Line data    Source code

       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      619527 : uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
      46      619527 :     bool mutation = false;
      47     1016320 :     while (hashes.size() > 1) {
      48      396793 :         if (mutated) {
      49     1172062 :             for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
      50      820748 :                 if (hashes[pos] == hashes[pos + 1]) mutation = true;
      51      820748 :             }
      52      351314 :         }
      53      396793 :         if (hashes.size() & 1) {
      54      139567 :             hashes.push_back(hashes.back());
      55      139567 :         }
      56      396793 :         SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
      57      396793 :         hashes.resize(hashes.size() / 2);
      58             :     }
      59      619527 :     if (mutated) *mutated = mutation;
      60      619527 :     if (hashes.size() == 0) return uint256();
      61      489479 :     return hashes[0];
      62      619527 : }
      63             : 
      64             : 
      65      407895 : uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
      66             : {
      67      407895 :     std::vector<uint256> leaves;
      68      407895 :     leaves.resize(block.vtx.size());
      69     1467405 :     for (size_t s = 0; s < block.vtx.size(); s++) {
      70     1059510 :         leaves[s] = block.vtx[s]->GetHash();
      71     1059510 :     }
      72      407895 :     return ComputeMerkleRoot(std::move(leaves), mutated);
      73      407895 : }
      74             :