    # File lib/chef/cookbook_uploader.rb, line 114
        def uploader_function_for(file, checksum, url, checksums_to_upload)
          lambda do
            # Checksum is the hexadecimal representation of the md5,
            # but we need the base64 encoding for the content-md5
            # header
            checksum64 = Base64.encode64([checksum].pack("H*")).strip
            timestamp = Time.now.utc.iso8601
            file_contents = File.open(file, "rb") {|f| f.read}
            # TODO - 5/28/2010, cw: make signing and sending the request streaming
            sign_obj = Mixlib::Authentication::SignedHeaderAuth.signing_object(
                                                                               :http_method => :put,
                                                                               :path        => URI.parse(url).path,
                                                                               :body        => file_contents,
                                                                               :timestamp   => timestamp,
                                                                               :user_id     => rest.client_name
                                                                               )
            headers = { 'content-type' => 'application/x-binary', 'content-md5' => checksum64, :accept => 'application/json' }
            headers.merge!(sign_obj.sign(OpenSSL::PKey::RSA.new(rest.signing_key)))

            begin
              RestClient::Resource.new(url, :headers=>headers, :timeout=>1800, :open_timeout=>1800).put(file_contents)
              checksums_to_upload.delete(checksum)
            rescue RestClient::Exception => e
              Chef::Knife.ui.error("Failed to upload #@cookbook : #{e.message}\n#{e.response.body}")
              raise
            end
          end
        end