The verification of the authenticity of software by an executing host has become a vital security issue in recent years with the original postulation and subsequent evolution of computer viruses. The CASS (Computer Architecture for Secure Systems) project addresses this issue by incorporating integrity checking at the operating system level. This paper describes three prototype implementations of the architecture, two of these at the kernel level targetting UNIX SVR4.2 and the Mach 3.0 microkernel, with the third-for reasons of generality-involving the implementation of a specialised shell which is then portable across UNIX-style platforms in general. The paper focusses on a description of the former, viz. the kernel-based implementations, and examines the design and implementation issues which had to be addressed in achieving kernel-based integrity checking of executables for the two platforms.