefaultArgs.publicKey.allowCredentials = idList;

var assertation = await navigator.credentials.get(getCredentialDefaultArgs);
console.log("ASSERTION", assertation);

// verify signature on server
var signature = await assertation.response.signature;
console.log("SIGNATURE", signature)

var clientDataJSON = await assertation.response.clientDataJSON;
console.log("clientDataJSON", clientDataJSON)

var authenticatorData = new Uint8Array(await assertation.response.authenticatorData);
console.log("authenticatorData", authenticatorData)

var clientDataHash = new Uint8Array(await crypto.subtle.digest("SHA-256", clientDataJSON));
console.log("clientDataHash", clientDataHash)

// concat authenticatorData and clientDataHash
var signedData = new Uint8Array(authenticatorData.length + clientDataHash.length);
signedData.set(authenticatorData);
signedData.set(clientDataHash, authenticatorData.length);
console.log("signedData", signedData)

// import key
var key = await crypto.subtle.importKey(
// The getPublicKey() operation thus returns the credential public key as a SubjectPublicKeyInfo. See:
// 
// https://w3c.github.io/webauthn/#sctn-public-key-easy
//
// crypto.subtle can import the spki format:
// 
// https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/importKey
  "spki", // "spki" Simple Public Key Infrastructure rfc2692

  cred.response.getPublicKey(),
  {
    // these are the algorithm options
    // await cred.response.getPublicKeyAlgorithm() // returns -7
    // -7 is ES256 with P-256 // search -7 in https://w3c.github.io/webauthn
    // the W3C webcrypto docs:
    //
    // https://www.w3.org/TR/WebCryptoAPI/#informative-references (scroll down a bit)
    //
    // ES256 corrisponds with the following AlgorithmIdentifier:
    name: "ECDSA",
    namedCurve: "P-256",
    hash: { name: "SHA-256" }
  },
  false, //whether the key is extractable (i.e. can be used in exportKey)
  ["verify"] //"verify" for public key import, "sign" for private key imports
);

// check signature with public key and signed data 
var verified = await crypto.subtle.verify(
  { name: "ECDSA", namedCurve: "P-256", hash: { name: "SHA-256" } },
  key,
  signature,
  signedData.buffer
);
// verified is false I want it to be true
console.log('verified', verified)

It wasn't the OS after all, sorry. I'm not sure why, but I now managed to produce the same through Chrome's virtual authenticator. Thanks anyway 🙂

I've put everything into a couple of highly-specific low-level libraries as well as a brief demo (without all the irrelevant boilerplate and cruft that the other ones I found carry).

I hope they can be helpful to others too. Any feedback is appreciated.

https://github.com/tjconcept/webauthn-tools
https://github.com/tjconcept/webauthn-json
https://github.com/tjconcept/webauthn-demo