学步园

定义：

In cryptography and computer
science a hash tree or Merkle
tree is a tree in which every non-leaf node is labelled with the hash of
the labels of its children nodes.

一棵树，它的非叶节点总是子节点内容的hash值。一个递归的定义。

比特币中应用

比特币中交易transactions是放置在块block中的，每一个tx可以看作一个叶子节点，对这样一个tx的序列计算hash（每2个一组)并构建merkle树，来简化校验块的一致性。

uint256 CBlock::BuildMerkleTree() const
{
    vMerkleTree.clear();
    BOOST_FOREACH(const CTransaction& tx, vtx)
        vMerkleTree.push_back(tx.GetHash());
    int j = 0;
    for (int nSize = vtx.size(); nSize > 1; nSize = (nSize + 1) / 2)
    {
        for (int i = 0; i < nSize; i += 2)
        {
            int i2 = std::min(i+1, nSize-1);
            vMerkleTree.push_back(Hash(BEGIN(vMerkleTree[j+i]),  END(vMerkleTree[j+i]),
                                       BEGIN(vMerkleTree[j+i2]), END(vMerkleTree[j+i2])));
        }
        j += nSize;
    }
    return (vMerkleTree.empty() ? 0 : vMerkleTree.back());
}

比特币的Hash实现是双重sha256sum (即 Double SHA256)

/** Compute the 256-bit hash of the concatenation of two objects. */
template<typename T1, typename T2>
inline uint256 Hash(const T1 p1begin, const T1 p1end,
                    const T2 p2begin, const T2 p2end) {
    static const unsigned char pblank[1] = {};
    uint256 result;
    CHash256().Write(p1begin == p1end ? pblank : (const unsigned char*)&p1begin[0], (p1end - p1begin) * sizeof(p1begin[0]))
              .Write(p2begin == p2end ? pblank : (const unsigned char*)&p2begin[0], (p2end - p2begin) * sizeof(p2begin[0]))
              .Finalize((unsigned char*)&result);
    return result;
}

/** A hasher class for Bitcoin's 256-bit hash (double SHA-256). */
class CHash256 {
private:
    CSHA256 sha;
public:
    static const size_t OUTPUT_SIZE = CSHA256::OUTPUT_SIZE;

    void Finalize(unsigned char hash[OUTPUT_SIZE]) {
        unsigned char buf[sha.OUTPUT_SIZE];
        sha.Finalize(buf);
        sha.Reset().Write(buf, sha.OUTPUT_SIZE).Finalize(hash);
    }

    CHash256& Write(const unsigned char *data, size_t len) {
        sha.Write(data, len);
        return *this;
    }

    CHash256& Reset() {
        sha.Reset();
        return *this;
    }
};