1  // Copyright (c) 2015-present 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  #include <util/check.h>
  8  
  9  /*     WARNING! If you're reading this because you're learning about crypto
 10         and/or designing a new system that will use merkle trees, keep in mind
 11         that the following merkle tree algorithm has a serious flaw related to
 12         duplicate txids, resulting in a vulnerability (CVE-2012-2459).
 13  
 14         The reason is that if the number of hashes in the list at a given level
 15         is odd, the last one is duplicated before computing the next level (which
 16         is unusual in Merkle trees). This results in certain sequences of
 17         transactions leading to the same merkle root. For example, these two
 18         trees:
 19  
 20                      A               A
 21                    /  \            /   \
 22                  B     C         B       C
 23                 / \    |        / \     / \
 24                D   E   F       D   E   F   F
 25               / \ / \ / \     / \ / \ / \ / \
 26               1 2 3 4 5 6     1 2 3 4 5 6 5 6
 27  
 28         for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
 29         6 are repeated) result in the same root hash A (because the hash of both
 30         of (F) and (F,F) is C).
 31  
 32         The vulnerability results from being able to send a block with such a
 33         transaction list, with the same merkle root, and the same block hash as
 34         the original without duplication, resulting in failed validation. If the
 35         receiving node proceeds to mark that block as permanently invalid
 36         however, it will fail to accept further unmodified (and thus potentially
 37         valid) versions of the same block. We defend against this by detecting
 38         the case where we would hash two identical hashes at the end of the list
 39         together, and treating that identically to the block having an invalid
 40         merkle root. Assuming no double-SHA256 collisions, this will detect all
 41         known ways of changing the transactions without affecting the merkle
 42         root.
 43  */
 44  
 45  
 46  uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
 47      bool mutation = false;
 48      while (hashes.size() > 1) {
 49          if (mutated) {
 50              for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
 51                  if (hashes[pos] == hashes[pos + 1]) mutation = true;
 52              }
 53          }
 54          if (hashes.size() & 1) {
 55              hashes.push_back(hashes.back());
 56          }
 57          SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
 58          hashes.resize(hashes.size() / 2);
 59      }
 60      if (mutated) *mutated = mutation;
 61      if (hashes.size() == 0) return uint256();
 62      return hashes[0];
 63  }
 64  
 65  
 66  uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
 67  {
 68      std::vector<uint256> leaves;
 69      leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
 70      for (size_t s = 0; s < block.vtx.size(); s++) {
 71          leaves.push_back(block.vtx[s]->GetHash().ToUint256());
 72      }
 73      return ComputeMerkleRoot(std::move(leaves), mutated);
 74  }
 75  
 76  uint256 BlockWitnessMerkleRoot(const CBlock& block)
 77  {
 78      std::vector<uint256> leaves;
 79      leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
 80      leaves.emplace_back(); // The witness hash of the coinbase is 0.
 81      for (size_t s = 1; s < block.vtx.size(); s++) {
 82          leaves.push_back(block.vtx[s]->GetWitnessHash().ToUint256());
 83      }
 84      return ComputeMerkleRoot(std::move(leaves));
 85  }
 86  
 87  /* This implements a constant-space merkle path calculator, limited to 2^32 leaves. */
 88  static void MerkleComputation(const std::vector<uint256>& leaves, uint32_t leaf_pos, std::vector<uint256>& path)
 89  {
 90      path.clear();
 91      Assume(leaves.size() <= UINT32_MAX);
 92      if (leaves.size() == 0) {
 93          return;
 94      }
 95      // count is the number of leaves processed so far.
 96      uint32_t count = 0;
 97      // inner is an array of eagerly computed subtree hashes, indexed by tree
 98      // level (0 being the leaves).
 99      // For example, when count is 25 (11001 in binary), inner[4] is the hash of
100      // the first 16 leaves, inner[3] of the next 8 leaves, and inner[0] equal to
101      // the last leaf. The other inner entries are undefined.
102      uint256 inner[32];
103      // Which position in inner is a hash that depends on the matching leaf.
104      int matchlevel = -1;
105      // First process all leaves into 'inner' values.
106      while (count < leaves.size()) {
107          uint256 h = leaves[count];
108          bool matchh = count == leaf_pos;
109          count++;
110          int level;
111          // For each of the lower bits in count that are 0, do 1 step. Each
112          // corresponds to an inner value that existed before processing the
113          // current leaf, and each needs a hash to combine it.
114          for (level = 0; !(count & ((uint32_t{1}) << level)); level++) {
115              if (matchh) {
116                  path.push_back(inner[level]);
117              } else if (matchlevel == level) {
118                  path.push_back(h);
119                  matchh = true;
120              }
121              h = Hash(inner[level], h);
122          }
123          // Store the resulting hash at inner position level.
124          inner[level] = h;
125          if (matchh) {
126              matchlevel = level;
127          }
128      }
129      // Do a final 'sweep' over the rightmost branch of the tree to process
130      // odd levels, and reduce everything to a single top value.
131      // Level is the level (counted from the bottom) up to which we've sweeped.
132      int level = 0;
133      // As long as bit number level in count is zero, skip it. It means there
134      // is nothing left at this level.
135      while (!(count & ((uint32_t{1}) << level))) {
136          level++;
137      }
138      uint256 h = inner[level];
139      bool matchh = matchlevel == level;
140      while (count != ((uint32_t{1}) << level)) {
141          // If we reach this point, h is an inner value that is not the top.
142          // We combine it with itself (Bitcoin's special rule for odd levels in
143          // the tree) to produce a higher level one.
144          if (matchh) {
145              path.push_back(h);
146          }
147          h = Hash(h, h);
148          // Increment count to the value it would have if two entries at this
149          // level had existed.
150          count += ((uint32_t{1}) << level);
151          level++;
152          // And propagate the result upwards accordingly.
153          while (!(count & ((uint32_t{1}) << level))) {
154              if (matchh) {
155                  path.push_back(inner[level]);
156              } else if (matchlevel == level) {
157                  path.push_back(h);
158                  matchh = true;
159              }
160              h = Hash(inner[level], h);
161              level++;
162          }
163      }
164  }
165  
166  static std::vector<uint256> ComputeMerklePath(const std::vector<uint256>& leaves, uint32_t position) {
167      std::vector<uint256> ret;
168      MerkleComputation(leaves, position, ret);
169      return ret;
170  }
171  
172  std::vector<uint256> TransactionMerklePath(const CBlock& block, uint32_t position)
173  {
174      std::vector<uint256> leaves;
175      leaves.resize(block.vtx.size());
176      for (size_t s = 0; s < block.vtx.size(); s++) {
177          leaves[s] = block.vtx[s]->GetHash().ToUint256();
178      }
179      return ComputeMerklePath(leaves, position);
180  }