UIP-4 Spend Backreferences (#4922)

## Describe your changes

This implements UIP-4 as described in
penumbra-zone/UIPs#2

## Issue ticket number and link

penumbra-zone/UIPs#2

## Checklist before requesting a review

- [ ] I have added guiding text to explain how a reviewer should test
these changes.

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

crates/bin/pcli/src/terminal.rs

@@ -11,7 +11,7 @@ use penumbra_keys::{
 };
 use penumbra_proof_params::GROTH16_PROOF_LENGTH_BYTES;
 use penumbra_sct::Nullifier;
-use penumbra_shielded_pool::{Note, NoteView};
+use penumbra_shielded_pool::{EncryptedBackref, Note, NoteView};
 use penumbra_tct::structure::Hash;
 use penumbra_transaction::{view, ActionPlan, ActionView, TransactionPlan, TransactionView};
 use termion::{color, input::TermRead};
@@ -82,6 +82,8 @@ fn pretty_print_transaction_plan(
                 balance_commitment: dummy_commitment(),
                 nullifier: Nullifier(Fq::default()),
                 rk: dummy_pk(),
+                encrypted_backref: EncryptedBackref::try_from([0u8; 0])
+                    .expect("can create dummy encrypted backref"),
             },
             auth_sig: dummy_sig(),
             proof: dummy_proof_spend(),

