Use of smart-phones and devices has proliferated almost all aspects of our lives. With that, each of us have become the producers and consumers of different types of data, of different levels of sensitivity, and of different levels of trustworthiness. One important aspect of managing such data on our devices and associated cloud (e.g., iCloud of Apple) is how to manage the authenticity such data. In this paper, we present a data model and a scheme towards assuring authenticity of the data. Our scheme is very efficient -- it computes only one signature.