fernetex

defmodule Fernet do
  @moduledoc """
  Generate or verify Fernet tokens based on https://github.com/fernet/spec
  """

  use Timex
  use Bitwise, only_operators: true

  @max_drift 60
  @default_ttl 60
  @version 0x80

  @doc """
  Generate a token for the given message using the secret to encrypt it.

  ## Options

  The accepted options are:

    * `:message` - message to be tokenized
    * `:secret`  - secret to use for encryptions (256 bits, defaults to env)

  """
  def generate(options) do
    generate(
      Dict.fetch!(options, :message),
      Dict.get(options, :secret, default_secret),
      Dict.get(options, :iv, new_iv),
      Dict.get(options, :now, now))
  end

  @doc """
  Verify a token using the given secret and optionally validate TTL

  ## Options

  The accepted options are:

    * `:token`       - token to be decrypted and verified
    * `:secret`      - secret to use for decryption (256 bits, defaults to env)
    * `:ttl`         - If `:enforce_ttl` then this is the time in seconds
    * `:enforce_ttl` - Should ttl be enforced (default to true)

  """
  def verify(options) do
    verify(
      Dict.fetch!(options, :token),
      Dict.get(options, :secret, default_secret),
      Dict.get(options, :ttl, @default_ttl),
      Dict.get(options, :enforce_ttl, true),
      Dict.get(options, :now, now))
  end

  defp verify(token, secret, ttl, enforce_ttl, now) when byte_size(secret) != 32 do
    verify(token, decode_secret!(secret), ttl, enforce_ttl, now)
  end

  defp verify(token, secret, ttl, enforce_ttl, now) when is_binary(now) do
    secs = now |> DateFormat.parse!("{ISO}") |> Date.to_secs
    verify(token, secret, ttl, enforce_ttl, secs)
  end

  defp verify(token, <<sig_key :: binary-size(16), enc_key :: binary-size(16)>>, ttl, enforce_ttl, now) do
    {:ok, plain_token, message_length} = parse_token(token)
    if message_length <= 0, do: raise "too short"
    <<version :: binary-size(1),
      issued_date :: 64-big-unsigned-integer-unit(1),
      iv :: binary-size(16),
      encrypted_message :: binary-size(message_length),
      mac :: binary-size(32)>> = plain_token
    if enforce_ttl do
      cond do
        (issued_date + ttl) <= now -> raise "expired TTL"
        issued_date > (now + @max_drift) -> raise "far-future TS (unacceptable clock skew)"
        true -> true
      end
    end
    payload = calculate_payload(version, issued_date, iv, encrypted_message)
    new_mac = :crypto.hmac(:sha256, sig_key, payload)
    unless mac == new_mac do
      raise "incorrect mac"
    end
    {:ok, decrypt(enc_key, encrypted_message, iv)}
  end

  defp parse_token(token) do
    try do
      plain_token = Base.url_decode64!(token)
      message_length = byte_size(plain_token) - 57
      {:ok, plain_token, message_length}
    rescue
      ArgumentError -> raise "invalid base64"
    end
  end

  defp generate(message, _secret, _iv, _now) when is_nil(message) or byte_size(message) == 0 do
    raise ArgumentError, "message must be provided"
  end

  defp generate(_message, secret, _iv, _now) when is_nil(secret) or byte_size(secret) < 32 do
    raise ArgumentError, "secret must be provided"
  end

  defp generate(message, secret, iv, now) when byte_size(secret) != 32 do
    generate(message, decode_secret!(secret), iv, now)
  end

  defp generate(message, secret, iv, now) when is_list(iv) do
    generate(message, secret, :erlang.list_to_binary(iv), now)
  end

  defp generate(message, secret, iv, now) when is_binary(now) do
    secs = now |> DateFormat.parse!("{ISO}") |> Date.to_secs
    generate(message, secret, iv, secs)
  end

  defp generate(message, <<sig_key :: binary-size(16), enc_key :: binary-size(16)>>, iv, now) do
    payload = calculate_payload(@version, now, iv, encrypt(enc_key, message, iv))
    mac = :crypto.hmac(:sha256, sig_key, payload)
    {:ok, iv, Base.url_encode64(payload <> mac)}
  end

  defp calculate_payload(version, now, iv, encrypted_message) do
    :erlang.list_to_binary [
      version,                   # Version
      pack_int64_bigindian(now), # Timestamp
      iv,                        # Initial Vector
      encrypted_message          # Message
    ]
  end

  defp pack_int64_bigindian(value) do
    0..7
    |> Enum.map(&((value >>> (&1 * 8)) &&& 0xff))
    |> Enum.reverse
    |> :erlang.list_to_binary
  end

  defp encrypt(key, message, iv) do
    :crypto.block_encrypt(:aes_cbc128, key, iv, pad(message))
  end

  defp decrypt(key, message, iv) do
    if rem(byte_size(message), 16) != 0, do: raise "payload size not multiple of block size"
    padded_message = :crypto.block_decrypt(:aes_cbc128, key, iv, message)
    pad_len = :binary.last(padded_message)
    msg_len = byte_size(padded_message) - pad_len
    <<plain_message :: binary-size(msg_len), the_padding :: binary-size(pad_len)>> = padded_message
    unless Enum.all?(:erlang.binary_to_list(the_padding), fn (p) -> p == pad_len end), do: raise "padding error"
    plain_message
  end

  defp pad(message) do
    case rem(byte_size(message), 16) do
      0 -> message
      r -> message <> padding(16 - r)
    end
  end

  defp padding(len) do
    1..len |> Enum.reduce <<>>, fn(_i, acc) -> acc <> <<len>> end
  end

  defp decode_secret!(secret) when byte_size(secret) == 32 do
    secret
  end

  defp decode_secret!(secret) do
    try do
      Base.decode64!(secret)
    rescue
      ArgumentError -> Base.url_decode64!(secret)
    end
  end

  defp default_secret, do: System.get_env("FERNET_SECRET")

  defp new_iv do
    :crypto.rand_bytes 16
  end

  defp now do
    Date.now |> DateFormat.format!("{ISO}")
  end
end