Note commitment derivation

src/address.rs

@@ -1,4 +1,9 @@
-use crate::keys::{DiversifiedTransmissionKey, Diversifier};
+use pasta_curves::pallas;
+
+use crate::{
+    keys::{DiversifiedTransmissionKey, Diversifier},
+    spec::diversify_hash,
+};
 
 /// A shielded payment address.
 ///
@@ -18,6 +23,18 @@ pub struct Address {
 
 impl Address {
     pub(crate) fn from_parts(d: Diversifier, pk_d: DiversifiedTransmissionKey) -> Self {
+        // We assume here that pk_d is correctly-derived from d. We ensure this for
+        // internal APIs. For parsing from raw byte encodings, we assume that users aren't
+        // modifying internals of encoded address formats. If they do, that can result in
+        // lost funds, but we can't defend against that from here.
         Address { d, pk_d }
     }
+
+    pub(crate) fn g_d(&self) -> pallas::Point {
+        diversify_hash(self.d.as_array())
+    }
+
+    pub(crate) fn pk_d(&self) -> &DiversifiedTransmissionKey {
+        &self.pk_d
+    }
 }

src/keys.rs

@@ -229,6 +229,13 @@ impl DiversifierKey {
 #[derive(Debug)]
 pub struct Diversifier([u8; 11]);
 
+impl Diversifier {
+    /// Returns the byte array corresponding to this diversifier.
+    pub fn as_array(&self) -> &[u8; 11] {
+        &self.0
+    }
+}
+
 /// A key that provides the capability to detect and decrypt incoming notes from the block
 /// chain, without being able to spend the notes or detect when they are spent.
 ///
@@ -290,7 +297,12 @@ impl DiversifiedTransmissionKey {
     ///
     /// [orchardkeycomponents]: https://zips.z.cash/protocol/nu5.pdf#orchardkeycomponents
     fn derive(ivk: &IncomingViewingKey, d: &Diversifier) -> Self {
-        let g_d = diversify_hash(&d.0);
+        let g_d = diversify_hash(&d.as_array());
         DiversifiedTransmissionKey(ka_orchard(&ivk.0, &g_d))
     }
+
+    /// $repr_P(self)$
+    pub(crate) fn to_bytes(&self) -> [u8; 32] {
+        self.0.to_bytes()
+    }
 }

src/note.rs

@@ -1,20 +1,33 @@
-use crate::{keys::FullViewingKey, value::NoteValue, Address};
+use group::GroupEncoding;
+use pasta_curves::pallas;
+
+use crate::{
+    keys::FullViewingKey,
+    spec::{prf_expand, to_base, to_scalar},
+    value::NoteValue,
+    Address,
+};
+
+mod commitment;
+pub use self::commitment::NoteCommitment;
 
 /// The ZIP 212 seed randomness for a note.
 #[derive(Debug)]
 struct RandomSeed([u8; 32]);
 
 impl RandomSeed {
-    fn psi(&self) -> () {
-        todo!()
+    /// Defined in [Zcash Protocol Spec § 4.7.3: Sending Notes (Orchard)][orchardsend].
+    ///
+    /// [orchardsend]: https://zips.z.cash/protocol/nu5.pdf#orchardsend
+    fn psi(&self) -> pallas::Base {
+        to_base(prf_expand(&self.0, &[0x09]))
     }
 
-    fn rcm(&self) -> () {
-        todo!()
-    }
-
-    fn esk(&self) -> () {
-        todo!()
+    /// Defined in [Zcash Protocol Spec § 4.7.3: Sending Notes (Orchard)][orchardsend].
+    ///
+    /// [orchardsend]: https://zips.z.cash/protocol/nu5.pdf#orchardsend
+    fn esk(&self) -> pallas::Scalar {
+        to_scalar(prf_expand(&self.0, &[0x04]))
     }
 }
 
