比特币POW算法分析

基本信息

官网：https://github.com/bitcoin/bitcoin

 

比特币节点pow大致流程

• 生成coinbase交易，并与其他所有准备打包进区块的交易组成交易列表，通过Merkle树算法生成Merkle根哈希；
• 把Merkle根哈希及其他相关字段组装成区块头，将区块头的80字节数据作为工作量证明的输入；
• 不停地变更区块头中的随机数，即nonce的数值，并对每次变更后的区块头做双重SHA256运算，将结果值与当前网络的目标值做对比，如果小于目标值则解题成功，工作量证明完成。

 

源码解析

比特币区块头结构

src/primitives/block.h

区块头长度为80字节

class CBlockHeader
{
public:
    // header
    int32_t nVersion;
    uint256 hashPrevBlock;
    uint256 hashMerkleRoot;
    uint32_t nTime;
    uint32_t nBits;
    uint32_t nNonce;

　　

比特币区块结构

src/primitives/block.h

class CBlock : public CBlockHeader
{
public:
    // network and disk（交易列表）
    std::vector<CTransactionRef> vtx;

    // memory only
    mutable bool fChecked;

　　

如上区块头长度为80字节，因此执行SHA256算法，分割成 64B和16B+填充的48B两段进行运算（？？）

挖矿的过程就是寻找符合规则的 nNonce ，使如下等式成立：

SHA256(SHA256(version + prev_hash + merkle_root + ntime + nbits + nNonce + 填充 )) < TARGET

　nNonce的范围为 0~2^32，当 nNonce 溢出仍然没有符合的值时，修改区块 coinbase 里面的 ExtraNonce

pow算法中生成coinbase交易以及创建区块

src/miner.cpp

std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& scriptPubKeyIn)
{
    int64_t nTimeStart = GetTimeMicros();

    resetBlock();

    pblocktemplate.reset(new CBlockTemplate());

    if(!pblocktemplate.get())
        return nullptr;
    CBlock* const pblock = &pblocktemplate->block; // pointer for convenience

    // Add dummy coinbase tx as first transaction
    pblock->vtx.emplace_back();
    pblocktemplate->vTxFees.push_back(-1); // updated at end
    pblocktemplate->vTxSigOpsCost.push_back(-1); // updated at end

    LOCK2(cs_main, m_mempool.cs);
    CBlockIndex* pindexPrev = ::ChainActive().Tip();
    assert(pindexPrev != nullptr);
    nHeight = pindexPrev->nHeight + 1;
    
     //版本号
    pblock->nVersion = ComputeBlockVersion(pindexPrev, chainparams.GetConsensus());
    // -regtest only: allow overriding block.nVersion with
    // -blockversion=N to test forking scenarios
    if (chainparams.MineBlocksOnDemand())
        pblock->nVersion = gArgs.GetArg("-blockversion", pblock->nVersion);

    //时间戳
    pblock->nTime = GetAdjustedTime();
    const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();

    nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
                       ? nMedianTimePast
                       : pblock->GetBlockTime();

    // Decide whether to include witness transactions
    // This is only needed in case the witness softfork activation is reverted
    // (which would require a very deep reorganization).
    // Note that the mempool would accept transactions with witness data before
    // IsWitnessEnabled, but we would only ever mine blocks after IsWitnessEnabled
    // unless there is a massive block reorganization with the witness softfork
    // not activated.
    // TODO: replace this with a call to main to assess validity of a mempool
    // transaction (which in most cases can be a no-op).
    fIncludeWitness = IsWitnessEnabled(pindexPrev, chainparams.GetConsensus());

    int nPackagesSelected = 0;
    int nDescendantsUpdated = 0;
    addPackageTxs(nPackagesSelected, nDescendantsUpdated);

    int64_t nTime1 = GetTimeMicros();

    m_last_block_num_txs = nBlockTx;
    m_last_block_weight = nBlockWeight;

