Biometric login (webauthn) in Go, how to verify signature

Question

With the very recent Windows Anniversary update, Edge now supports biometric authentication using Windows Hello (cf. https://developer.microsoft.com/en-us/microsoft-edge/platform/documentation/dev-guide/device/web-authentication/ , https://blogs.windows.com/msedgedev/2016/04/12/a-world-without-passwords-windows-hello-in-microsoft-edge/ )

I have some samples in C#, PHP and Node.js, and am trying to make it work in Go.

The following works in JS (I have hardcoded in the challenge and the key):

function parseBase64(s) {
    s = s.replace(/-/g, "+").replace(/_/g, "/").replace(/\s/g, '');  
    return new Uint8Array(Array.prototype.map.call(atob(s), function (c) { return c.charCodeAt(0) }));  
}

function concatUint8Array(a1,a2) {
    var d = new Uint8Array(a1.length + a2.length);
    d.set(a1);
    d.set(a2,a1.length);
    return d;
}

var credAlgorithm = "RSASSA-PKCS1-v1_5";
var id,authenticatorData,signature,hash;
webauthn.getAssertion("chalenge").then(function(assertion) {
    id = assertion.credential.id;
    authenticatorData = assertion.authenticatorData;
    signature = assertion.signature;
    return crypto.subtle.digest("SHA-256",parseBase64(assertion.clientData));
}).then(function(h) {
    hash = new Uint8Array(h);
    var publicKey = "{\"kty\":\"RSA\",\"alg\":\"RS256\",\"ext\":false,\"n\":\"mEqGJwp0GL1oVwjRikkNfzd-Rkpb7vIbGodwQkTDsZT4_UE02WDaRa-PjxzL4lPZ4rUpV5SqVxM25aEIeGkEOR_8Xoqx7lpNKNOQs3E_o8hGBzQKpGcA7de678LeAUZdJZcnnQxXYjNf8St3aOIay7QrPoK8wQHEvv8Jqg7O1-pKEKCIwSKikCFHTxLhDDRo31KFG4XLWtLllCfEO6vmQTseT-_8OZPBSHOxR9VhIbY7VBhPq-PeAWURn3G52tQX-802waGmKBZ4B87YtEEPxCNbyyvlk8jRKP1KIrI49bgJhAe5Mow3yycQEnGuPDwLzmJ1lU6I4zgkyL1jI3Ghsw\",\"e\":\"AQAB\"}";
    return crypto.subtle.importKey("jwk",JSON.parse(publicKey),credAlgorithm,false,["verify"]);
}).then(function(key) {
    return crypto.subtle.verify({name:credAlgorithm, hash: { name: "SHA-256" }},key,parseBase64(signature),concatUint8Array(parseBase64(authenticatorData),hash));
}).then(function(result) {
    console.log("ID=" + id + "\r\n" + result);
}).catch(function(err) {
    console.log('got err: ', err);
});

In go I have the following code, meant to match the above JS code (req is a struct with strings from a JSON request body):

func webauthnSigninConversion(g string) ([]byte, error) {
    g = strings.Replace(g, "-", "+", -1)
    g = strings.Replace(g, "_", "/", -1)
    switch(len(g) % 4) { // Pad with trailing '='s
    case 0:
        // No pad chars in this case
    case 2:
        // Two pad chars
        g = g + "=="
    case 3:
        // One pad char
        g = g + "=";
    default:
        return nil, fmt.Errorf("invalid string in public key")
    }
    b, err := base64.StdEncoding.DecodeString(g)
    if err != nil {
        return nil, err
    }
    return b, nil
}


clientData, err := webauthnSigninConversion(req.ClientData)
if err != nil {
    return err
}

authenticatorData, err := webauthnSigninConversion(req.AuthenticatorData)
if err != nil {
    return err
}

signature, err := webauthnSigninConversion(req.Signature)
if err != nil {
    return err
}

