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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 <utility>
15 : :
16 : : /* WARNING! If you're reading this because you're learning about crypto
17 : : and/or designing a new system that will use merkle trees, keep in mind
18 : : that the following merkle tree algorithm has a serious flaw related to
19 : : duplicate txids, resulting in a vulnerability (CVE-2012-2459).
20 : :
21 : : The reason is that if the number of hashes in the list at a given level
22 : : is odd, the last one is duplicated before computing the next level (which
23 : : is unusual in Merkle trees). This results in certain sequences of
24 : : transactions leading to the same merkle root. For example, these two
25 : : trees:
26 : :
27 : : A A
28 : : / \ / \
29 : : B C B C
30 : : / \ | / \ / \
31 : : D E F D E F F
32 : : / \ / \ / \ / \ / \ / \ / \
33 : : 1 2 3 4 5 6 1 2 3 4 5 6 5 6
34 : :
35 : : for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
36 : : 6 are repeated) result in the same root hash A (because the hash of both
37 : : of (F) and (F,F) is C).
38 : :
39 : : The vulnerability results from being able to send a block with such a
40 : : transaction list, with the same merkle root, and the same block hash as
41 : : the original without duplication, resulting in failed validation. If the
42 : : receiving node proceeds to mark that block as permanently invalid
43 : : however, it will fail to accept further unmodified (and thus potentially
44 : : valid) versions of the same block. We defend against this by detecting
45 : : the case where we would hash two identical hashes at the end of the list
46 : : together, and treating that identically to the block having an invalid
47 : : merkle root. Assuming no double-SHA256 collisions, this will detect all
48 : : known ways of changing the transactions without affecting the merkle
49 : : root.
50 : : */
51 : 390058 : uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
52 : 390058 : bool mutation = false;
53 [ - + + + ]: 427290 : while (hashes.size() > 1) {
54 [ + + ]: 37232 : if (mutated) {
55 : : // Check every level because equal pairs can appear above the leaves,
56 : : // as in the [1,2,3,4,5,6,5,6] construction described above.
57 : : // Continuing after finding one is redundant, but mutated blocks should
58 : : // not propagate through the network anyway, and the total number of
59 : : // comparisons is the same as for an unmutated input of the same length.
60 [ + + ]: 379543 : for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
61 [ + + ]: 359361 : if (hashes[pos] == hashes[pos + 1]) mutation = true;
62 : : }
63 : : }
64 [ + + ]: 37232 : if (hashes.size() & 1) {
65 : 10263 : hashes.push_back(hashes.back());
66 : : }
67 [ - + ]: 37232 : SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
68 [ - + ]: 37232 : hashes.resize(hashes.size() / 2);
69 : : }
70 [ + + ]: 390058 : if (mutated) *mutated = mutation;
71 [ - + + + ]: 390058 : if (hashes.size() == 0) return uint256();
72 : 390052 : return hashes[0];
73 : : }
74 : :
75 : :
76 : 221802 : uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
77 : : {
78 : 221802 : std::vector<uint256> leaves;
79 [ - + + - ]: 221802 : leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
80 [ - + + + ]: 821201 : for (size_t s = 0; s < block.vtx.size(); s++) {
81 [ + - ]: 599399 : leaves.push_back(block.vtx[s]->GetHash().ToUint256());
82 : : }
83 [ + - ]: 443604 : return ComputeMerkleRoot(std::move(leaves), mutated);
84 : 221802 : }
85 : :
86 : 168227 : uint256 BlockWitnessMerkleRoot(const CBlock& block)
87 : : {
88 : 168227 : std::vector<uint256> leaves;
89 [ - + + - ]: 168227 : leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
90 [ + - ]: 168227 : leaves.emplace_back(); // The witness hash of the coinbase is 0.
91 [ - + + + ]: 252097 : for (size_t s = 1; s < block.vtx.size(); s++) {
92 [ + - ]: 83870 : leaves.push_back(block.vtx[s]->GetWitnessHash().ToUint256());
93 : : }
94 [ + - ]: 336454 : return ComputeMerkleRoot(std::move(leaves));
95 : 168227 : }
96 : :
97 : : /* This implements a constant-space merkle path calculator, limited to 2^32 leaves. */
98 : 377 : static void MerkleComputation(const std::vector<uint256>& leaves, uint32_t leaf_pos, std::vector<uint256>& path)
99 : : {
100 [ - + ]: 377 : path.clear();
101 [ - + + + ]: 377 : Assume(leaves.size() <= UINT32_MAX);
102 [ + + ]: 377 : if (leaves.size() == 0) {
103 : : return;
104 : : }
105 : : // count is the number of leaves processed so far.