crates/core/component/shielded-pool/src/backref.rs

@@ -0,0 +1,217 @@
+use anyhow::Result;
+use chacha20poly1305::{
+    aead::{Aead, NewAead},
+    ChaCha20Poly1305, Nonce,
+};
+
+use penumbra_keys::BackreferenceKey;
+use penumbra_sct::Nullifier;
+use penumbra_tct as tct;
+
+pub const ENCRYPTED_BACKREF_LEN: usize = 48;
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct Backref {
+    note_commitment: tct::StateCommitment,
+}
+
+#[derive(Clone, Debug)]
+pub struct EncryptedBackref {
+    /// The inner bytes can either have 0 or `ENCRYPTED_BACKREF_LEN` bytes.
+    bytes: Vec<u8>,
+}
+
+impl Backref {
+    pub fn new(note_commitment: tct::StateCommitment) -> Self {
+        Self { note_commitment }
+    }
+
+    pub fn encrypt(&self, brk: &BackreferenceKey, nullifier: &Nullifier) -> EncryptedBackref {
+        let cipher = ChaCha20Poly1305::new(&brk.0);
+
+        // Nonce is the first 12 bytes of the nullifier
+        let nonce_bytes = &nullifier.to_bytes()[..12];
+        let nonce = Nonce::from_slice(&nonce_bytes);
+
+        let plaintext = self.note_commitment.0.to_bytes();
+
+        let ciphertext = cipher
+            .encrypt(nonce, plaintext.as_ref())
+            .expect("encryption should succeed  ");
+
+        EncryptedBackref { bytes: ciphertext }
+    }
+}
+
+impl EncryptedBackref {
+    pub fn is_empty(&self) -> bool {
+        self.bytes.is_empty()
+    }
+
+    pub fn len(&self) -> usize {
+        self.bytes.len()
+    }
+
+    pub fn dummy() -> Self {
+        Self { bytes: vec![] }
+    }
+
+    /// Decrypts the encrypted backref, returning a backref if the decryption is successful,
+    /// or `None` if the encrypted backref is zero-length.
+    pub fn decrypt(
+        &self,
+        brk: &BackreferenceKey,
+        nullifier: &Nullifier,
+    ) -> Result<Option<Backref>> {
+        // We might have a 0-length encrypted backref, which
+        // is treated as a valid value and means that the note has no backref.
+        if self.is_empty() {
+            return Ok(None);
+        }
+
+        let cipher = ChaCha20Poly1305::new(&brk.0);
+
+        let nonce_bytes = &nullifier.to_bytes()[..12];
+        let nonce = Nonce::from_slice(&nonce_bytes);
+
+        let plaintext = cipher
+            .decrypt(nonce, self.bytes.as_ref())
+            .map_err(|_| anyhow::anyhow!("decryption error"))?;
+
+        let note_commitment_bytes: [u8; 32] = plaintext
+            .try_into()
+            .map_err(|_| anyhow::anyhow!("decryption error"))?;
+
+        let backref = Backref::try_from(note_commitment_bytes)
+            .map_err(|_| anyhow::anyhow!("decryption error"))?;
+
+        Ok(Some(backref))
+    }
+}
+
+impl TryFrom<[u8; 32]> for Backref {
+    type Error = anyhow::Error;
+
+    fn try_from(bytes: [u8; 32]) -> Result<Self> {
+        Ok(Self {
+            note_commitment: tct::StateCommitment::try_from(bytes)
+                .map_err(|_| anyhow::anyhow!("invalid note commitment"))?,
+        })
+    }
+}
+
+// EncryptedBackrefs can either have 0 or ENCRYPTED_BACKREF_LEN bytes.
+
+impl TryFrom<[u8; ENCRYPTED_BACKREF_LEN]> for EncryptedBackref {
+    type Error = anyhow::Error;
+
+    fn try_from(bytes: [u8; ENCRYPTED_BACKREF_LEN]) -> Result<Self> {
+        Ok(Self {
+            bytes: bytes.to_vec(),
+        })
+    }
+}
+
+impl TryFrom<[u8; 0]> for EncryptedBackref {
+    type Error = anyhow::Error;
+
+    fn try_from(bytes: [u8; 0]) -> Result<Self> {
+        Ok(Self {
+            bytes: bytes.to_vec(),
+        })
+    }
+}
+
+impl From<EncryptedBackref> for Vec<u8> {
+    fn from(encrypted_backref: EncryptedBackref) -> Vec<u8> {
+        encrypted_backref.bytes
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proptest::prelude::*;
+
+    use penumbra_asset::{asset, Value};
+    use penumbra_keys::keys::{Bip44Path, SeedPhrase, SpendKey};
+
+    use crate::{Note, Rseed};
+
+    proptest! {
+        #[test]
+        fn encrypted_backref_zero_length(seed_phrase_randomness in any::<[u8; 32]>(), amount_to_send in any::<u64>(), rseed_randomness in any::<[u8; 32]>()) {
+            let seed_phrase = SeedPhrase::from_randomness(&seed_phrase_randomness);
+            let sk = SpendKey::from_seed_phrase_bip44(seed_phrase, &Bip44Path::new(0));
+            let fvk = sk.full_viewing_key();
+            let brk = fvk.backref_key();
+
+            let ivk = fvk.incoming();
+            let (sender, _dtk_d) = ivk.payment_address(0u32.into());
+
+            let value_to_send = Value {
+                amount: amount_to_send.into(),
+                asset_id: asset::Cache::with_known_assets()
+                    .get_unit("upenumbra")
+                    .unwrap()
+                    .id(),
+            };
+            let rseed = Rseed(rseed_randomness);
+
+            let note = Note::from_parts(sender, value_to_send, rseed).expect("valid note");
+            let note_commitment: penumbra_tct::StateCommitment = note.commit();
+            let nk = *sk.nullifier_key();
+            let mut sct = tct::Tree::new();
+
+            sct.insert(tct::Witness::Keep, note_commitment).unwrap();
+            let state_commitment_proof = sct.witness(note_commitment).unwrap();
+            let nullifier = Nullifier::derive(&nk, state_commitment_proof.position(), &note_commitment);
+
+            let encrypted_backref = EncryptedBackref::dummy();
+            assert!(encrypted_backref.is_empty());
+            assert_eq!(encrypted_backref.len(), 0);
+
+            // Decrypting a zero-length encrypted backref should return `None`.
+            let decrypted_backref = encrypted_backref.decrypt(&brk, &nullifier).unwrap();
+            assert_eq!(decrypted_backref, None);
+        }
+    }
+
+    proptest! {
+        #[test]
+        fn encrypted_backref_round_trip(seed_phrase_randomness in any::<[u8; 32]>(), amount_to_send in any::<u64>(), rseed_randomness in any::<[u8; 32]>()) {
+            let seed_phrase = SeedPhrase::from_randomness(&seed_phrase_randomness);
+            let sk = SpendKey::from_seed_phrase_bip44(seed_phrase, &Bip44Path::new(0));
+            let fvk = sk.full_viewing_key();
+            let brk = fvk.backref_key();
+
+            let ivk = fvk.incoming();
+            let (sender, _dtk_d) = ivk.payment_address(0u32.into());
+
+            let value_to_send = Value {
+                amount: amount_to_send.into(),
+                asset_id: asset::Cache::with_known_assets()
+                    .get_unit("upenumbra")
+                    .unwrap()
+                    .id(),
+            };
+            let rseed = Rseed(rseed_randomness);
+
+            let note = Note::from_parts(sender, value_to_send, rseed).expect("valid note");
+            let note_commitment: penumbra_tct::StateCommitment = note.commit();
+            let nk = *sk.nullifier_key();
+            let mut sct = tct::Tree::new();
+
+            sct.insert(tct::Witness::Keep, note_commitment).unwrap();
+            let state_commitment_proof = sct.witness(note_commitment).unwrap();
+            let nullifier = Nullifier::derive(&nk, state_commitment_proof.position(), &note_commitment);
+
+            let backref = Backref::new(note_commitment);
+            let encrypted_backref = backref.encrypt(&brk, &nullifier);
+
+            let decrypted_backref = encrypted_backref.decrypt(&brk, &nullifier).unwrap();
+
+            assert_eq!(Some(backref), decrypted_backref);
+        }
+    }
+}

