This paper shows that the scheme described in Haber and Stornetta [Haber and Stornetta Jr., 1994] for extending the validity of a cryptographic timestamp for a Time Stamping Service contains security shortcomings. A modification is proposed to rectify the identified shortcomings.

1 Institut für Angewandte und Physikalische Chemie, Arbeitsgruppe Chemische Synergetik, Universität Bremen, Bibliothekstraße NW 2, 28359 Bremen, Germany 2Department of Physical Chemistry, Spatiotemporal Self-organization Group, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany 3Department of Physical Chemistry Complex Systems Group, Fritz-Haber-Institut der...

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