fix: zcash sapling checking

diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index 6f5e640..c35203c 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -473,8 +473,10 @@ dependencies = [
  "hex",
  "keystore",
  "pczt",
+ "postcard",
  "rand_core 0.6.4",
  "rust_tools",
+ "serde",
  "thiserror-core",
  "zcash_note_encryption",
  "zcash_primitives",
diff --git a/rust/apps/zcash/Cargo.toml b/rust/apps/zcash/Cargo.toml
index c37b2b3..66a03e0 100644
--- a/rust/apps/zcash/Cargo.toml
+++ b/rust/apps/zcash/Cargo.toml
@@ -22,6 +22,8 @@ zcash_note_encryption = "0.4.1"
 keystore = { path = "../../keystore" }
 pczt = { version = "0.2.1", default-features = false, features = ["orchard", "sapling", "transparent", "zcp-builder"] }
 zcash_primitives = { version = "0.22", default-features = false, features = ["circuits", "test-dependencies", "transparent-inputs"] }
+postcard = { version = "1.0.3", features = ["alloc"] }
+serde = { workspace = true }
 
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage_nightly)'] }
diff --git a/rust/apps/zcash/src/lib.rs b/rust/apps/zcash/src/lib.rs
index ed6d102..6fefeb0 100644
--- a/rust/apps/zcash/src/lib.rs
+++ b/rust/apps/zcash/src/lib.rs
@@ -210,10 +210,13 @@ pub fn sign_pczt(pczt: &[u8], seed: &[u8]) -> Result<Vec<u8>> {
 #[cfg(feature = "cypherpunk")]
 #[cfg(test)]
 mod tests {
+    use alloc::vec::Vec;
+
     use consensus::MainNetwork;
     use keystore::algorithms::zcash::{calculate_seed_fingerprint, derive_ufvk};
     use ::pczt::roles::creator::Creator;
     use rand_core::OsRng;
+    use serde::{Deserialize, Serialize};
     use zcash_primitives::transaction::{
         builder::{BuildConfig, Builder, PcztResult},
         fees::zip317,
@@ -221,12 +224,58 @@ mod tests {
     use zcash_vendor::{
         orchard,
         transparent::{bundle as transparent, keys::IncomingViewingKey},
-        zcash_protocol::{memo::MemoBytes, value::Zatoshis},
+        zcash_protocol::{
+            consensus::{BranchId, NetworkConstants},
+            memo::MemoBytes,
+            value::Zatoshis,
+        },
         zip32,
     };
 
     use super::*;
     extern crate std;
+
+    const EMPTY_SAPLING_BUNDLE_ERROR: &str =
+        "sapling value_sum must be zero when Sapling bundle is empty";
+
+    #[derive(Serialize, Deserialize)]
+    struct PcztMirror {
+        global: ::pczt::common::Global,
+        transparent: ::pczt::transparent::Bundle,
+        sapling: SaplingBundleMirror,
+        orchard: ::pczt::orchard::Bundle,
+    }
+
+    #[derive(Serialize, Deserialize)]
+    struct SaplingBundleMirror {
+        spends: Vec<::pczt::sapling::Spend>,
+        outputs: Vec<::pczt::sapling::Output>,
+        value_sum: i128,
+        anchor: [u8; 32],
+        bsk: Option<[u8; 32]>,
+    }
+
+    fn malformed_pczt_with_empty_sapling_bundle_and_nonzero_value_sum() -> Vec<u8> {
+        let mut bytes = Creator::new(BranchId::Nu6.into(), 10, MainNetwork.coin_type(), [0; 32], [0; 32])
+            .build()
+            .serialize();
+        let mut pczt: PcztMirror = postcard::from_bytes(&bytes[8..]).unwrap();
+        assert!(pczt.sapling.spends.is_empty());
+        assert!(pczt.sapling.outputs.is_empty());
+
+        pczt.sapling.value_sum = 1;
+
+        bytes.truncate(8);
+        postcard::to_extend(&pczt, bytes).unwrap()
+    }
+
+    fn assert_empty_sapling_bundle_error<T: core::fmt::Debug>(result: Result<T>) {
+        assert_eq!(
+            result.unwrap_err(),
+            ZcashError::InvalidPczt(EMPTY_SAPLING_BUNDLE_ERROR.to_string())
+        );
+    }
+
     #[test]
     fn test_get_address() {
         let address = get_address(&MainNetwork, "uview1s2e0495jzhdarezq4h4xsunfk4jrq7gzg22tjjmkzpd28wgse4ejm6k7yfg8weanaghmwsvc69clwxz9f9z2hwaz4gegmna0plqrf05zkeue0nevnxzm557rwdkjzl4pl4hp4q9ywyszyjca8jl54730aymaprt8t0kxj8ays4fs682kf7prj9p24dnlcgqtnd2vnskkm7u8cwz8n0ce7yrwx967cyp6dhkc2wqprt84q0jmwzwnufyxe3j0758a9zgk9ssrrnywzkwfhu6ap6cgx3jkxs3un53n75s3");
@@ -360,6 +409,24 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_check_pczt_rejects_empty_sapling_bundle_with_nonzero_value_sum() {
+        let seed = [9u8; 32];
+        let malformed_pczt = malformed_pczt_with_empty_sapling_bundle_and_nonzero_value_sum();
+        let ufvk = derive_ufvk(&MainNetwork, &seed, "m/32'/133'/0'").unwrap();
+        let seed_fingerprint = calculate_seed_fingerprint(&seed).unwrap();
+
+        let result = check_pczt_cypherpunk(
+            &MainNetwork,
+            &malformed_pczt,
+            &ufvk.to_string(),
+            &seed_fingerprint,
+            0,
+        );
+
+        assert_empty_sapling_bundle_error(result);
+    }
+
     #[test]
     fn test_get_address_invalid_ufvk() {
         let invalid_ufvk = "invalid_ufvk_string";
diff --git a/rust/apps/zcash/src/pczt/check.rs b/rust/apps/zcash/src/pczt/check.rs
index 2fc656f..791a070 100644
--- a/rust/apps/zcash/src/pczt/check.rs
+++ b/rust/apps/zcash/src/pczt/check.rs
@@ -1,5 +1,7 @@
 // checking logic for PCZT
 