crates/core/component/shielded-pool/src/lib.rs

@@ -25,6 +25,9 @@ pub mod nullifier_derivation;
 pub mod output;
 pub mod spend;
 
+pub mod backref;
+pub use backref::{Backref, EncryptedBackref};
+
 pub use convert::{ConvertCircuit, ConvertProof, ConvertProofPrivate, ConvertProofPublic};
 pub use nullifier_derivation::{
     NullifierDerivationCircuit, NullifierDerivationProof, NullifierDerivationProofPrivate,

crates/core/component/shielded-pool/src/spend/action.rs

@@ -9,6 +9,7 @@ use penumbra_txhash::{EffectHash, EffectingData};
 use serde::{Deserialize, Serialize};
 
 use crate::SpendProof;
+use crate::{backref::ENCRYPTED_BACKREF_LEN, EncryptedBackref};
 
 #[derive(Clone, Debug)]
 pub struct Spend {
@@ -23,6 +24,7 @@ pub struct Body {
     pub balance_commitment: balance::Commitment,
     pub nullifier: Nullifier,
     pub rk: VerificationKey<SpendAuth>,
+    pub encrypted_backref: EncryptedBackref,
 }
 
 impl EffectingData for Body {
@@ -91,6 +93,7 @@ impl From<Body> for pb::SpendBody {
             balance_commitment: Some(msg.balance_commitment.into()),
             nullifier: Some(msg.nullifier.into()),
             rk: Some(msg.rk.into()),
+            encrypted_backref: msg.encrypted_backref.into(),
         }
     }
 }
@@ -117,10 +120,26 @@ impl TryFrom<pb::SpendBody> for Body {
             .try_into()
             .context("malformed rk")?;
 
+        // `EncryptedBackref` must have 0 or `ENCRYPTED_BACKREF_LEN` bytes.
+        let encrypted_backref: EncryptedBackref;
+        if proto.encrypted_backref.len() == ENCRYPTED_BACKREF_LEN {
+            let bytes: [u8; ENCRYPTED_BACKREF_LEN] = proto
+                .encrypted_backref
+                .try_into()
+                .map_err(|_| anyhow::anyhow!("invalid encrypted backref"))?;
+            encrypted_backref = EncryptedBackref::try_from(bytes)
+                .map_err(|_| anyhow::anyhow!("invalid encrypted backref"))?;
+        } else if proto.encrypted_backref.len() == 0 {
+            encrypted_backref = EncryptedBackref::dummy();
+        } else {
+            return Err(anyhow::anyhow!("invalid encrypted backref length"));
+        }
+
         Ok(Body {
             balance_commitment,
             nullifier,
             rk,
+            encrypted_backref,
         })
     }
 }

crates/core/component/shielded-pool/src/spend/plan.rs

@@ -9,7 +9,7 @@ use rand_core::{CryptoRng, RngCore};
 use serde::{Deserialize, Serialize};
 
 use super::{Body, Spend, SpendProof};
-use crate::{Note, Rseed, SpendProofPrivate, SpendProofPublic};
+use crate::{Backref, Note, Rseed, SpendProofPrivate, SpendProofPublic};
 
