Correct the prior documented TOCTOU

Now, if a malicious validator set publishes a malicious `Batch` at the last
moment, it'll cause all future `Batch`s signed by the next validator set to
require a bool being set (yet they never will set it).

This will prevent the handover.

The only overhead is having two distinct `batch_message` calls on-chain.

substrate/client/tests/common/in_instructions.rs

@@ -39,7 +39,8 @@ pub async fn provide_batch(batch: Batch) -> [u8; 32] {
 
   let block = publish_tx(&Serai::execute_batch(SignedBatch {
     batch: batch.clone(),
-    signature: pair.sign(&batch_message(&batch)),
+    // TODO: This `batch.id == 0` line only works when session == 0
+    signature: pair.sign(&batch_message(batch.id == 0, &batch)),
   }))
   .await;
 

substrate/in-instructions/pallet/src/lib.rs

@@ -155,14 +155,21 @@ pub mod pallet {
       // verify the signature
       let network = batch.batch.network;
       let (current_session, prior, current) = keys_for_network::<T>(network)?;
-      let batch_message = batch_message(&batch.batch);
       // Check the prior key first since only a single `Batch` (the last one) will be when prior is
       // Some yet prior wasn't the signing key
-      let valid_by_prior =
-        if let Some(key) = prior { key.verify(&batch_message, &batch.signature) } else { false };
+      let valid_by_prior = if let Some(key) = prior {
+        key.verify(&batch_message(false, &batch.batch), &batch.signature)
+      } else {
+        false
+      };
       let valid = valid_by_prior ||
         (if let Some(key) = current {
-          key.verify(&batch_message, &batch.signature)
+          key.verify(
+            // This `== 0` is valid as either it'll be the first Batch for the first set, or if
+            // they never had a Batch, the first Batch for the next set
+            &batch_message((batch.batch.id == 0) || prior.is_some(), &batch.batch),
+            &batch.signature,
+          )
         } else {
           false
         });

substrate/in-instructions/primitives/src/lib.rs

@@ -84,6 +84,6 @@ impl Zeroize for SignedBatch {
 
 // TODO: Make this an associated method?
 /// The message for the batch signature.
-pub fn batch_message(batch: &Batch) -> Vec<u8> {
-  [b"InInstructions-batch".as_ref(), &batch.encode()].concat()
+pub fn batch_message(is_first_batch_of_set: bool, batch: &Batch) -> Vec<u8> {
+  [b"InInstructions-batch".as_ref(), &(is_first_batch_of_set, batch).encode()].concat()
 }