“…By contrast, more challenging proposals address both problems, namely, they build a PoW scheme and offer protection against honest-but-curious servers (Xu and Zhou, 2014;Li et al, 2014a;Stanek et al, 2013;jin et al, 2013;Ng et al, 2012). Furthermore, Xu and Zhou (2014), Stanek et al (2013), and Ng et al (2012) provide a formal security analysis and Stanek et al (2013) focus on server storage efficiency as well.…”