test: add decryption for 0 length UIP 4 encrypted backrefs

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

@@ -142,6 +142,44 @@ mod tests {
 
     use crate::Note;
 
+    #[test]
+    fn encrypted_backref_zero_length() {
+        let rng = OsRng;
+
+        let seed_phrase = SeedPhrase::generate(rng);
+        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: 1u64.into(),
+            asset_id: asset::Cache::with_known_assets()
+                .get_unit("upenumbra")
+                .unwrap()
+                .id(),
+        };
+
+        let note = Note::generate(&mut OsRng, &sender, value_to_send);
+        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);
+    }
+
     #[test]
     fn encrypted_backref_round_trip() {
         let rng = OsRng;
@@ -176,6 +214,6 @@ mod tests {
 
         let decrypted_backref = encrypted_backref.decrypt(&brk, &nullifier).unwrap();
 
-        assert_eq!(backref, decrypted_backref);
+        assert_eq!(Some(backref), decrypted_backref);
     }
 }