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+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "merkle.h"
+#include "hash.h"
+#include "utilstrencodings.h"
+
+/* WARNING! If you're reading this because you're learning about crypto
+ and/or designing a new system that will use merkle trees, keep in mind
+ that the following merkle tree algorithm has a serious flaw related to
+ duplicate txids, resulting in a vulnerability (CVE-2012-2459).
+
+ The reason is that if the number of hashes in the list at a given time
+ is odd, the last one is duplicated before computing the next level (which
+ is unusual in Merkle trees). This results in certain sequences of
+ transactions leading to the same merkle root. For example, these two
+ trees:
+
+ A A
+ / \ / \
+ B C B C
+ / \ | / \ / \
+ D E F D E F F
+ / \ / \ / \ / \ / \ / \ / \
+ 1 2 3 4 5 6 1 2 3 4 5 6 5 6
+
+ for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
+ 6 are repeated) result in the same root hash A (because the hash of both
+ of (F) and (F,F) is C).
+
+ The vulnerability results from being able to send a block with such a
+ transaction list, with the same merkle root, and the same block hash as
+ the original without duplication, resulting in failed validation. If the
+ receiving node proceeds to mark that block as permanently invalid
+ however, it will fail to accept further unmodified (and thus potentially
+ valid) versions of the same block. We defend against this by detecting
+ the case where we would hash two identical hashes at the end of the list
+ together, and treating that identically to the block having an invalid
+ merkle root. Assuming no double-SHA256 collisions, this will detect all
+ known ways of changing the transactions without affecting the merkle
+ root.
+*/
+
+/* This implements a constant-space merkle root/path calculator, limited to 2^32 leaves. */
+static void MerkleComputation(const std::vector<uint256>& leaves, uint256* proot, bool* pmutated, uint32_t branchpos, std::vector<uint256>* pbranch) {
+ if (pbranch) pbranch->clear();
+ if (leaves.size() == 0) {
+ if (pmutated) *pmutated = false;
+ if (proot) *proot = uint256();
+ return;
+ }
+ bool mutated = false;
+ // count is the number of leaves processed so far.
+ uint32_t count = 0;
+ // inner is an array of eagerly computed subtree hashes, indexed by tree
+ // level (0 being the leaves).
+ // For example, when count is 25 (11001 in binary), inner[4] is the hash of
+ // the first 16 leaves, inner[3] of the next 8 leaves, and inner[0] equal to
+ // the last leaf. The other inner entries are undefined.
+ uint256 inner[32];
+ // Which position in inner is a hash that depends on the matching leaf.
+ int matchlevel = -1;
+ // First process all leaves into 'inner' values.
+ while (count < leaves.size()) {
+ uint256 h = leaves[count];
+ bool matchh = count == branchpos;
+ count++;
+ int level;
+ // For each of the lower bits in count that are 0, do 1 step. Each
+ // corresponds to an inner value that existed before processing the
+ // current leaf, and each needs a hash to combine it.
+ for (level = 0; !(count & (((uint32_t)1) << level)); level++) {
+ if (pbranch) {
+ if (matchh) {
+ pbranch->push_back(inner[level]);
+ } else if (matchlevel == level) {
+ pbranch->push_back(h);
+ matchh = true;
+ }
+ }
+ mutated |= (inner[level] == h);
+ CHash256().Write(inner[level].begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
+ }
+ // Store the resulting hash at inner position level.
+ inner[level] = h;
+ if (matchh) {
+ matchlevel = level;
+ }
+ }
+ // Do a final 'sweep' over the rightmost branch of the tree to process
+ // odd levels, and reduce everything to a single top value.
+ // Level is the level (counted from the bottom) up to which we've sweeped.
+ int level = 0;
+ // As long as bit number level in count is zero, skip it. It means there
+ // is nothing left at this level.
+ while (!(count & (((uint32_t)1) << level))) {
+ level++;
+ }
+ uint256 h = inner[level];
+ bool matchh = matchlevel == level;
+ while (count != (((uint32_t)1) << level)) {
+ // If we reach this point, h is an inner value that is not the top.
+ // We combine it with itself (Bitcoin's special rule for odd levels in
+ // the tree) to produce a higher level one.
+ if (pbranch && matchh) {
+ pbranch->push_back(h);
+ }
+ CHash256().Write(h.begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
+ // Increment count to the value it would have if two entries at this
+ // level had existed.
+ count += (((uint32_t)1) << level);
+ level++;
+ // And propagate the result upwards accordingly.
+ while (!(count & (((uint32_t)1) << level))) {
+ if (pbranch) {
+ if (matchh) {
+ pbranch->push_back(inner[level]);
+ } else if (matchlevel == level) {
+ pbranch->push_back(h);
+ matchh = true;
+ }
+ }
+ CHash256().Write(inner[level].begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
+ level++;
+ }
+ }
+ // Return result.
+ if (pmutated) *pmutated = mutated;
+ if (proot) *proot = h;
+}
+
+uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated) {
+ uint256 hash;
+ MerkleComputation(leaves, &hash, mutated, -1, NULL);
+ return hash;
+}
+
+std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position) {
+ std::vector<uint256> ret;
+ MerkleComputation(leaves, NULL, NULL, position, &ret);
+ return ret;
+}
+
+uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint256>& vMerkleBranch, uint32_t nIndex) {
+ uint256 hash = leaf;
+ for (std::vector<uint256>::const_iterator it = vMerkleBranch.begin(); it != vMerkleBranch.end(); ++it) {
+ if (nIndex & 1) {
+ hash = Hash(BEGIN(*it), END(*it), BEGIN(hash), END(hash));
+ } else {
+ hash = Hash(BEGIN(hash), END(hash), BEGIN(*it), END(*it));
+ }
+ nIndex >>= 1;
+ }
+ return hash;
+}
+
+uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
+{
+ std::vector<uint256> leaves;
+ leaves.resize(block.vtx.size());
+ for (size_t s = 0; s < block.vtx.size(); s++) {
+ leaves[s] = block.vtx[s].GetHash();
+ }
+ return ComputeMerkleRoot(leaves, mutated);
+}
+
+std::vector<uint256> BlockMerkleBranch(const CBlock& block, uint32_t position)
+{
+ std::vector<uint256> leaves;
+ leaves.resize(block.vtx.size());
+ for (size_t s = 0; s < block.vtx.size(); s++) {
+ leaves[s] = block.vtx[s].GetHash();
+ }
+ return ComputeMerkleBranch(leaves, position);
+}