Hiding sensitive information with base64 encoding and binary serialization

Base64Dictionary.cs

class Program
{
    static void Main(string[] args)
    {
        var foo = new Base64Dictionary();
        foo["bar"] = "baz";
        foo["foo"] = "bar";

        foo.WriteTo("test.bin");

        var bar = Base64Dictionary.ReadFrom("test.bin");
        Console.WriteLine("foo = " + bar["foo"]);
        Console.WriteLine("bar = " + bar["bar"]);

        Console.Read();
    }
}

[Serializable]
public class Base64Dictionary: Dictionary<string, string>
{
    public string this[string key]
    {
        get
        {
            var encoded_key = Convert.ToBase64String(Encoding.UTF8.GetBytes(key));
            var encoded_value = base[encoded_key];
            return Encoding.UTF8.GetString(Convert.FromBase64String(encoded_value));
        }
        set 
        { 
            var encoded_key = Convert.ToBase64String(Encoding.UTF8.GetBytes(key));
            var encoded_value = Convert.ToBase64String(Encoding.UTF8.GetBytes(value));
            base[encoded_key] = encoded_value;
        }
    }

    public Base64Dictionary() { }

    public Base64Dictionary(SerializationInfo info, StreamingContext context) : base(info, context) { }

    public void WriteTo(string file)
    {
        IFormatter formatter = new BinaryFormatter();
        using (Stream stream = new FileStream(file, FileMode.Create, FileAccess.Write, FileShare.None))
        {
            formatter.Serialize(stream, this);
        }
    }
        
    public static Base64Dictionary ReadFrom(string file)
    {
        Base64Dictionary obj;
        IFormatter formatter = new BinaryFormatter();
        using (Stream stream = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.Read))
        {
            obj = (Base64Dictionary) formatter.Deserialize(stream);
        }
        return obj;
    }
}