fix: only allow a monero header if it serializes back to the same data (

tari-project#5716)

Description
---
Adds a check that the serialized monero POWdata matches the deserialized
data

Co-authored-by: SW van Heerden <swvheerden@gmail.com>

base_layer/core/src/proof_of_work/monero_rx/error.rs

@@ -28,6 +28,8 @@ use crate::proof_of_work::DifficultyError;
 /// Errors that can occur when merging Monero PoW data with Tari PoW data
 #[derive(Debug, thiserror::Error)]
 pub enum MergeMineError {
+    #[error("Serialized POWData does not match provided data: {0}")]
+    SerializedPowDataDoesNotMatch(String),
     #[error("Serialization error: {0}")]
     SerializeError(String),
     #[error("Error deserializing Monero data: {0}")]

base_layer/core/src/proof_of_work/monero_rx/merkle_tree.rs

@@ -173,6 +173,9 @@ impl MerkleProof {
     }
 
     /// Calculates the merkle root hash from the provide Monero hash
+    /// The coinbase must be the first transaction in the block, so
+    /// that you can't have multiple coinbases in a block. That means the coinbase
+    /// is always the leftmost branch in the merkle tree
     pub fn calculate_root(&self, hash: &Hash) -> Hash {
         if self.branch.is_empty() {
             return *hash;

base_layer/core/src/proof_of_work/monero_rx/pow_data.rs

@@ -103,6 +103,21 @@ impl MoneroPowData {
                 v.len()
             )));
         }
+        let mut test_serialized_data = vec![];
+
+        // This is an inefficient test, so maybe it can be removed in future, but because we rely
+        // on third party parsing libraries, there could be a case where the data we deserialized
+        // can be generated from multiple input data. This way we test that there is only one of those
+        // inputs that is allowed. Remember that the data in powdata is used for the hash, so having
+        // multiple pow_data that generate the same randomx difficulty could be a problem.
+        BorshSerialize::serialize(&pow_data, &mut test_serialized_data)
+            .map_err(|e| MergeMineError::SerializeError(format!("{:?}", e)))?;
+        if test_serialized_data != tari_header.pow.pow_data {
+            return Err(MergeMineError::SerializedPowDataDoesNotMatch(
+                "Serialized pow data does not match original pow data".to_string(),
+            ));
+        }
+
         Ok(pow_data)
     }