International Association
for Cryptologic Research

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.