    // Create coinbase transaction. 创建coinbase交易
    CMutableTransaction coinbaseTx;
    coinbaseTx.vin.resize(1);
    coinbaseTx.vin[0].prevout.SetNull();
    coinbaseTx.vout.resize(1);
    coinbaseTx.vout[0].scriptPubKey = scriptPubKeyIn;
    //挖矿奖励 GetBlockSubsidy(）和手续费
    coinbaseTx.vout[0].nValue = nFees + GetBlockSubsidy(nHeight, chainparams.GetConsensus());
    coinbaseTx.vin[0].scriptSig = CScript() << nHeight << OP_0;
    //第一笔交易即为矿工获得奖励和手续费的特殊交易
    pblock->vtx[0] = MakeTransactionRef(std::move(coinbaseTx));
    pblocktemplate->vchCoinbaseCommitment = GenerateCoinbaseCommitment(*pblock, pindexPrev, chainparams.GetConsensus());
    pblocktemplate->vTxFees[0] = -nFees;

    LogPrintf("CreateNewBlock(): block weight: %u txs: %u fees: %ld sigops %d\n", GetBlockWeight(*pblock), nBlockTx, nFees, nBlockSigOpsCost);

    // Fill in header 将区块头数据补齐
    pblock->hashPrevBlock  = pindexPrev->GetBlockHash();
    UpdateTime(pblock, chainparams.GetConsensus(), pindexPrev);
    pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock, chainparams.GetConsensus());
    pblock->nNonce         = 0;  //随机数 nNonce 先重置为0
    pblocktemplate->vTxSigOpsCost[0] = WITNESS_SCALE_FACTOR * GetLegacySigOpCount(*pblock->vtx[0]);

    BlockValidationState state;
    if (!TestBlockValidity(state, chainparams, *pblock, pindexPrev, false, false)) {
        throw std::runtime_error(strprintf("%s: TestBlockValidity failed: %s", __func__, state.ToString()));
    }
    int64_t nTime2 = GetTimeMicros();

    LogPrint(BCLog::BENCH, "CreateNewBlock() packages: %.2fms (%d packages, %d updated descendants), validity: %.2fms (total %.2fms)\n", 0.001 * (nTime1 - nTimeStart), nPackagesSelected, nDescendantsUpdated, 0.001 * (nTime2 - nTime1), 0.001 * (nTime2 - nTimeStart));

    return std::move(pblocktemplate);
}

POW的实现

src/rpc/mining.cpp

static bool GenerateBlock(ChainstateManager& chainman, CBlock& block, uint64_t& max_tries, unsigned int& extra_nonce, uint256& block_hash)
{
    block_hash.SetNull();

    {
        LOCK(cs_main);
        IncrementExtraNonce(&block, ::ChainActive().Tip(), extra_nonce);
    }

    CChainParams chainparams(Params());

      //不断变更区块头中的随机数 Nonce

      //对变更后的区块头做双重SHA256哈希运算     

//CheckProofOfWork 函数 与当前难度的目标值做比对，如果小于目标难度，即Pow完成

       //uint64_t nMaxTries = 1000000;即重试100万次<br>

    while (max_tries > 0 && block.nNonce < std::numeric_limits<uint32_t>::max() && !CheckProofOfWork(block.GetHash(), block.nBits, chainparams.GetConsensus()) && !ShutdownRequested()) {
        ++block.nNonce;
        --max_tries;
    }
    if (max_tries == 0 || ShutdownRequested()) {
        return false;
    }
    if (block.nNonce == std::numeric_limits<uint32_t>::max()) {
        return true;
    }

//函数验证合法性
    std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(block);
    if (!chainman.ProcessNewBlock(chainparams, shared_pblock, true, nullptr)) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "ProcessNewBlock, block not accepted");
    }

    block_hash = block.GetHash();
    return true;
}

static UniValue generateBlocks(ChainstateManager& chainman, const CTxMemPool& mempool, const CScript& coinbase_script, int nGenerate, uint64_t nMaxTries)
{
    int nHeightEnd = 0;
    int nHeight = 0;

