# File lib/aws/s3/request.rb, line 167
      def add_authorization! credentials
        if token = credentials.session_token
          headers["x-amz-security-token"] = token
        end

        secret = credentials.secret_access_key
        signature = Core::Signer.sign(secret, string_to_sign, 'sha1')
        signature = URI.escape(signature)
        headers["authorization"] = "AWS #{credentials.access_key_id}:#{signature}"
      end