Possible compatibility issue between this library and the .NET SDK

[balazserdeszlogmein]

[Note: this has also been posted on the .NET SDK as an issue here.]

Hi,

I have noticed a possible compatibility issue between this library and your .NET SDK.

I am creating (registering) a U2F credential with the libfido2 command line tool like this on a macOS operating system.

bash

% echo credential challenge | openssl sha256 -binary | base64 > cred_param    
% echo relying_party >> cred_param                   
% echo username >> cred_param                       
% dd if=/dev/urandom bs=1 count=32 | base64 >> cred_param  

then bash:

% fido2-cred -M -u -i cred_param ioreg://4295049894 | fido2-cred -V -o cred

which results in a file called "cred" in the following format:

Hy8EHTL3JjA0UpNavz[...]TiLCi/5WeGnvzHyq8bonq7y0YQ==
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj[...]09K4FOmgk3dHXzSgcA==
-----END PUBLIC KEY-----

Then, we also have a credential assertion (authentication) application, written with this .NET SDK, roughly doing the following on a Windows operating system:

1. Getting two inputs from the user, private key handle and public key as bas64 strings:

C#

var privateKeyHandle = Convert.FromBase64String(s: PrivateKeyHandleBase64);
var publicKey = Convert.FromBase64String(s: PublicKeyBase64);

2. Authenticating with these credentials

C#

IYubiKeyDevice yubiKey = YubiKeyDevice.FindAll().FirstOrDefault();

if (yubiKey is null) { return null; }

using (var session = new U2fSession(yubiKey: yubiKey))
{
    byte[] hashOrigin = U2fSession.EncodeAndHashString(data: "relying_party");

    var clientData = new FidoU2fClientData
    {
        Challenge = GenerateChallengeDataBase64(), // Not putting the implementation here, it is unnecessary
        Origin = "relying_party"
    };

    string jsonClientData = JsonConvert.SerializeObject(value: clientData);

    byte[] hashClientData = U2fSession.EncodeAndHashString(data: jsonClientData);

    AuthenticationData authenticationData = session.Authenticate(
        applicationId: hashOrigin,
        clientDataHash: hashClientData,
        keyHandle: privateKeyHandle,
        timeout: TimeSpan.FromSeconds(10)
    );
}

When passing the credential id (private key handle) created on macOS with fido2-cred, calling the last session.Authenticate function throws an error, namely new ArgumentException(ExceptionMessages.UnsupportedAlgorithm); in line 401 of the EcdsaVerify.cs file, because at line 368 in the private static ECDsa ConvertPublicKey(ReadOnlyMemory<byte> encodedEccPoint) the (encodedEccPoint.Span[0] == EncodedPointTag) condition is not met.

Based on this, what we realized after a while, is that if we base64 decode the string of the credential id (key handle), prepend a 04 byte, then base64 encode it back to a string, the above will work and the authentication is going to be successful. Why is there a difference between the U2F implementation for LibFido2 and the .NET SDK?

Let me know if anything else is needed from my side to investigate this issue.

Thanks in advance, Balazs

[LDVG]

The fido2-cred output contains a PEM-encoded public key, itself encoded in the SubjectPublicKeyInfo format. Furthermore, libfido2's API allows you to pick a suitable serialization yourself, depending on your needs.

I see @GregDomzalski has got you covered in your other issue, so I'll be closing this.