From 9b7cb688ed615e75ba6a56e3c27b0993739cf0d7 Mon Sep 17 00:00:00 2001
From: Luke Parker <lukeparker5132@gmail.com>
Date: Thu, 31 Aug 2023 23:04:37 -0400
Subject: [PATCH] Have Batch contain Block and batch ID, ensuring eclipsed
 validators don't publish invalid shares

See prior commit message for more info.

With the plan for the batch sign ID to be just 5 bytes (potentially 4), this
does incur a +5 bytes cost compared to the ExternalBlock system *even in the
standard case*. The simplicity remains preferred at this time.
---
 coordinator/src/main.rs                |  2 +-
 coordinator/src/tests/tributary/mod.rs |  4 +++-
 coordinator/src/tributary/handle.rs    |  2 +-
 coordinator/src/tributary/mod.rs       | 13 ++++++++-----
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/coordinator/src/main.rs b/coordinator/src/main.rs
index 04489680..19208cb3 100644
--- a/coordinator/src/main.rs
+++ b/coordinator/src/main.rs
@@ -600,7 +600,7 @@ pub async fn handle_processors<D: Db, Pro: Processors, P: P2p>(
             MainDb::<D>::save_first_preprocess(&mut txn, id.id, preprocess);
             txn.commit();
 
-            Some(Transaction::Batch(id.id))
+            Some(Transaction::Batch(block.0, id.id))
           } else {
             Some(Transaction::BatchPreprocess(SignData {
               plan: id.id,
diff --git a/coordinator/src/tests/tributary/mod.rs b/coordinator/src/tests/tributary/mod.rs
index 4ed87fd8..62ea82e7 100644
--- a/coordinator/src/tests/tributary/mod.rs
+++ b/coordinator/src/tests/tributary/mod.rs
@@ -103,9 +103,11 @@ fn serialize_transaction() {
   ));
 
   {
+    let mut block = [0; 32];
+    OsRng.fill_bytes(&mut block);
     let mut batch = [0; 32];
     OsRng.fill_bytes(&mut batch);
-    test_read_write(Transaction::Batch(batch));
+    test_read_write(Transaction::Batch(block, batch));
   }
   test_read_write(Transaction::SubstrateBlock(OsRng.next_u64()));
 
diff --git a/coordinator/src/tributary/handle.rs b/coordinator/src/tributary/handle.rs
index 7355b457..3c223867 100644
--- a/coordinator/src/tributary/handle.rs
+++ b/coordinator/src/tributary/handle.rs
@@ -443,7 +443,7 @@ pub async fn handle_application_tx<
       }
     }
 
-    Transaction::Batch(batch) => {
+    Transaction::Batch(_, batch) => {
       // Because this Batch has achieved synchrony, its batch ID should be authorized
       TributaryDb::<D>::recognize_id(txn, Zone::Batch.label(), genesis, batch);
       recognized_id(spec.set().network, genesis, RecognizedIdType::Batch, batch).await;
diff --git a/coordinator/src/tributary/mod.rs b/coordinator/src/tributary/mod.rs
index 3329b7e7..f7858ff0 100644
--- a/coordinator/src/tributary/mod.rs
+++ b/coordinator/src/tributary/mod.rs
@@ -232,7 +232,7 @@ pub enum Transaction {
   DkgConfirmed(u32, [u8; 32], Signed),
 
   // When we have synchrony on a batch, we can allow signing it
-  Batch([u8; 32]),
+  Batch([u8; 32], [u8; 32]),
   // When a Serai block is finalized, with the contained batches, we can allow the associated plan
   // IDs
   SubstrateBlock(u64),
@@ -331,9 +331,11 @@ impl ReadWrite for Transaction {
       }
 
       3 => {
+        let mut block = [0; 32];
+        reader.read_exact(&mut block)?;
         let mut batch = [0; 32];
         reader.read_exact(&mut batch)?;
-        Ok(Transaction::Batch(batch))
+        Ok(Transaction::Batch(block, batch))
       }
 
       4 => {
@@ -430,8 +432,9 @@ impl ReadWrite for Transaction {
         signed.write(writer)
       }
 
-      Transaction::Batch(batch) => {
+      Transaction::Batch(block, batch) => {
         writer.write_all(&[3])?;
+        writer.write_all(block)?;
         writer.write_all(batch)
       }
 
@@ -475,7 +478,7 @@ impl TransactionTrait for Transaction {
       Transaction::DkgShares { signed, .. } => TransactionKind::Signed(signed),
       Transaction::DkgConfirmed(_, _, signed) => TransactionKind::Signed(signed),
 
-      Transaction::Batch(_) => TransactionKind::Provided("batch"),
+      Transaction::Batch(_, _) => TransactionKind::Provided("batch"),
       Transaction::SubstrateBlock(_) => TransactionKind::Provided("serai"),
 
       Transaction::BatchPreprocess(data) => TransactionKind::Signed(&data.signed),
@@ -534,7 +537,7 @@ impl Transaction {
         Transaction::DkgShares { ref mut signed, .. } => signed,
         Transaction::DkgConfirmed(_, _, ref mut signed) => signed,
 
-        Transaction::Batch(_) => panic!("signing Batch"),
+        Transaction::Batch(_, _) => panic!("signing Batch"),
         Transaction::SubstrateBlock(_) => panic!("signing SubstrateBlock"),
 
         Transaction::BatchPreprocess(ref mut data) => &mut data.signed,