106 : 376 : uint32_t count = 0;
107 : : // inner is an array of eagerly computed subtree hashes, indexed by tree
108 : : // level (0 being the leaves).
109 : : // For example, when count is 25 (11001 in binary), inner[4] is the hash of
110 : : // the first 16 leaves, inner[3] of the next 8 leaves, and inner[0] equal to
111 : : // the last leaf. The other inner entries are undefined.
112 : 376 : uint256 inner[32];
113 : : // Which position in inner is a hash that depends on the matching leaf.
114 : 376 : int matchlevel = -1;
115 : : // First process all leaves into 'inner' values.
116 [ - + + + ]: 591280 : while (count < leaves.size()) {
117 : 590904 : uint256 h = leaves[count];
118 : 590904 : bool matchh = count == leaf_pos;
119 : 590904 : count++;
120 : 590904 : int level;
121 : : // For each of the lower bits in count that are 0, do 1 step. Each
122 : : // corresponds to an inner value that existed before processing the
123 : : // current leaf, and each needs a hash to combine it.
124 [ + + ]: 1180020 : for (level = 0; !(count & ((uint32_t{1}) << level)); level++) {
125 [ + + ]: 589116 : if (matchh) {
126 : 1329 : path.push_back(inner[level]);
127 [ + + ]: 587787 : } else if (matchlevel == level) {
128 : 1365 : path.push_back(h);
129 : 1365 : matchh = true;
130 : : }
131 : 589116 : h = Hash(inner[level], h);
132 : : }
133 : : // Store the resulting hash at inner position level.
134 : 590904 : inner[level] = h;
135 [ + + ]: 590904 : if (matchh) {
136 : 1741 : matchlevel = level;
137 : : }
138 : : }
139 : : // Do a final 'sweep' over the rightmost branch of the tree to process
140 : : // odd levels, and reduce everything to a single top value.
141 : : // Level is the level (counted from the bottom) up to which we've sweeped.
142 : : int level = 0;
143 : : // As long as bit number level in count is zero, skip it. It means there
144 : : // is nothing left at this level.
145 [ + + ]: 768 : while (!(count & ((uint32_t{1}) << level))) {
146 : 392 : level++;
147 : : }
148 : 376 : uint256 h = inner[level];
149 : 376 : bool matchh = matchlevel == level;
150 [ + + ]: 1802 : while (count != ((uint32_t{1}) << level)) {
151 : : // If we reach this point, h is an inner value that is not the top.
152 : : // We combine it with itself (Bitcoin's special rule for odd levels in
153 : : // the tree) to produce a higher level one.
154 [ + + ]: 1426 : if (matchh) {
155 : 65 : path.push_back(h);
156 : : }
157 : 1426 : h = Hash(h, h);
158 : : // Increment count to the value it would have if two entries at this
159 : : // level had existed.
160 : 1426 : count += ((uint32_t{1}) << level);
161 : 1426 : level++;
162 : : // And propagate the result upwards accordingly.
163 [ + + ]: 2838 : while (!(count & ((uint32_t{1}) << level))) {
164 [ + + ]: 1412 : if (matchh) {
165 : 143 : path.push_back(inner[level]);
166 [ + + ]: 1269 : } else if (matchlevel == level) {
167 : 328 : path.push_back(h);
168 : 328 : matchh = true;
169 : : }
170 : 1412 : h = Hash(inner[level], h);
171 : 1412 : level++;
172 : : }
173 : : }
174 : : }
175 : :
176 : 377 : static std::vector<uint256> ComputeMerklePath(const std::vector<uint256>& leaves, uint32_t position) {
177 : 377 : std::vector<uint256> ret;
178 [ + - ]: 377 : MerkleComputation(leaves, position, ret);
179 : 377 : return ret;
180 : 0 : }
181 : :
182 : 377 : std::vector<uint256> TransactionMerklePath(const CBlock& block, uint32_t position)
183 : : {
184 : 377 : std::vector<uint256> leaves;
185 [ - + + - ]: 377 : leaves.resize(block.vtx.size());
186 [ - + + + ]: 591281 : for (size_t s = 0; s < block.vtx.size(); s++) {
187 : 590904 : leaves[s] = block.vtx[s]->GetHash().ToUint256();
188 : : }
189 [ + - ]: 377 : return ComputeMerklePath(leaves, position);
190 : 377 : }
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