Future satellite missions like Earth Observation, Telecommunication or any other kind are likely to be exposed to various threats aiming at exploiting vulnerabilities of the involved systems and communications. Moreover, the growing complexity of systems coupled with more ambitious types of operational scenarios imply increased security vulnerabilities in the future. In the paper we will describe an architecture and software elements to ensure high level of security on-board a spacecraft. First the threats to the Security Partition Communication Controller (SPCC) will be addressed including the identification of specific vulnerabilities to the SPCC. Furthermore, appropriate security objectives and security requirements are identified to be counter the identified threats. The security evaluation of the SPCC will be done in accordance to the Common Criteria (CC). The Software Elements for SPCC has been implemented on flight representative hardware which consists of two major elements: the I/O board and the SPCC board. The SPCC board provides the interfaces with ground while the I/O board interfaces with typical spacecraft equipment busses. Both boards are physically interconnected by a high speed spacewire (SpW) link.