    {   // Don't keep cs_main locked
        LOCK(cs_main);  ////用于更改 coinbase交易中的 ExtraNonce
        nHeight = ::ChainActive().Height();
        nHeightEnd = nHeight+nGenerate;
    }
    unsigned int nExtraNonce = 0;
    UniValue blockHashes(UniValue::VARR);
    while (nHeight < nHeightEnd && !ShutdownRequested())
    {
        std::unique_ptr<CBlockTemplate> pblocktemplate(BlockAssembler(mempool, Params()).CreateNewBlock(coinbase_script));
        if (!pblocktemplate.get())
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
        CBlock *pblock = &pblocktemplate->block;

        uint256 block_hash;
        if (!GenerateBlock(chainman, *pblock, nMaxTries, nExtraNonce, block_hash)) {
            break;
        }

        if (!block_hash.IsNull()) {
            ++nHeight;
            blockHashes.push_back(block_hash.GetHex());
        }
    }
    return blockHashes;
}

　　

 

 双SHA256验证过程

区块头长度为80字节，因此执行SHA256算法，分割成 64B和16B+填充的48B两段进行运算（？？）

挖矿的过程就是寻找符合规则的 nNonce ，使如下等式成立：

SHA256(SHA256(version + prev_hash + merkle_root + ntime + nbits + nNonce + 填充 )) < TARGET

src/pow.cpp

bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)
{
    bool fNegative;
    bool fOverflow;
    arith_uint256 bnTarget;

    bnTarget.SetCompact(nBits, &fNegative, &fOverflow);

    // Check range
    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(params.powLimit))
        return false;

    // Check proof of work matches claimed amount
    if (UintToArith256(hash) > bnTarget)
        return false;

    return true;
}

　　

修改区块 coinbase 里面的 ExtraNonce

nNonce的范围为 0~2^32，当 nNonce 溢出仍然没有符合的值时，使用IncrementExtraNonce（）函数修改区块 coinbase 里面的 ExtraNonce

 src/miner.cpp

void IncrementExtraNonce(CBlock* pblock, const CBlockIndex* pindexPrev, unsigned int& nExtraNonce)
{
    // Update nExtraNonce// Update nExtraNonce 更新
    static uint256 hashPrevBlock;
    if (hashPrevBlock != pblock->hashPrevBlock)
    {
        nExtraNonce = 0;
        hashPrevBlock = pblock->hashPrevBlock;
    }
    ++nExtraNonce; //加 1
    unsigned int nHeight = pindexPrev->nHeight+1; // Height first in coinbase required for block.version=2
    CMutableTransaction txCoinbase(*pblock->vtx[0]);
   //重新生成签名
    txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce));
    assert(txCoinbase.vin[0].scriptSig.size() <= 100);
    //重新计算 pBlock 区块头中的 hashMerkleRoot
    pblock->vtx[0] = MakeTransactionRef(std::move(txCoinbase));
    pblock->hashMerkleRoot = BlockMerkleRoot(*pblock);
}

　

难度值计算 - 源码见 GetNextWorkRequired 函数，位置 src/pow.cpp

规则大致为每创建2016个块后将计算新的难度，此后的2016个块使用新的难度。计算步骤如下： 

1. 找到前2016个块的第一个块，计算生成这2016个块花费的时间。即最后一个块的时间与第一个块的时间差。时间差不小于3.5天，不大于56天。
2. 计算前2016个块的难度总和，即单个块的难度x总时间。
3. 计算新的难度，即2016个块的难度总和/14天的秒数，得到每秒的难度值。
4. 要求新的难度，难度不低于参数定义的最小难度。

简易模式

 

参考资料

https://www.cnblogs.com/zhang-qc/p/10451817.html