 /// A planned [`Spend`](Spend).
 #[derive(Clone, Debug, Deserialize, Serialize)]
@@ -75,10 +75,14 @@ impl SpendPlan {
 
     /// Construct the [`spend::Body`] described by this [`SpendPlan`].
     pub fn spend_body(&self, fvk: &FullViewingKey) -> Body {
+        // Construct the backreference for this spend.
+        let backref = Backref::new(self.note.commit());
+        let encrypted_backref = backref.encrypt(&fvk.backref_key(), &self.nullifier(fvk));
         Body {
             balance_commitment: self.balance().commit(self.value_blinding),
             nullifier: self.nullifier(fvk),
             rk: self.rk(fvk),
+            encrypted_backref,
         }
     }
 

crates/core/keys/src/keys/fvk.rs

@@ -12,7 +12,7 @@ use crate::keys::wallet_id::WalletId;
 use crate::{
     fmd, ka, prf,
     rdsa::{SpendAuth, VerificationKey},
-    Address, AddressView,
+    Address, AddressView, BackreferenceKey,
 };
 
 use super::{AddressIndex, DiversifierKey, IncomingViewingKey, NullifierKey, OutgoingViewingKey};
@@ -120,6 +120,11 @@ impl FullViewingKey {
         &self.ak
     }
 
+    /// Construct the backreference key for this full viewing key.
+    pub fn backref_key(&self) -> BackreferenceKey {
+        BackreferenceKey::derive(self.outgoing()).clone()
+    }
+
     /// Hashes the full viewing key into an [`WalletId`].
     pub fn wallet_id(&self) -> WalletId {
         let hash_result = hash_2(

crates/core/keys/src/lib.rs

@@ -14,7 +14,7 @@ pub mod test_keys;
 
 pub use address::{Address, AddressVar, AddressView};
 pub use keys::FullViewingKey;
-pub use symmetric::PayloadKey;
+pub use symmetric::{BackreferenceKey, PayloadKey};
 
 fn fmt_hex<T: AsRef<[u8]>>(data: T, f: &mut std::fmt::Formatter) -> std::fmt::Result {
     write!(f, "{}", hex::encode(data))

crates/core/keys/src/symmetric.rs

@@ -344,3 +344,20 @@ impl TryFrom<&[u8]> for WrappedMemoKey {
         Ok(Self(bytes))
     }
 }
+
+/// Represents a symmetric `ChaCha20Poly1305` key used for Spend backreferences.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct BackreferenceKey(pub Key);
+
+impl BackreferenceKey {
+    pub fn derive(ovk: &OutgoingViewingKey) -> Self {
+        let mut kdf_params = blake2b_simd::Params::new();
+        kdf_params.personal(b"Penumbra_Backref");
+        kdf_params.hash_length(32);
+        let mut kdf = kdf_params.to_state();
+        kdf.update(&ovk.to_bytes());
+
+        let key = kdf.finalize();
+        Self(*Key::from_slice(key.as_bytes()))
+    }
+}

crates/proto/src/gen/penumbra.core.component.shielded_pool.v1.rs

@@ -420,6 +420,9 @@ pub struct SpendBody {
     pub rk: ::core::option::Option<
         super::super::super::super::crypto::decaf377_rdsa::v1::SpendVerificationKey,
     >,
+    /// An encryption of the commitment of the input note to the sender's OVK.
+    #[prost(bytes = "vec", tag = "7")]
+    pub encrypted_backref: ::prost::alloc::vec::Vec<u8>,
 }
 impl ::prost::Name for SpendBody {
     const NAME: &'static str = "SpendBody";