1 THE DIGITAL TIMESTAMPING PROBLEM Bart Van Rompay ∗ , Bart Preneel † , Joos Vandewalle Katholieke Universiteit Leuven, ESAT–SISTA/COSIC K. Mercierlaan 94, B-3001 Heverlee, Belgium bart.vanrompay@esat.kuleuven.ac.be In this paper we examine the relation between time and cryptography. Time- stamping systems provide temporal authentication of electronic documents, they certify the date a document was created or last modified. Important applications are found in legal situations, electronic contracts, the protection of Intellectual Property Rights, and Internet security in general. We give an overview of the problem and discuss several state-of-the-art techniques. INTRODUCTION The interaction between time and cryptography is a relatively new subject. [HS91] is the reference work in this area. A study was made in the framework of the Belgian Timesec project (see [MQ97] and [PRQ + 98]). Most cryptographic research has been focused on secure messaging systems. Symmetric and public key encryption are used for protecting confidentiality. Authenticity (data integrity and data origin authentication) can be obtained by applying keyed hash functions (for the symmetric key case) or digital signatures (in the public key scenario). However, the secure maintenance of documents with a long lifetime is more complicated. Key pairs used in public key systems have a limited lifetime and may be revoked if the private key has been compromized. In a non-repudiation service we must be able to determine, at a later time, if a document was signed (using the owner’s private signature key) within the validity period of the certificate. In other applications the time itself can be important, for example the date of an inventor’s patent claim to establish precedence over competing claims. Digital timestamping provides the solution to this problem, by certifying the date and time on which a particular document is submitted to the timestamping * Sponsored by the Timesec project of the Federal Office for Scientific, Technical and Cultural Affairs (OSTC), Belgium. † F.W.O. postdoctoral researcher, sponsored by the Fund for Scientific Research – Flanders (Bel- gium). Appeared in Proceedings of the 20th Symposium on Information Theory in the Benelux, A. Barb´ e, E. C. van der Meulen, and P. Vanroose (eds.), Werkgemeenschap voor Informatie- en Communicatietheorie, pp. 71–78, 1999.