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diff --git a/lib/pleroma/web/oauth/scopes.ex b/lib/pleroma/web/oauth/scopes.ex
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+++ b/lib/pleroma/web/oauth/scopes.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.OAuth.Scopes do
+  @moduledoc """
+  Functions for dealing with scopes.
+  """
+
+  @doc """
+  Fetch scopes from requiest params.
+
+  Note: `scopes` is used by Mastodon — supporting it but sticking to
+  OAuth's standard `scope` wherever we control it
+  """
+  @spec fetch_scopes(map(), list()) :: list()
+  def fetch_scopes(params, default) do
+    parse_scopes(params["scope"] || params["scopes"], default)
+  end
+
+  def parse_scopes(scopes, _default) when is_list(scopes) do
+    Enum.filter(scopes, &(&1 not in [nil, ""]))
+  end
+
+  def parse_scopes(scopes, default) when is_binary(scopes) do
+    scopes
+    |> to_list
+    |> parse_scopes(default)
+  end
+
+  def parse_scopes(_, default) do
+    default
+  end
+
+  @doc """
+  Convert scopes string to list
+  """
+  @spec to_list(binary()) :: [binary()]
+  def to_list(nil), do: []
+
+  def to_list(str) do
+    str
+    |> String.trim()
+    |> String.split(~r/[\s,]+/)
+  end
+
+  @doc """
+  Convert scopes list to string
+  """
+  @spec to_string(list()) :: binary()
+  def to_string(scopes), do: Enum.join(scopes, " ")
+
+  @doc """
+  Validates scopes.
+  """
+  @spec validates(list() | nil, list()) ::
+          {:ok, list()} | {:error, :missing_scopes | :unsupported_scopes}
+  def validates([], _app_scopes), do: {:error, :missing_scopes}
+  def validates(nil, _app_scopes), do: {:error, :missing_scopes}
+
+  def validates(scopes, app_scopes) do
+    case scopes -- app_scopes do
+      [] -> {:ok, scopes}
+      _ -> {:error, :unsupported_scopes}
+    end
+  end
+end