### user.email_hash 

the UserDatabase sample provides a means to maintain a local user database and as described you may filter this dataset by `user.email_hash` to locate a user by email.

Here is a C# method to properly construct the `email_hash` which may be used to filter User records as well as constructing a Gravatar image.

    // using System.Security.Cryptography;

    public static string CalculateEmailHash(string email)
    {
        return string.Join("",
            MD5
            .Create()
            .ComputeHash(Encoding.ASCII.GetBytes(email.ToLower()))
            .Select(v => v.ToString("x2")).ToArray());
    }