[Resource Topic] 2013/730: Stamp \& Extend -- Instant but Undeniable Timestamping based on Lazy Trees

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Stamp & Extend – Instant but Undeniable Timestamping based on Lazy Trees

Authors: Łukasz Krzywiecki, Przemys{\l}aw Kubiak, Miros{\l}aw Kuty{\l}owski


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.

ePrint: https://eprint.iacr.org/2013/730

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