Chapter

Trusted Systems

Volume 7711 of the series Lecture Notes in Computer Science pp 5-24

Stamp and Extend – Instant But Undeniable Timestamping Based on Lazy Trees

  • Łukasz KrzywieckiAffiliated withFaculty of Fundamental Problems of Technology, Wrocław University of Technology
  • , Przemysław KubiakAffiliated withFaculty of Fundamental Problems of Technology, Wrocław University of Technology
  • , Mirosław KutyłowskiAffiliated withFaculty of Fundamental Problems of Technology, Wrocław University of Technology

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Abstract

We present a Stamp&Extend time-stamping scheme based on linking via modified creation of Schnorr signatures. The scheme is based on lazy construction of a tree of signatures.

Stamp&Extend returns a timestamp immediately after the request, unlike the schemes based on the concept of timestamping rounds. Despite the fact that all timestamps are linearly linked, verification of a timestamp requires a logarithmic number of steps with respect to the chain length. An extra feature of the scheme is that any attempt to forge a timestamp by the Time Stamping Authority (TSA) results in revealing its secret key, providing an undeniable cryptographic evidence of misbehavior of TSA.

Breaking Stamp&Extend requires not only breaking Schnorr signatures, but to some extend also breaking Pedersen commitments.

Keywords

timestamping undeniability forgery evidence Schnorr signature