<configuration>
  <configSections>
    <section name="mystartup" type="mystartup.Configuration.EncryptionConfigurationSection, mystartup" />
  </configSections>
</configuration>

namespace mystartup.Configuration
{
    using System;
    using System.Configuration;

    public class EncryptionConfigurationSection : ConfigurationSection // Наследование от базового класса
    {
        ...
    }
}