Add shielded_balances_match check

zebra-consensus/src/transaction.rs

@@ -53,6 +53,8 @@ pub enum VerifyTransactionError {
     WrongVersion,
     /// A transaction MUST move money around.
     NoTransfer,
+    /// The balance of money moving around doesn't compute.
+    BadBalance,
     /// Could not verify a transparent script
     Script(#[from] zebra_script::Error),
     /// Could not verify a Groth16 proof of a JoinSplit/Spend/Output description
@@ -180,6 +182,7 @@ where
                     }
 
                     if let Some(shielded_data) = shielded_data {
+                        check::shielded_balances_match(&shielded_data, *value_balance)?;
                         for spend in shielded_data.spends() {
                             // TODO: check that spend.cv and spend.rk are NOT of small
                             // order.

zebra-consensus/src/transaction/check.rs

@@ -1,7 +1,7 @@
-use std::convert::TryFrom;
+use std::convert::{TryFrom, TryInto};
 
 use zebra_chain::{
-    amount::Amount,
+    amount::{self, Amount, NegativeAllowed},
     primitives::{
         ed25519,
         redjubjub::{self, Binding},
@@ -76,3 +76,19 @@ pub fn any_coinbase_inputs_no_transparent_outputs(
         _ => return Err(VerifyTransactionError::WrongVersion),
     }
 }
+
+/// Check that if there are no Spends or Outputs, that valueBalance is also 0.
+///
+/// https://zips.z.cash/protocol/canopy.pdf#consensusfrombitcoin
+pub fn shielded_balances_match(
+    shielded_data: &ShieldedData,
+    value_balance: Amount,
+) -> Result<(), VerifyTransactionError> {
+    if shielded_data.spends().count() + shielded_data.outputs().count() != 0 {
+        return Ok(());
+    } else if i64::from(value_balance) == 0 {
+        return Ok(());
+    } else {
+        return Err(VerifyTransactionError::BadBalance);
+    }
+}