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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 : :
7 : : #include <crypto/sha256.h>
8 : : #include <hash.h>
9 : : #include <primitives/block.h>
10 : : #include <primitives/transaction.h>
11 : : #include <util/check.h>
12 : :
13 : : #include <cstddef>
14 : : #include <memory>
15 : : #include <utility>
16 : :
17 : : /* WARNING! If you're reading this because you're learning about crypto
18 : : and/or designing a new system that will use merkle trees, keep in mind
19 : : that the following merkle tree algorithm has a serious flaw related to
20 : : duplicate txids, resulting in a vulnerability (CVE-2012-2459).
21 : :
22 : : The reason is that if the number of hashes in the list at a given level
23 : : is odd, the last one is duplicated before computing the next level (which
24 : : is unusual in Merkle trees). This results in certain sequences of
25 : : transactions leading to the same merkle root. For example, these two
26 : : trees:
27 : :
28 : : A A
29 : : / \ / \
30 : : B C B C
31 : : / \ | / \ / \
32 : : D E F D E F F
33 : : / \ / \ / \ / \ / \ / \ / \
34 : : 1 2 3 4 5 6 1 2 3 4 5 6 5 6
35 : :
36 : : for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
37 : : 6 are repeated) result in the same root hash A (because the hash of both
38 : : of (F) and (F,F) is C).
39 : :
40 : : The vulnerability results from being able to send a block with such a
41 : : transaction list, with the same merkle root, and the same block hash as
42 : : the original without duplication, resulting in failed validation. If the
43 : : receiving node proceeds to mark that block as permanently invalid
44 : : however, it will fail to accept further unmodified (and thus potentially
45 : : valid) versions of the same block. We defend against this by detecting
46 : : the case where we would hash two identical hashes at the end of the list
47 : : together, and treating that identically to the block having an invalid
48 : : merkle root. Assuming no double-SHA256 collisions, this will detect all
49 : : known ways of changing the transactions without affecting the merkle
50 : : root.
51 : : */
52 : :
53 : :
54 : 47893 : uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
55 : 47893 : bool mutation = false;
56 [ - + + + ]: 49995 : while (hashes.size() > 1) {
57 [ + + ]: 2102 : if (mutated) {
58 [ + + ]: 279371 : for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
59 [ + + ]: 277595 : if (hashes[pos] == hashes[pos + 1]) mutation = true;
60 : : }
61 : : }
62 [ + + ]: 2102 : if (hashes.size() & 1) {
63 : 793 : hashes.push_back(hashes.back());
64 : : }
65 [ - + ]: 2102 : SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
66 [ - + ]: 2102 : hashes.resize(hashes.size() / 2);
67 : : }
68 [ + + ]: 47893 : if (mutated) *mutated = mutation;
69 [ - + + + ]: 47893 : if (hashes.size() == 0) return uint256();
70 : 47890 : return hashes[0];
71 : : }
72 : :
73 : :
74 : 22808 : uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
75 : : {
76 : 22808 : std::vector<uint256> leaves;
77 [ - + + - ]: 22808 : leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
78 [ - + + + ]: 329747 : for (size_t s = 0; s < block.vtx.size(); s++) {
79 [ + - ]: 306939 : leaves.push_back(block.vtx[s]->GetHash().ToUint256());
80 : : }
81 [ + - ]: 45616 : return ComputeMerkleRoot(std::move(leaves), mutated);
82 : 22808 : }
83 : :
84 : 25077 : uint256 BlockWitnessMerkleRoot(const CBlock& block)
85 : : {
86 : 25077 : std::vector<uint256> leaves;
87 [ - + + - ]: 25077 : leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
88 [ + - ]: 25077 : leaves.emplace_back(); // The witness hash of the coinbase is 0.
89 [ - + + + ]: 25436 : for (size_t s = 1; s < block.vtx.size(); s++) {
90 [ + - ]: 359 : leaves.push_back(block.vtx[s]->GetWitnessHash().ToUint256());
91 : : }
92 [ + - ]: 50154 : return ComputeMerkleRoot(std::move(leaves));
93 : 25077 : }
94 : :
95 : : /* This implements a constant-space merkle path calculator, limited to 2^32 leaves. */
96 : 377 : static void MerkleComputation(const std::vector<uint256>& leaves, uint32_t leaf_pos, std::vector<uint256>& path)
97 : : {
98 [ - + ]: 377 : path.clear();
99 [ - + + + ]: 377 : Assume(leaves.size() <= UINT32_MAX);
100 [ + + ]: 377 : if (leaves.size() == 0) {
101 : : return;
102 : : }
103 : : // count is the number of leaves processed so far.
