Xilinx encrypts IP HDL files with the IEEE Recommended Practice for Encryption and Management of Electronic Design Intellectual Property (IP) (IEEE Std P1735).
Video: See the Vivado Design Suite QuickTake Video: Using IP Encryption Tool in
Vivado Design Suite for more information about encryption.
Also, see this link to Vivado Design Suite User Guide: Creating and Packaging Custom IP (UG1118).