publicKey := "{\"kty\":\"RSA\",\"alg\":\"RS256\",\"ext\":false,\"n\":\"mEqGJwp0GL1oVwjRikkNfzd-Rkpb7vIbGodwQkTDsZT4_UE02WDaRa-PjxzL4lPZ4rUpV5SqVxM25aEIeGkEOR_8Xoqx7lpNKNOQs3E_o8hGBzQKpGcA7de678LeAUZdJZcnnQxXYjNf8St3aOIay7QrPoK8wQHEvv8Jqg7O1-pKEKCIwSKikCFHTxLhDDRo31KFG4XLWtLllCfEO6vmQTseT-_8OZPBSHOxR9VhIbY7VBhPq-PeAWURn3G52tQX-802waGmKBZ4B87YtEEPxCNbyyvlk8jRKP1KIrI49bgJhAe5Mow3yycQEnGuPDwLzmJ1lU6I4zgkyL1jI3Ghsw\",\"e\":\"AQAB\"}" // this is really from a db, not hardcoded
// load json from public key, extract modulus and public exponent
obj := strings.Replace(publicKey, "\\", "", -1) // remove escapes
var k struct {
    N string `json:"n"`
    E string `json:"e"`
}
if err = json.Unmarshal([]byte(obj), &k); err != nil {
    return err
}
n, err := webauthnSigninConversion(k.N)
if err != nil {
    return err
}
e, err := webauthnSigninConversion(k.E)
if err != nil {
    return err
}
pk := &rsa.PublicKey{
    N: new(big.Int).SetBytes(n), // modulus
    E: int(new(big.Int).SetBytes(e).Uint64()), // public exponent
}
 
hash := sha256.Sum256(clientData)

// Create data buffer to verify signature over
b := append(authenticatorData, hash[:]...)
 
if err = rsa.VerifyPKCS1v15(pk, crypto.SHA256, b, signature); err != nil {
    return err
}

// if no error, signature matches

This code fails with crypto/rsa: input must be hashed message. If I change to using hash[:] instead of b in rsa.VerifyPKCS1v15, it fails with crypto/rsa: verification error. The reason I believe I need to combine authenticatorData and hash is because that is what happens in the C# and PHP sample codes (cf,  https://github.com/adrianba/fido-snippets/blob/master/csharp/app.cs , https://github.com/adrianba/fido-snippets/blob/master/php/fido-authenticator.php ).

Maybe Go does it a different way?

I have printed the byte arrays in JS and Go, and verified that clientData, signatureData, authenticatorData and hash (and the combined array of the latter two) have the exact same values. I have not been able to extract the n and e fields from JS after creating the public key, so there might be a problem in how I create the public key.

Answer 1

I'm not a crypto expert but I have some experience in Go, including verifying signatures that were signed with PHP. So, assuming the compared byte values are the same I would say that Your problem is probably the public key creation. I would suggest to try my solution of creating public keys from modulus and exponent with this function:

func CreatePublicKey(nStr, eStr string)(pubKey *rsa.PublicKey, err error){

    decN, err := base64.StdEncoding.DecodeString(nStr)
    n := big.NewInt(0)
    n.SetBytes(decN)

    decE, err := base64.StdEncoding.DecodeString(eStr)
    if err != nil {
        fmt.Println(err)
        return nil, err
    }
    var eBytes []byte
    if len(decE) < 8 {
        eBytes = make([]byte, 8-len(decE), 8)
        eBytes = append(eBytes, decE...)
    } else {
        eBytes = decE
    }
    eReader := bytes.NewReader(eBytes)
    var e uint64
    err = binary.Read(eReader, binary.BigEndian, &e)
    if err != nil {
        fmt.Println(err)
        return nil, err
    }
    pKey := rsa.PublicKey{N: n, E: int(e)}
    return &pKey, nil
}

I compared my public key and Yours (Playground), and they have different values. Could You please give me feedback of the solution I suggested with Your code if it's working?

Edit 1: URLEncoding example Playground 2

Edit 2: This is how I verify the signature:

hasher := sha256.New()
hasher.Write([]byte(data))
err = rsa.VerifyPKCS1v15(pubKey, crypto.SHA256, hasher.Sum(nil), signature)

So the 'data' variable in Edit 2 snippet is the same data(message) that has been used for signing on PHP side.