Investigate support for X509 Certs and EC Keys

[josephdecock]

EC Keys in X509 certs came up in DuendeSoftware/Support#1154, and I'm curious if the situation has improved since we originally implemented the key manager. Or possibly, can we use Microsoft.IdentityModel.Tokens.ECDsaSecurityKey instead of Microsoft.IdentityModel.Tokens.X509SecurityKey?

Alternatively, is there some other helper we could build that would make dealing with EC keys more pleasant?

Here's a bit of code from the KeyManager.cs that comments on the issue:

 else if (alg.IsEcKey)
        {
            var ec = CryptoHelper.CreateECDsaSecurityKey(CryptoHelper.GetCurveNameFromSigningAlgorithm(alg.Name));
            // X509 certs don't currently work with EC keys.
            container = //_options.KeyManagement.WrapKeysInX509Certificate ? //new X509KeyContainer(ec, alg, now, _options.KeyManagement.KeyRetirementAge, iss) :
                (KeyContainer) new EcKeyContainer(ec, alg.Name, now);
        }

[joegoldman2]

X509SecurityKey will support EC keys with AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet#2377.