authorize_multipart_upload.rb

def authorize
  encoded_md5 = [[md5].pack("H*")].pack("m0")
  auth_date = Time.now.httpdate
  amz_headers = "x-amz-date:#{auth_date}\n"
  resource = "/#{upload.bucket}/#{upload.filename}?partNumber=#{part_number}&uploadId=upload_id"

  string_to_sign =
    "PUT" + "\n" + # HTTP-Verb
    encoded_md5 + "\n" + # Content-MD5
    upload.mime_type + "\n" + # Content-Type
    "\n" + # Expires
    amz_headers + # CanonicalizedAmzHeaders
    resource # CanonicalizedResource

  { auth_header: "AWS #{ENV['AWS_ACCESS_KEY_ID']}:#{sign_string(string_to_sign)}",
    auth_date: auth_date,
    content_md5: encoded_md5 }
end

def sign_string(string)
  Base64.strict_encode64(OpenSSL::HMAC.digest("sha1", ENV["AWS_SECRET_ACCESS_KEY"], string))
end