eece63fa72
This switches the Merkle tree logic for blocks to one that runs in constant (small) space. The old code is moved to tests, and a new test is added that for various combinations of block sizes, transaction positions to compute a branch for, and mutations: * Verifies that the old code and new code agree for the Merkle root. * Verifies that the old code and new code agree for the Merkle branch. * Verifies that the computed Merkle branch is valid. * Verifies that mutations don't change the Merkle root. * Verifies that mutations are correctly detected.
172 lines
7 KiB
C++
172 lines
7 KiB
C++
#include "merkle.h"
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#include "hash.h"
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#include "utilstrencodings.h"
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/* WARNING! If you're reading this because you're learning about crypto
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and/or designing a new system that will use merkle trees, keep in mind
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that the following merkle tree algorithm has a serious flaw related to
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duplicate txids, resulting in a vulnerability (CVE-2012-2459).
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The reason is that if the number of hashes in the list at a given time
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is odd, the last one is duplicated before computing the next level (which
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is unusual in Merkle trees). This results in certain sequences of
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transactions leading to the same merkle root. For example, these two
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trees:
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A A
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/ \ / \
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B C B C
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/ \ | / \ / \
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D E F D E F F
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/ \ / \ / \ / \ / \ / \ / \
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1 2 3 4 5 6 1 2 3 4 5 6 5 6
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for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
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6 are repeated) result in the same root hash A (because the hash of both
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of (F) and (F,F) is C).
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The vulnerability results from being able to send a block with such a
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transaction list, with the same merkle root, and the same block hash as
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the original without duplication, resulting in failed validation. If the
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receiving node proceeds to mark that block as permanently invalid
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however, it will fail to accept further unmodified (and thus potentially
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valid) versions of the same block. We defend against this by detecting
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the case where we would hash two identical hashes at the end of the list
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together, and treating that identically to the block having an invalid
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merkle root. Assuming no double-SHA256 collisions, this will detect all
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known ways of changing the transactions without affecting the merkle
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root.
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*/
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/* This implements a constant-space merkle root/path calculator, limited to 2^32 leaves. */
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static void MerkleComputation(const std::vector<uint256>& leaves, uint256* proot, bool* pmutated, uint32_t branchpos, std::vector<uint256>* pbranch) {
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if (pbranch) pbranch->clear();
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if (leaves.size() == 0) {
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if (pmutated) *pmutated = false;
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if (proot) *proot = uint256();
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return;
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}
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bool mutated = false;
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// count is the number of leaves processed so far.
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uint32_t count = 0;
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// inner is an array of eagerly computed subtree hashes, indexed by tree
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// level (0 being the leaves).
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// For example, when count is 25 (11001 in binary), inner[4] is the hash of
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// the first 16 leaves, inner[3] of the next 8 leaves, and inner[0] equal to
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// the last leaf. The other inner entries are undefined.
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uint256 inner[32];
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// Which position in inner is a hash that depends on the matching leaf.
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int matchlevel = -1;
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// First process all leaves into 'inner' values.
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while (count < leaves.size()) {
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uint256 h = leaves[count];
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bool matchh = count == branchpos;
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count++;
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int level;
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// For each of the lower bits in count that are 0, do 1 step. Each
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// corresponds to an inner value that existed before processing the
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// current leaf, and each needs a hash to combine it.
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for (level = 0; !(count & (((uint32_t)1) << level)); level++) {
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if (pbranch) {
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if (matchh) {
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pbranch->push_back(inner[level]);
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} else if (matchlevel == level) {
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pbranch->push_back(h);
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matchh = true;
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}
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}
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mutated |= (inner[level] == h);
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CHash256().Write(inner[level].begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
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}
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// Store the resulting hash at inner position level.
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inner[level] = h;
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if (matchh) {
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matchlevel = level;
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}
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}
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// Do a final 'sweep' over the rightmost branch of the tree to process
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// odd levels, and reduce everything to a single top value.
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// Level is the level (counted from the bottom) up to which we've sweeped.
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int level = 0;
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// As long as bit number level in count is zero, skip it. It means there
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// is nothing left at this level.
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while (!(count & (((uint32_t)1) << level))) {
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level++;
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}
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uint256 h = inner[level];
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bool matchh = matchlevel == level;
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while (count != (((uint32_t)1) << level)) {
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// If we reach this point, h is an inner value that is not the top.
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// We combine it with itself (Bitcoin's special rule for odd levels in
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// the tree) to produce a higher level one.
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if (pbranch && matchh) {
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pbranch->push_back(h);
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}
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CHash256().Write(h.begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
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// Increment count to the value it would have if two entries at this
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// level had existed.
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count += (((uint32_t)1) << level);
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level++;
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// And propagate the result upwards accordingly.
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while (!(count & (((uint32_t)1) << level))) {
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if (pbranch) {
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if (matchh) {
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pbranch->push_back(inner[level]);
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} else if (matchlevel == level) {
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pbranch->push_back(h);
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matchh = true;
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}
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}
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CHash256().Write(inner[level].begin(), 32).Write(h.begin(), 32).Finalize(h.begin());
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level++;
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}
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}
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// Return result.
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if (pmutated) *pmutated = mutated;
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if (proot) *proot = h;
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}
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uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated) {
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uint256 hash;
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MerkleComputation(leaves, &hash, mutated, -1, NULL);
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return hash;
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}
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std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position) {
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std::vector<uint256> ret;
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MerkleComputation(leaves, NULL, NULL, position, &ret);
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return ret;
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}
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uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint256>& vMerkleBranch, uint32_t nIndex) {
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uint256 hash = leaf;
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for (std::vector<uint256>::const_iterator it = vMerkleBranch.begin(); it != vMerkleBranch.end(); ++it) {
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if (nIndex & 1) {
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hash = Hash(BEGIN(*it), END(*it), BEGIN(hash), END(hash));
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} else {
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hash = Hash(BEGIN(hash), END(hash), BEGIN(*it), END(*it));
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}
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nIndex >>= 1;
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}
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return hash;
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}
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uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
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{
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std::vector<uint256> leaves;
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leaves.resize(block.vtx.size());
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for (size_t s = 0; s < block.vtx.size(); s++) {
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leaves[s] = block.vtx[s].GetHash();
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}
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return ComputeMerkleRoot(leaves, mutated);
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}
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std::vector<uint256> BlockMerkleBranch(const CBlock& block, uint32_t position)
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{
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std::vector<uint256> leaves;
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leaves.resize(block.vtx.size());
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for (size_t s = 0; s < block.vtx.size(); s++) {
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leaves[s] = block.vtx[s].GetHash();
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}
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return ComputeMerkleBranch(leaves, position);
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}
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