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[Wallet] Switch to a constant-space Merkle root/branch algorithm
Backport of bitcoin#6508 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.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,168 @@ | ||
| #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); | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,32 @@ | ||
| // 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. | ||
|
|
||
| #ifndef BITCOIN_MERKLE | ||
| #define BITCOIN_MERKLE | ||
|
|
||
| #include <stdint.h> | ||
| #include <vector> | ||
|
|
||
| #include "primitives/transaction.h" | ||
| #include "primitives/block.h" | ||
| #include "uint256.h" | ||
|
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||
| uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated = NULL); | ||
| std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position); | ||
| uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint256>& branch, uint32_t position); | ||
|
|
||
| /* | ||
| * Compute the Merkle root of the transactions in a block. | ||
| * *mutated is set to true if a duplicated subtree was found. | ||
| */ | ||
| uint256 BlockMerkleRoot(const CBlock& block, bool* mutated = NULL); | ||
|
|
||
| /* | ||
| * Compute the Merkle branch for the tree of transactions in a block, for a | ||
| * given position. | ||
| * This can be verified using ComputeMerkleRootFromBranch. | ||
| */ | ||
| std::vector<uint256> BlockMerkleBranch(const CBlock& block, uint32_t position); | ||
|
|
||
| #endif |
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