This paper presents a novel method to perform on-the-fly attestation of hardware structures loaded to reconfigurable devices. Given that a loadable hardware structure to a reconfigurable device is described by a binary bitstream, the hash value of this bitstream can be calculated to validate the hardware structure. To optimize this attestation, the hash value computation is implemented in hardware on the FPGA itself. To guarantee the integrity of the existing computation architecture, the proposed hardware module also enforces region delimitation. With the region delimitation, only the regions intended to be reconfigured can be modified. Implementation results suggest that this bitstream attestation can be performed without imposing an extra delay to the reconfigurable process and at an area cost of less that 10% of a Virtex II Pro 30 FPGA device.