Support blind indexes

[kdawgwilk]

I realize the readme calls out the ability to use cloak_ecto to create hashed searchable columns but it would be nice to offer more of a first-class option for this similar to this ruby lib https://github.com/ankane/blind_index

They reference this article which breaks down what this would look like https://www.sitepoint.com/how-to-search-on-securely-encrypted-database-fields

[danielberkompas]

@kdawgwilk Cloak.Ecto already supports blind indexes. The differences from blind_index are mainly due to Elixir / Ecto doing things differently than ActiveRecord.

As you noted, you can already use this library to create fields for blind indexes and keep them up to date, like so:

schema "table" do
  field :field_name, MyApp.Encrypted.Binary
  field :field_name_hash, MyApp.Hashed.HMAC
end

def changeset(struct, attrs \\ %{}) do
  struct
  |> cast(attrs, [:field_name, :field_name_hash])
  |> put_hashed_fields()
end

defp put_hashed_fields(changeset) do
  changeset
  |> put_change(:field_name_hash, get_field(changeset, :field_name))
end

This is functionally the same as the blind_index gem, and I don't currently see any way to make it simpler within the constraints of Ecto's design. If you have any specific suggestions, let me know!