[zcash/zcash] Some question about shielded transaction (#3428)
@NickyYangYang nf is almost certainly unique to a given note, because it’s derived using a collision-resistant PRF applied to ρ and ask, and ρ is also almost certainly unique as described in the section «Faerie Gold attack and fix» of the protocol spec.
If any of the nf values revealed by a transaction have already appeared then the transaction should be rejected.