defmodule ExDjango.Utils.Signing do
  @moduledoc """
  Functions for creating and restoring url-safe signed JSON objects.

  Compatible with django/core/signing.py
  """

  def default_salt() do
    "django.contrib.sessions.backends.signed_cookiessigner"
  end

  def b64_encode(s) do
    String.rstrip(Base.url_encode64(s), ?=)
  end

  def b64_decode(s) do
    case rem(String.length(s), 4) do
      0 -> s
      x -> s <> String.duplicate("=", 4 - x)
    end |> Base.url_decode64!()
  end

  def salted_hmac(salt, secret, value) do
    sha1_hash = :crypto.hash(:sha, salt <> secret)
    :crypto.hmac(:sha, sha1_hash, value)
  end

  def base64_hmac(salt, secret, value) do
    b64_encode(salted_hmac(salt, secret, value))
  end

end