Establishing contractual liabilities in case of litigation is generally a delicate matter. It becomes even more challenging when IT systems are involved. At the core of the problem lies the issue of the evidence provided by the opposing parties. We believe that the means to constitute evidence that could be used in case of conflict should be considered from the onset of IT projects and be part of the requirements for the design of IT systems. This paper proposes criteria for acceptable log architectures depending on the features of the system and the potential claims between the parties. We establish properties guaranteed by acceptable architectures and illustrate our framework with a travel booking system.