public static void VerifySignature(PublicKeyCredential cred, byte[] authData, byte[] clientData, byte[] signature, string rpid)
{
if (cred == null)
{
throw new ArgumentNullException(nameof(cred));
}
var payload = authData.Concat(CredentialUtility.Hash(clientData)).ToArray();
var rawSignature = DERSignature.Deserialize(signature);
var authDataSpan = authData.AsSpan();
var rpidHash = authDataSpan.Slice(0, 32);
var flags = (AuthenticatorDataFlags)authDataSpan[32];
var counterSpan = authDataSpan.Slice(33, 4);
counterSpan.Reverse();
uint counter = BitConverter.ToUInt32(counterSpan);
if ((flags & AuthenticatorDataFlags.UserPresent) == 0)
{
throw new Exception("user does not present");
}
if (!rpidHash.SequenceEqual(CredentialUtility.Hash(Encoding.UTF8.GetBytes(rpid))))
{
throw new Exception("RP ID Hash does not match");
}
var publicKey = CBORObject.DecodeFromBytes(cred.PublicKey);
var x = publicKey.MapGet(-2).GetByteString();
var y = publicKey.MapGet(-3).GetByteString();
var ecDsa = ECDsa.Create(new ECParameters
{
Curve = ECCurve.NamedCurves.nistP256,
Q = new ECPoint {
X = x, Y = y
}
});
var isValid = ecDsa.VerifyData(payload, rawSignature, HashAlgorithmName.SHA256);
if (!isValid)
{
throw new Exception("invalid signature");
}
if (cred.SignCounter >= counter)
{
throw new Exception("invalid signature counter");
}
cred.SignCounter = counter;
}
public void Add(PublicKeyCredential cred) => _data.Add(cred);