International Association
for Cryptologic Research

With popularity of cloud storage, efficiently proving the integrity of data stored at an untrusted server has become significant. Authenticated Skip Lists and Rank-based Authenticated Skip Lists (RBASL) have been used in cloud storage to provide support for provable data update operations. In a dynamic file scenario, an RBASL falls short when updates are not proportional to a fixed block size; such an update to the file, however small, may translate to O(n) many block updates to the RBASL, for a file with n blocks.

To overcome this problem, we introduce FlexList: Flexible Length-Based Authenticated Skip List. FlexList translates even variable-size updates to O(u) insertions, removals, or modifications, where u is the size of the update divided by the block size. We present various optimizations on the four types of skip lists (regular, authenticated, rank-based authenticated, and FlexList). We compute one single proof to answer multiple (non-)membership queries and obtain efficiency gains of 35%, 35% and 40% in terms of proof time, energy, and size, respectively. We also deployed our implementation of FlexDPDP (DPDP with FlexList instead of RBASL) on PlanetLab, demonstrating that FlexDPDP performs comparable to the most efficient static storage scheme (PDP), while providing dynamic data support.