This is a greatly simplified and slightly tweaked version of
af2e3713e2
Their version will detect duplicate zkproofs across transactions while
this code will only detect duplicate zkproofs in a single ztx. If dupes
are found, the tx will be denied entry into the mempool.
This provides most of the benefit (increased CPU cost to attackers) with the
least code change and no annoyance to full node operators. Detecting
duplicate zkproofs across transactions requires a one-time reindex of
all of history, which means significant downtime for nodes.
Since Hush + HSCs have a much more strict policy on number of shielded
outputs and shielded inputs, only detecting duplicate zkproofs in
individual ztxs seems sufficient for now.
No correctly functioning node or wallet will ever create duplicate
zkproofs, so there is no worry of this accidentally affecting normal
users. Currently this is not a consensus rule but it could become one
in the future.
Duke