defmodule WalletPasses.Google.CallbackVerifier do
  @moduledoc """
  Verifies Google Wallet callback envelopes per the `ECv2SigningOnly` protocol.

  Pure-OTP implementation using `:public_key` and `:crypto` — no Tink
  dependency. Fetches Google's published root keys from
  `https://pay.google.com/gp/m/issuer/keys` (overridable via
  `:wallet_passes, :google_keys_url`) and caches them via
  `WalletPasses.TokenCache`. See [Google Wallet](guides/google-wallet.md).
  """

  alias WalletPasses.TokenCache

  @sender_id "GooglePayPasses"
  @protocol_version "ECv2SigningOnly"
  @default_keys_url "https://pay.google.com/gp/m/issuer/keys"
  @cache_key :google_callback_root_keys

  # ECv2SigningOnly is fixed to NIST P-256, so we hardcode the curve OID rather
  # than parse the AlgorithmIdentifier. `:public_key.der_decode/2` returns a
  # low-level tuple that `:public_key.verify/4` doesn't accept directly — we
  # extract the point bytes and rebuild the high-level `{{:ECPoint, _}, params}`
  # form that verify needs.
  @p256_oid {1, 2, 840, 10_045, 3, 1, 7}

  @spec verify(map(), String.t()) :: {:ok, String.t()} | {:error, atom()}
  def verify(envelope, issuer_id) when is_map(envelope) and is_binary(issuer_id) do
    case do_verify(envelope, issuer_id) do
      {:ok, _} = ok -> ok
      {:error, atom} when is_atom(atom) -> {:error, atom}
      _ -> {:error, :verification_failed}
    end
  end

  defp do_verify(envelope, issuer_id) do
    with {:ok, signed_message} <- get_string(envelope, "signedMessage"),
         {:ok, signature_b64} <- get_string(envelope, "signature"),
         {:ok, ik} <- get_map(envelope, "intermediateSigningKey"),
         {:ok, protocol_version} <- get_string(envelope, "protocolVersion"),
         :ok <- check_protocol(protocol_version),
         {:ok, intermediate_pub_b64} <- verify_intermediate(ik, protocol_version),
         :ok <-
           verify_message(
             signed_message,
             signature_b64,
             intermediate_pub_b64,
             issuer_id,
             protocol_version
           ) do
      {:ok, signed_message}
    end
  end

  defp check_protocol(@protocol_version), do: :ok
  defp check_protocol(_), do: {:error, :unsupported_protocol}

  defp verify_intermediate(%{"signedKey" => signed_key, "signatures" => sigs}, protocol_version)
       when is_binary(signed_key) and is_list(sigs) do
    with {:ok, %{"keyValue" => intermediate_pub_b64, "keyExpiration" => exp_str}} <-
           Jason.decode(signed_key),
         :ok <- check_expiration(exp_str),
         {:ok, roots} <- fetch_roots() do
      signed_data = sig_input([@sender_id, protocol_version, signed_key])

      if Enum.any?(sigs, fn sig_b64 ->
           Enum.any?(roots, fn root ->
             root.protocol_version == protocol_version and not_expired?(root.expiration) and
               verify_sig(signed_data, sig_b64, root.pub)
           end)
         end) do
        {:ok, intermediate_pub_b64}
      else
        {:error, :verification_failed}
      end
    end
  end

  defp verify_intermediate(_, _), do: {:error, :verification_failed}

  defp verify_message(
         signed_message,
         signature_b64,
         intermediate_pub_b64,
         issuer_id,
         protocol_version
       ) do
    with {:ok, intermediate_pub} <- decode_pub(intermediate_pub_b64),
         signed_data =
           sig_input([@sender_id, issuer_id, protocol_version, signed_message]),
         true <- verify_sig(signed_data, signature_b64, intermediate_pub) do
      :ok
    else
      _ -> {:error, :verification_failed}
    end
  end

  defp verify_sig(signed_data, sig_b64, pub_key) do
    case Base.decode64(sig_b64) do
      {:ok, sig_der} -> :public_key.verify(signed_data, :sha256, sig_der, pub_key)
      _ -> false
    end
  rescue
    _ -> false
  end

  defp decode_pub(b64) do
    with {:ok, der} <- Base.decode64(b64),
         {:SubjectPublicKeyInfo, _algo, point_bytes} <-
           :public_key.der_decode(:SubjectPublicKeyInfo, der) do
      {:ok, {{:ECPoint, point_bytes}, {:namedCurve, @p256_oid}}}
    else
      _ -> {:error, :verification_failed}
    end
  rescue
    _ -> {:error, :verification_failed}
  end

  defp check_expiration(exp_str) when is_binary(exp_str) do
    case Integer.parse(exp_str) do
      {millis, ""} ->
        if millis > System.system_time(:millisecond),
          do: :ok,
          else: {:error, :verification_failed}

      _ ->
        {:error, :verification_failed}
    end
  end

  defp check_expiration(_), do: {:error, :verification_failed}

  defp not_expired?(millis) when is_integer(millis),
    do: millis > System.system_time(:millisecond)

  defp not_expired?(_), do: false

  defp sig_input(parts) do
    Enum.map_join(parts, fn s -> <<byte_size(s)::little-32, s::binary>> end)
  end

  defp get_string(map, key) do
    case Map.get(map, key) do
      v when is_binary(v) -> {:ok, v}
      _ -> {:error, :verification_failed}
    end
  end

  defp get_map(map, key) do
    case Map.get(map, key) do
      v when is_map(v) -> {:ok, v}
      _ -> {:error, :verification_failed}
    end
  end

  # -- Root key fetch + cache --

  defp fetch_roots do
    case TokenCache.get(@cache_key) do
      {:ok, roots} ->
        {:ok, roots}

      :miss ->
        with {:ok, %{status: 200, body: body}} <- Req.get(keys_url(), decode_body: :json),
             %{"keys" => raw} when is_list(raw) <- body,
             {:ok, parsed} <- parse_roots(raw) do
          TokenCache.put(@cache_key, parsed)
          {:ok, parsed}
        else
          _ -> {:error, :verification_failed}
        end
    end
  end

  defp parse_roots(raw) do
    parsed =
      Enum.flat_map(raw, fn
        %{"keyValue" => v, "protocolVersion" => p, "keyExpiration" => e}
        when is_binary(v) and is_binary(p) and is_binary(e) ->
          with {:ok, pub} <- decode_pub(v),
               {millis, ""} <- Integer.parse(e) do
            [%{pub: pub, protocol_version: p, expiration: millis}]
          else
            _ -> []
          end

        _ ->
          []
      end)

    case parsed do
      [] -> {:error, :verification_failed}
      _ -> {:ok, parsed}
    end
  end

  defp keys_url do
    Application.get_env(:wallet_passes, :google_keys_url, @default_keys_url)
  end
end