For modern safety-critical systems we aim to simultaneously maintain safety whilst taking advantage of the benefits of system interconnectedness and faster communications. Many standards have recognised and responded to the serious security implications of making these connections between systems that have traditionally been closed. In addition, there have been several advances in developing techniques to combine the two attributes, however, the problem of integrated assurance remains. What is missing is a systematic approach to reasoning about alignment. In this paper, the Safety-Security Assurance Framework(SSAF) is presented as a candidate solution. SSAF is a two-part framework based on the concept of independent co-assurance (i.e. allowing separate assurance processes, but enabling the timely exchange of correct and necessary information).To demonstrate SSAF’s application, a case study is given using requirements from widely-adopted standards (IEC 61508 and Common Criteria) and a Bayesian Belief Network. With a clear understanding of the trade-offs and the interactions, it is possible to create better models for alignment and therefore improve safety-security co-assurance.