Optimally Efficient Accountable Time-Stamping

Efficient secure time-stamping schemes employ a 2-level approach in which the time-stamping service operates in rounds. We say that a time-stamping service is accountable if if it makes the TSA and other authorities accountable for their actions by enabling a principal to detect and later prove to a judge any frauds, including attempts to reorder time-stamps from the same round. We investigate the paradigm of time-stamping services based on simply connected graphs, and propose a simple, yet optimal, accountable time-stamping service, using what we call threaded tree schemes. We improve upon the previously best scheme by Buldas and Laud by reducing the size of a time stamp by a factor of about 3.786 and show that our construction is optimal in a strict sense. The new protocols also increase the trustworthiness of the publication process, which takes place at the end of each round.