Why do the ASP.NET Identity interfaces use strings for primary and foreign keys?

The intent was to allow both arbitrary id types (i.e. int, guid, string), but also avoid having serialization/casting issues for the id property.

So you can define your keys however you like and just implement the interface method

public class MyUser : IUser {
  public int Id { get; set; }
  string IUser.Id { get { return Id.ToString(); } }
}