defmodule Kcl do

  @moduledoc """
  pure Elixir NaCL crypto suite substitute

  The `box` and `unbox` functions exposed here are the equivalent
  of NaCl's:

  - `crypto_box_curve25519xsalsa20poly1305`
  - `crypto_box_curve25519xsalsa20poly1305_open`
  """

  @typedoc """
  shared nonce
  """
  @type nonce :: <<_ :: 24 * 8>>

  @typedoc """
  public or private key
  """
  @type key :: <<_ :: 32 * 8>>

  @typedoc """
  computed signature
  """
  @type signature :: <<_:: 64 * 8>>

  @typedoc """
  key varieties
  """
  @type key_variety :: :sign | :encrypt

  defp first_level_key(k), do: k |> pad(16) |> Salsa20.hash(sixteen_zeroes)
  defp second_level_key(k,n) when byte_size(n) == 24, do: k |> Salsa20.hash(binary_part(n,0,16))

  defp pad(s, n) when byte_size(s) >= n, do: s
  defp pad(s, n) when byte_size(s) <  n, do: pad(<<0>><>s, n)

  defp sixteen_zeroes,   do: <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0>>
  defp thirtytwo_zeroes, do: sixteen_zeroes<>sixteen_zeroes

  @doc """
  generate a `{private, public}` key pair

  """
  @spec generate_key_pair(key_variety) :: {key, key} | :error
  def generate_key_pair(variety \\ :encrypt)
  def generate_key_pair(:encrypt), do: Curve25519.generate_key_pair
  def generate_key_pair(:sign), do: Ed25519.generate_key_pair

  @doc """
  derive a public key from a private key
  """
  @spec derive_public_key(key, key_variety) :: key | :error
  def derive_public_key(private_key, variety \\ :encrypt)
  def derive_public_key(private_key, :encrypt), do: Curve25519.derive_public_key(private_key)
  def derive_public_key(private_key, :sign), do: Ed25519.derive_public_key(private_key)

  @doc """
  pre-compute a shared key

  Mainly useful in a situation where many messages will be exchanged.
  """
  def shared_secret(our_private, their_public), do: Curve25519.derive_shared_secret(our_private,their_public) |> first_level_key

  @doc """
  box up an authenticated packet

  """
  @spec box(binary, nonce, key, key) :: {binary, Kcl.State.t}
  def box(msg,nonce,our_private,their_public), do: box(msg,nonce,Kcl.State.init(our_private) |> Kcl.State.new_peer(their_public))
  @spec box(binary,nonce,Kcl.State.t) :: {binary, Kcl.State.t}
  def box(msg,nonce,state) when is_map(state), do: {secretbox(msg,nonce,state.shared_secret), struct(state, [previous_nonce: nonce])}
  @spec secretbox(binary, nonce, key) :: binary
  @doc """
  box based on a shared secret
  """
  def secretbox(msg,nonce,key) do
      <<pnonce::binary-size(32), c::binary>> = Salsa20.crypt(thirtytwo_zeroes<>msg,second_level_key(key,nonce),binary_part(nonce,16,8))
      Poly1305.hmac(c,pnonce)<>c
  end

  @doc """
  unbox an authenticated packet

  Returns `:error` when the packet contents cannot be authenticated, otherwise
  the decrypted payload and updated state.
  """
  @spec unbox(binary, nonce, key, key) :: {binary, Kcl.State.t} | :error
  def unbox(packet,nonce,our_private,their_public), do: unbox(packet,nonce,Kcl.State.init(our_private) |> Kcl.State.new_peer(their_public))
  def unbox(packet,nonce,state) do
      case {nonce > state.previous_nonce, secretunbox(packet, nonce, state.shared_secret)} do
        {false, _}     -> {:error, "nonce"}
        {true, :error} -> {:error, "decode"}
        {true, m}      -> {m, struct(state, [previous_nonce: nonce])}
      end
  end

  @doc """
  unbox based on a shared secret
  """
  @spec secretunbox(binary, nonce, key) :: binary | :error
  def secretunbox(packet,nonce,key)
  def secretunbox(<<mac::binary-size(16),c::binary>>,nonce,key) do
      <<pnonce::binary-size(32), m::binary>> = Salsa20.crypt(thirtytwo_zeroes<>c,second_level_key(key,nonce),binary_part(nonce,16,8))
      case (c |> Poly1305.hmac(pnonce) |> Poly1305.same_hmac?(mac)) do
          true ->  m
          _    ->  :error
      end
  end

  @doc """
  create an inital state for a peer connection

  A convenience wrapper around `Kcl.State.init` and `Kcl.State.new_peer`
  """
  @spec new_connection_state(key, key | nil, key) :: Kcl.State.t
  def new_connection_state(our_private, our_public \\ nil, their_public) do
    Kcl.State.init(our_private, our_public) |> Kcl.State.new_peer(their_public)
  end

  @doc """
  sign a message

  If only the secret key is provided, the public key will be derived therefrom.
  This can add significant overhead to the signing operation.
  """
  @spec sign(binary, key, key) :: signature
  def sign(message, secret_key, public_key \\ nil), do: Ed25519.signature(message, secret_key, public_key)

  @doc """
  validate a message signature
  """
  @spec valid_signature?(signature, binary, key) :: boolean
  def valid_signature?(signature, message, public_key), do: Ed25519.valid_signature?(signature, message, public_key)

end