Stamp and Extend – Instant But Undeniable Timestamping Based on Lazy Trees
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.
Keywordstimestamping undeniability forgery evidence Schnorr signature
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