SPION: Secure Protocols in OSI Networks This report describes how security services can be realized in a computer network using the protocols of the Open Systems Interconnection (OSI) reference model for communication. The report starts with defining security requirements for a "typical" local area network in a company, university or similar organization. It is assumed that the organization does not use the network for transfer of extremely sensitive information, such as military secrets. A set of security services, as specified in the OSI security architecture, are selected in order to satisfy the requirements. The selected services are then placed in suitable layers of the OSI model according to the criteria in the security architecture, and to the taste of the authors. The report concentrates on the transport layer. An extension of the OSI transport protocol, class 4, including security services is described in detail. The protocol is a fully compatible extension of the standard transport protocol. Key management is another topic which is included in the report. A key management system for handling public keys and digital signatures based on an article by Dorothy E. Denning is described. The system includes functions for distributing and validating public keys, and registering and later verifying digital signatures. A key management protocol supporting these functions is defined for communication between ordinary open systems and special key server systems.