Version 12.0.0, which is due for release in late November, will make the protocol accessible in the IUserInfo interface (and, by extension, the IAuthenticationInfo interface), as shown below:
public interface IUserInfo
{
    IPEndPoint RemoteEndPoint { get; }
    IPEndPoint LocalEndPoint { get; }
    string Protocol { get; }
    X509Certificate2 ClientCertificate { get; }
    string UserName { get; }
    bool IsValidUserName { get; set; }
    string SiteName { get; }
    Guid SiteID { get; }
    ISession Session { get; }
    string DefaultDomain { get; }
    string HomeDirectory { get; set; }
    List<string> Groups { get; }
}
Note the LocalEndPoint and ClientCertificate properties, which are also new.