Merge pull request #224 from anoma/xuyang/update_nullifier_derivation…

…_spec

update nullifier derivation in spec

book/src/spec.md

@@ -95,7 +95,7 @@ Note commitments are stored in a global commitment tree. The global commitment t
 |`cm` |$E_p$|$cm = \mathrm{NoteCom}(note, rcm\_{note})$|
 
 ### 2.4 Nullifier
-Note nullifiers are stored in a global nullifier set. Adding a note's nullifier to the set invalidates the note. We use the same nullifier derivation algorithm as in Orchard: $\mathrm{DeriveNullifier}_{nk}(ρ, ψ, cm) = \mathrm{Extract}([PRF^{nf}_{nk}(ρ) + ψ \mod{q}]K + cm)$.
+Note nullifiers are stored in a global nullifier set. Adding a note's nullifier to the set invalidates the note. We use the same nullifier derivation algorithm as in Orchard: $\mathrm{DeriveNullifier}_{nk}(ρ, ψ, cm) = PRF^{nf}_{nk}(ρ, ψ, cm)$.
 
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