text: // Copyright (c) 2015-2017 The DigiByte Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include <consensus/merkle.h>
  #include <hash.h>
  #include <utilstrencodings.h>
  
  /*     WARNING! If you're reading this because you're learning about crypto
         and/or designing a new system that will use merkle trees, keep in mind
         that the following merkle tree algorithm has a serious flaw related to
         duplicate txids, resulting in a vulnerability (CVE-2012-2459).
  
         The reason is that if the number of hashes in the list at a given time
         is odd, the last one is duplicated before computing the next level (which
         is unusual in Merkle trees). This results in certain sequences of
         transactions leading to the same merkle root. For example, these two
         trees:
  
                      A               A
                    /  \            /   \
                  B     C         B       C
                 / \    |        / \     / \
                D   E   F       D   E   F   F
               / \ / \ / \     / \ / \ / \ / \
               1 2 3 4 5 6     1 2 3 4 5 6 5 6
  
         for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
         6 are repeated) result in the same root hash A (because the hash of both
         of (F) and (F,F) is C).
  
         The vulnerability results from being able to send a block with such a
         transaction list, with the same merkle root, and the same block hash as
         the original without duplication, resulting in failed validation. If the
         receiving node proceeds to mark that block as permanently invalid
         however, it will fail to accept further unmodified (and thus potentially
         valid) versions of the same block. We defend against this by detecting
         the case where we would hash two identical hashes at the end of the list
         together, and treating that identically to the block having an invalid
         merkle root. Assuming no double-SHA256 collisions, this will detect all
         known ways of changing the transactions without affecting the merkle
         root.
  */
  
  
  uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
      bool mutation = false;
      while (hashes.size() > 1) {
          if (mutated) {
              for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
                  if (hashes[pos] == hashes[pos + 1]) mutation = true;
              }
          }
          if (hashes.size() & 1) {
              hashes.push_back(hashes.back());
          }
          SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
          hashes.resize(hashes.size() / 2);
      }
      if (mutated) *mutated = mutation;
      if (hashes.size() == 0) return uint256();
      return hashes[0];
  }
  
  
  uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
  {
      std::vector<uint256> leaves;
      leaves.resize(block.vtx.size());
      for (size_t s = 0; s < block.vtx.size(); s++) {
          leaves[s] = block.vtx[s]->GetHash();
      }
      return ComputeMerkleRoot(std::move(leaves), mutated);
  }
  
  uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated)
  {
      std::vector<uint256> leaves;
      leaves.resize(block.vtx.size());
      leaves[0].SetNull(); // The witness hash of the coinbase is 0.
      for (size_t s = 1; s < block.vtx.size(); s++) {
          leaves[s] = block.vtx[s]->GetWitnessHash();
      }
      return ComputeMerkleRoot(std::move(leaves), mutated);
  }
  
  