Sign a request for Aliyun OSS (i.e. how to create the Authorization header) in Elixir

make_signature.ex

#first, we need to obtain a converter to GMT, needed for the Date header
# GMT format: "Sun, 22 Nov 2015 08:16:38 GMT"
  defp utc_to_gmt(utc_datetime) do
    # format = "{WDshort}, {0D} {Mshort} {YYYY} {h24}:{m}:{s} GMT"
    days = ["Mon","Tue","Wed","Thu","Fri","Sat","Sun"]
    months = ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
    day_of_week = Date.day_of_week(utc_datetime)
    month = Enum.at(months,utc_datetime.month-1)
    year = utc_datetime.year
    time = to_string(Time.truncate(DateTime.to_time(utc_datetime),:second))
    day_of_month = String.length(to_string(utc_datetime.day))==1 && "0#{utc_datetime.day}" || to_string(utc_datetime.day)
    day_of_week = Enum.at(days,day_of_week-1)
    "#{day_of_week}, #{day_of_month} #{month} #{year} #{time} GMT"
  end
  
  # this is valid for e.g. get requests.
  # Based on: https://www.alibabacloud.com/help/doc-detail/31951.htm?spm=a2c63.p38356.b99.152.36eb66fc9Iw1Iu#h2-url-5
  
  defp build_auth_signature(
    method \\ :post,                       # REST protocol used
    date \\ get_gmt_date(),                # time of the request
    content_type \\ "application/json",    # the format of the request
    can_headers \\ "/BucketName/Resource", # the absolute url of the resource requested
    oss_headers \\ ["x-oss-sth": "", ...], # optional, can stay empty for most requests
    body \\ ""                             # optional, for simple get requests may stay empty
    ) do
    verb =  Atom.to_string(method) |> String.upcase()
    md5_header = (body == "") && "" || md5_content(body)
    oss_string = (Enum.count(oss_headers) == 0) && "" || (
      (Enum.map(oss_headers, fn{k,v} -> k <> ":" <> v end) |> Enum.join("\n")) <> "\n")
    string_to_sign = verb  <> "\n"
      <> md5_header        <> "\n"
      <> content_type      <> "\n"
      <> date              <> "\n"
      <> oss_string
      <> can_headers
    signature = :crypto.hmac(:sha, @access_key_secret,string_to_sign)
    |> Base.encode64()

    "OSS " <> @access_key_id <> ":" <> signature
  end