  Trusted timestamping consists in proving that certain data existed at a
particular point in time. Existing timestamping methods require either a
centralized and dedicated trusted service or the collaboration of other
participants using the timestamping service. We propose a novel trusted
timestamping scheme, called DNStamp, that does not require a dedicated service
nor collaboration between participants. DNStamp produces shortlived timestamps
with a validity period of several days. The generation and verification
involves a large number of Domain Name System cache resolvers, thus removing
any single point of failure and any single point of trust. Any host with
Internet access may request or verify a timestamp, with no need to register to
any timestamping service. We provide a full description and analysis of
DNStamp. We analyze the security against various adversaries and show
resistance to forward-dating, back-dating and erasure attacks. Experiments with
our implementation of DNStamp show that one can set and then reliably verify
timestamps even under continuous attack conditions.
