Constructing high-assurance, secure hardware remains a challenge, because to do so relies on both a verifiable means of hardware description and implementation. However, production hardware description languages (HDL) lack the formal underpinnings required by formal methods in security. Still, there is no such thing as high-assurance systems without high-assurance hardware. We present a core calculus of secure hardware description with its formal semantics, security type system, and mechanization in Coq. This calculus is the core of the functional HDL, ReWire, shown in previous work to have useful applications in reconfigurable computing. This work supports a full-fledged, formal methodology for producing high-assurance hardware.