104 : 376 : uint32_t count = 0;
105 : : // inner is an array of eagerly computed subtree hashes, indexed by tree
106 : : // level (0 being the leaves).
107 : : // For example, when count is 25 (11001 in binary), inner[4] is the hash of
108 : : // the first 16 leaves, inner[3] of the next 8 leaves, and inner[0] equal to
109 : : // the last leaf. The other inner entries are undefined.
110 : 376 : uint256 inner[32];
111 : : // Which position in inner is a hash that depends on the matching leaf.
112 : 376 : int matchlevel = -1;
113 : : // First process all leaves into 'inner' values.
114 [ - + + + ]: 553136 : while (count < leaves.size()) {
115 : 552760 : uint256 h = leaves[count];
116 : 552760 : bool matchh = count == leaf_pos;
117 : 552760 : count++;
118 : 552760 : int level;
119 : : // For each of the lower bits in count that are 0, do 1 step. Each
120 : : // corresponds to an inner value that existed before processing the
121 : : // current leaf, and each needs a hash to combine it.
122 [ + + ]: 1103684 : for (level = 0; !(count & ((uint32_t{1}) << level)); level++) {
123 [ + + ]: 550924 : if (matchh) {
124 : 1338 : path.push_back(inner[level]);
125 [ + + ]: 549586 : } else if (matchlevel == level) {
126 : 1394 : path.push_back(h);
127 : 1394 : matchh = true;
128 : : }
129 : 550924 : h = Hash(inner[level], h);
130 : : }
131 : : // Store the resulting hash at inner position level.
132 : 552760 : inner[level] = h;
133 [ + + ]: 552760 : if (matchh) {
134 : 1770 : matchlevel = level;
135 : : }
136 : : }
137 : : // Do a final 'sweep' over the rightmost branch of the tree to process
138 : : // odd levels, and reduce everything to a single top value.
139 : : // Level is the level (counted from the bottom) up to which we've sweeped.
140 : : int level = 0;
141 : : // As long as bit number level in count is zero, skip it. It means there
142 : : // is nothing left at this level.
143 [ + + ]: 624 : while (!(count & ((uint32_t{1}) << level))) {
144 : 248 : level++;
145 : : }
146 : 376 : uint256 h = inner[level];
147 : 376 : bool matchh = matchlevel == level;
148 [ + + ]: 1930 : while (count != ((uint32_t{1}) << level)) {
149 : : // If we reach this point, h is an inner value that is not the top.
150 : : // We combine it with itself (Bitcoin's special rule for odd levels in
151 : : // the tree) to produce a higher level one.
152 [ + + ]: 1554 : if (matchh) {
153 : 71 : path.push_back(h);
154 : : }
155 : 1554 : h = Hash(h, h);
156 : : // Increment count to the value it would have if two entries at this
157 : : // level had existed.
158 : 1554 : count += ((uint32_t{1}) << level);
159 : 1554 : level++;
160 : : // And propagate the result upwards accordingly.
161 [ + + ]: 3014 : while (!(count & ((uint32_t{1}) << level))) {
162 [ + + ]: 1460 : if (matchh) {
163 : 131 : path.push_back(inner[level]);
164 [ + + ]: 1329 : } else if (matchlevel == level) {
165 : 328 : path.push_back(h);
166 : 328 : matchh = true;
167 : : }
168 : 1460 : h = Hash(inner[level], h);
169 : 1460 : level++;
170 : : }
171 : : }
172 : : }
173 : :
174 : 377 : static std::vector<uint256> ComputeMerklePath(const std::vector<uint256>& leaves, uint32_t position) {
175 : 377 : std::vector<uint256> ret;
176 [ + - ]: 377 : MerkleComputation(leaves, position, ret);
177 : 377 : return ret;
178 : 0 : }
179 : :
180 : 377 : std::vector<uint256> TransactionMerklePath(const CBlock& block, uint32_t position)
181 : : {
182 : 377 : std::vector<uint256> leaves;
183 [ - + + - ]: 377 : leaves.resize(block.vtx.size());
184 [ - + + + ]: 553137 : for (size_t s = 0; s < block.vtx.size(); s++) {
185 : 552760 : leaves[s] = block.vtx[s]->GetHash().ToUint256();
186 : : }
187 [ + - ]: 377 : return ComputeMerklePath(leaves, position);
188 : 377 : }
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