@@ -38,8 +51,21 @@ pub struct Note {
 
 impl Note {
     /// Derives the commitment to this note.
+    ///
+    /// Defined in [Zcash Protocol Spec § 3.2: Notes][notes].
+    ///
+    /// [notes]: https://zips.z.cash/protocol/nu5.pdf#notes
     pub fn commitment(&self) -> NoteCommitment {
-        todo!()
+        let g_d = self.recipient.g_d();
+
+        NoteCommitment::derive(
+            g_d.to_bytes(),
+            self.recipient.pk_d().to_bytes(),
+            self.value,
+            self.rho.0,
+            self.rseed.psi(),
+            (&self.rseed).into(),
+        )
     }
 
     /// Derives the nullifier for this note.
@@ -52,10 +78,6 @@ impl Note {
 #[derive(Debug)]
 pub struct EncryptedNote;
 
-/// A commitment to a note.
-#[derive(Debug)]
-pub struct NoteCommitment;
-
 /// A unique nullifier for a note.
 #[derive(Debug)]
-pub struct Nullifier;
+pub struct Nullifier(pallas::Base);

src/note/commitment.rs

@@ -0,0 +1,55 @@
+use std::iter;
+
+use bitvec::{array::BitArray, order::Lsb0};
+use ff::PrimeField;
+use pasta_curves::pallas;
+
+use crate::{
+    constants::L_ORCHARD_BASE,
+    primitives::sinsemilla,
+    spec::{prf_expand, to_scalar},
+    value::NoteValue,
+};
+
+use super::RandomSeed;
+
+pub(super) struct NoteCommitTrapdoor(pallas::Scalar);
+
+impl From<&RandomSeed> for NoteCommitTrapdoor {
+    fn from(rseed: &RandomSeed) -> Self {
+        NoteCommitTrapdoor(to_scalar(prf_expand(&rseed.0, &[0x05])))
+    }
+}
+
+/// A commitment to a note.
+#[derive(Debug)]
+pub struct NoteCommitment(pallas::Point);
+
+impl NoteCommitment {
+    /// $NoteCommit^Orchard$.
+    ///
+    /// Defined in [Zcash Protocol Spec § 5.4.8.4: Sinsemilla commitments][concretesinsemillacommit].
+    ///
+    /// [concretesinsemillacommit]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillacommit
+    pub(super) fn derive(
+        g_d: [u8; 32],
+        pk_d: [u8; 32],
+        v: NoteValue,
+        rho: pallas::Base,
+        psi: pallas::Base,
+        rcm: NoteCommitTrapdoor,
+    ) -> Self {
+        let domain = sinsemilla::CommitDomain::new("z.cash:Orchard-NoteCommit");
+        NoteCommitment(
+            domain.commit(
+                iter::empty()
+                    .chain(BitArray::<Lsb0, _>::new(g_d).iter().by_val())
+                    .chain(BitArray::<Lsb0, _>::new(pk_d).iter().by_val())
+                    .chain(v.to_le_bits().iter().by_val())
+                    .chain(rho.to_le_bits().iter().by_val().take(L_ORCHARD_BASE))
+                    .chain(psi.to_le_bits().iter().by_val().take(L_ORCHARD_BASE)),
+                &rcm.0,
+            ),
+        )
+    }
+}

src/value.rs

@@ -13,10 +13,18 @@
 //! [`Action`]: crate::bundle::Action
 //! [`Bundle`]: crate::bundle::Bundle
 
+use bitvec::{array::BitArray, order::Lsb0};
+
 /// The value of an individual Orchard note.
-#[derive(Debug)]
+#[derive(Clone, Copy, Debug)]
 pub struct NoteValue(u64);
 
+impl NoteValue {
+    pub(crate) fn to_le_bits(&self) -> BitArray<Lsb0, [u8; 8]> {
+        BitArray::<Lsb0, _>::new(self.0.to_le_bytes())
+    }
+}
+
 /// A sum of Orchard note values.
 #[derive(Debug)]
 pub struct ValueSum(i64);