Stamp&Extend  
     - Instant but Undeniable Timestamping based on Lazy Trees
      
       Lukasz Krzywiecki, Przemyslaw Kubiak, Miroslaw Kutylowski

                (paper accepted for INTRUST 2012)

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 is returned immediately after the request, unlike
the schemes based on the round concept. Nevertheless, all timestamps are 
linearly linked and timestamp verification requires a logarithmic number of steps
and  a logarithmic number of signatures from Time Stamping Authority (TSA).
An extra feature of the scheme is that any attempt  to forge a timestamp by
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.