The goal of the Computer Architecture for Secure Systems (CASS) project [1] is to develop an architecture and tools to ensure the security and integrity of software in distributed systems. CASS makes use of various cryptographic techniques at the operating system kernel level to authenticate software integrity. The CASS shell, the work described in this paper, is on the other hand a secure shell implemented on top of UNIX1 System V Release 4.2 (UNIX SVR4.2) to achieve the same purpose but in an operating system independent manner. The CASS shell carries out cryptographic authentication of executable files based on the MD5 Message-Digest algorithm [2] and presents a closed computing environment in which system utilities are safeguarded against unauthorised alteration and users are prevented from executing unsafe commands. In order to provide cryptographic authentication and other cryptographic functions such as public-key based signatures, in hardware, the work has also involved the incorporation of an encryption hardware sub-system into SVR4.2 operating on an Intel 80×86 hardware platform. The paper describes the structure and features of the CASS shell and the development and performance of both the hardware and software implementations of the cryptographic functions it uses.