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Schnorr signature scheme

The Topos protocol adapts a Schnorr signature scheme for efficient verification inside the Topos STARK proving system, by replacing the commonly used binary hash function to an algebraically efficient one, the Rescue-Prime hash function.

The internal design of the signing process (and hence of the verification algorithm as well) follows the specification used in libsecp256k1, deviating from the original version by a difference in the output of the signing phase. This design choice is justified by Bellare, et al.1 showing that Schnorr signatures as implemented in libsecp256k1 can achieve semi-strong unforgeability, while the original construction of Claus Schnorr can only provide normal unforgeability.