A Secure Round-Based Timestamping Scheme with Absolute Timestamps (Short Paper)

The aim of timestamping systems is to provide a proof-ofexistence of a digital document at a given time. Such systems are important to ensure integrity and non-repudiation of digital data over time. Most of the existing timestamping schemes use the notions of round (a period of time) and round token (a single value aggregating the timestamping requests received during one round). Such schemes have the following drawbacks: (i) Clients who have submitted a timestamping request must wait for the end of the round before receiving their timestamping certificate (ii) TimeStamping Authorities (TSA) based on such schemes are discrete-time systems and provide relative temporal authentication only, i.e. all the documents submitted during the same round are timestamped with the same date and time. (iii) the TSA can tamper timestamps before the round token is published in a widely distributed media. In this paper, we define a new timestamping scheme which overcomes these drawbacks. First, we use chameleon hash functions to compute the round token at the beginning of the round. Therefore, timestamping certificates can be returned to clients immediately after receiving their request. There is no need to wait for the end of the round. Second, our scheme is distributed, i.e. several servers cooperate to compute round tokens and timestamping certificates. Such a distribution of servers allows us to define a continuous time scheme secure against all kind of attacks and providing us with provable absolute temporal authentication.