1  // Copyright (c) 2015-2020 The Bitcoin Core developers
 2  // Distributed under the MIT software license, see the accompanying
 3  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 4  
 5  #include <consensus/merkle.h>
 6  #include <hash.h>
 7  
 8  /*     WARNING! If you're reading this because you're learning about crypto
 9         and/or designing a new system that will use merkle trees, keep in mind
10         that the following merkle tree algorithm has a serious flaw related to
11         duplicate txids, resulting in a vulnerability (CVE-2012-2459).
12  
13         The reason is that if the number of hashes in the list at a given level
14         is odd, the last one is duplicated before computing the next level (which
15         is unusual in Merkle trees). This results in certain sequences of
16         transactions leading to the same merkle root. For example, these two
17         trees:
18  
19                      A               A
20                    /  \            /   \
21                  B     C         B       C
22                 / \    |        / \     / \
23                D   E   F       D   E   F   F
24               / \ / \ / \     / \ / \ / \ / \
25               1 2 3 4 5 6     1 2 3 4 5 6 5 6
26  
27         for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
28         6 are repeated) result in the same root hash A (because the hash of both
29         of (F) and (F,F) is C).
30  
31         The vulnerability results from being able to send a block with such a
32         transaction list, with the same merkle root, and the same block hash as
33         the original without duplication, resulting in failed validation. If the
34         receiving node proceeds to mark that block as permanently invalid
35         however, it will fail to accept further unmodified (and thus potentially
36         valid) versions of the same block. We defend against this by detecting
37         the case where we would hash two identical hashes at the end of the list
38         together, and treating that identically to the block having an invalid
39         merkle root. Assuming no double-SHA256 collisions, this will detect all
40         known ways of changing the transactions without affecting the merkle
41         root.
42  */
43  
44  
45  uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
46      bool mutation = false;
47      while (hashes.size() > 1) {
48          if (mutated) {
49              for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
50                  if (hashes[pos] == hashes[pos + 1]) mutation = true;
51              }
52          }
53          if (hashes.size() & 1) {
54              hashes.push_back(hashes.back());
55          }
56          SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
57          hashes.resize(hashes.size() / 2);
58      }
59      if (mutated) *mutated = mutation;
60      if (hashes.size() == 0) return uint256();
61      return hashes[0];
62  }
63  
64  
65  uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
66  {
67      std::vector<uint256> leaves;
68      leaves.resize(block.vtx.size());
69      for (size_t s = 0; s < block.vtx.size(); s++) {
70          leaves[s] = block.vtx[s]->GetHash();
71      }
72      return ComputeMerkleRoot(std::move(leaves), mutated);
73  }
74  
75  uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated)
76  {
77      std::vector<uint256> leaves;
78      leaves.resize(block.vtx.size());
79      leaves[0].SetNull(); // The witness hash of the coinbase is 0.
80      for (size_t s = 1; s < block.vtx.size(); s++) {
81          leaves[s] = block.vtx[s]->GetWitnessHash();
82      }
83      return ComputeMerkleRoot(std::move(leaves), mutated);
84  }
85