In this paper, a procedure for intellectual property protection (IPP) of digital circuits called IPP@HDL is presented. Its aim is to protect the author rights in the development and distribution of reusable modules by means of an electronic signature. The technique relies on hosting the bits of the digital signature within memory structures or combinational logic that are part of the system, at the high level description of the design. Thus, the area of the system is not increased and the signature is difficult to change or to remove without damaging the design. The technique also includes a procedure for secure signature extraction requiring minimal modifications to the system and without interfering its normal operation. The benefits of the presented procedure are illustrated with programmable logic and cell-based application-specific integrated circuit examples with several signature lengths. These design examples show no performance degradation and a negligible area increase, while probabilistic analyses show that the proposed IPP scheme offers high resistance against attacks.