defmodule RNS.Crypto do
  @moduledoc """
  Crypto primitives for Reticulum.
  Lazily copied from https://github.com/Sgiath/reticulum.
  Usually, you won't use this module but the Identity module, which then uses this module.
  """
  @type hash() :: <<_::256>>
  @type truncated_hash() :: <<_::128>>
  @type key() :: <<_::256>>
  @type signature() :: <<_::512>>
  @type ratchet() :: <<_::256>>

  @hash_len 32

  @doc """
  https://soatok.blog/2021/11/17/understanding-hkdf/
  https://en.wikipedia.org/wiki/HKDF
  """
  @spec hkdf(binary(), binary(), binary(), non_neg_integer()) :: <<_::512>>
  def hkdf(ikm, salt \\ <<>>, info \\ <<>>, len \\ 64)
      when len > 0 and is_binary(ikm) and byte_size(ikm) > 0 do
    salt
    |> hkdf_extract(ikm)
    |> hkdf_expand(info, len)
  end

  @spec hkdf_extract(binary(), binary()) :: binary()
  defp hkdf_extract(<<>>, ikm), do: hkdf_extract(<<0::size(256)>>, ikm)
  defp hkdf_extract(salt, ikm), do: hmac(salt, ikm)

  @spec hkdf_expand(binary(), binary(), non_neg_integer()) :: binary()
  defp hkdf_expand(prk, info, len) do
    {<<okm::binary-size(len), _rest::binary>>, _t} =
      Enum.reduce(1..ceil(len / @hash_len), {<<>>, <<>>}, fn i, {okm, t} ->
        t = hmac(prk, t <> info <> <<i>>)
        {okm <> t, t}
      end)

    okm
  end

  @spec truncate(binary(), non_neg_integer()) :: binary()
  def truncate(data, length) do
    <<truncated_data::binary-size(length), _rest::binary>> = data
    truncated_data
  end

  @spec sha256(binary()) :: hash()
  def sha256(data), do: :crypto.hash(:sha256, data)

  @spec truncated_sha256(binary()) :: truncated_hash()
  def truncated_sha256(data) do
    truncate(:crypto.hash(:sha256, data), 16)
  end

  @spec hmac(iodata(), binary()) :: <<_::256>>
  def hmac(key, data) do
    :crypto.mac(:hmac, :sha256, key, data)
  end

  @spec ed25519() :: {key(), key()}
  def ed25519() do
    :crypto.generate_key(:eddsa, :ed25519)
  end

  @spec x25519() :: {key(), key()}
  def x25519() do
    :crypto.generate_key(:eddh, :x25519)
  end

  @spec compute(key(), key()) :: key()
  def compute(secret, public) do
    :crypto.compute_key(:eddh, public, secret, :x25519)
  end

  @spec sign(key(), binary()) :: signature()
  def sign(key, data) do
    :crypto.sign(:eddsa, :none, data, [key, :ed25519])
  end

  @spec validate?(key(), binary(), signature()) :: boolean()
  def validate?(key, data, signature) do
    :crypto.verify(:eddsa, :none, data, signature, [key, :ed25519])
  end
end