+use alloc::string::ToString;
+
 use super::*;
 
 #[cfg(feature = "cypherpunk")]
@@ -11,10 +13,34 @@ use zcash_vendor::{
     sha2::{Digest, Sha256},
     transparent::{self, address::TransparentAddress, keys::AccountPubKey},
     zcash_address::{ToAddress, ZcashAddress},
-    zcash_protocol::consensus::{self, NetworkConstants},
+    zcash_protocol::{
+        consensus::{self, NetworkConstants},
+        value::ZatBalance,
+    },
     zip32,
 };
 
+fn validate_sapling_bundle_consistency(pczt: &Pczt) -> Result<(), ZcashError> {
+    let value_balance = (*pczt.sapling().value_sum())
+        .try_into()
+        .ok()
+        .and_then(|v| ZatBalance::from_i64(v).ok())
+        .ok_or(ZcashError::InvalidPczt(
+            "sapling value_sum is invalid".to_string(),
+        ))?;
+    let sapling_value_sum: i64 = value_balance.into();
+    let has_sapling_bundle =
+        !pczt.sapling().spends().is_empty() || !pczt.sapling().outputs().is_empty();
+
+    if !has_sapling_bundle && sapling_value_sum != 0 {
+        return Err(ZcashError::InvalidPczt(
+            "sapling value_sum must be zero when Sapling bundle is empty".to_string(),
+        ));
+    }
+
+    Ok(())
+}
+
 #[cfg(feature = "cypherpunk")]
 pub fn check_pczt_orchard<P: consensus::Parameters>(
     params: &P,
@@ -23,6 +49,7 @@ pub fn check_pczt_orchard<P: consensus::Parameters>(
     ufvk: &UnifiedFullViewingKey,
     pczt: &Pczt,
 ) -> Result<(), ZcashError> {
+    validate_sapling_bundle_consistency(pczt)?;
     // checking orchard keys.
     let orchard = ufvk.orchard().ok_or(ZcashError::InvalidDataError(
         "orchard fvk is not present".to_string(),
@@ -44,6 +71,7 @@ pub fn check_pczt_transparent<P: consensus::Parameters>(
     pczt: &Pczt,
     check_sfp: bool,
 ) -> Result<(), ZcashError> {
+    validate_sapling_bundle_consistency(pczt)?;
     Verifier::new(pczt.clone())
         .with_transparent(|bundle| {
             check_transparent(
diff --git a/rust/apps/zcash/src/pczt/parse.rs b/rust/apps/zcash/src/pczt/parse.rs
index 7bdd02b..be0b9f8 100644
--- a/rust/apps/zcash/src/pczt/parse.rs
+++ b/rust/apps/zcash/src/pczt/parse.rs
@@ -28,7 +28,6 @@ use zcash_vendor::orchard::{
 };
 
 use crate::errors::ZcashError;
-
 use super::structs::{ParsedFrom, ParsedOrchard, ParsedPczt, ParsedTo, ParsedTransparent};
 
 const ZEC_DIVIDER: u32 = 100_000_000;