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

@@ -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,

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()));
 

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;

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,