Post
Topic
Board Altcoin Discussion
how fix diff adjustment attack ?
by
tseries
on 25/07/2017, 01:44:05 UTC
How possible fix adjustment atack ?



the sample of pow code used on altcoin ( scrypt  coin based on old sources of litecoin)

main.cpp

Code:
int64 static GetBlockValue(int nHeight, int64 nFees)
{
    int64 nSubsidy = 500 * COIN;

            if(nHeight == 1)
            {
            nSubsidy = 160000000 * COIN;
            }

    // Subsidy is cut in half every 840000 blocks
    nSubsidy >>= (nHeight / 840000);

    return nSubsidy + nFees;
}

static const int64 nTargetTimespan = 10 * 60;
static const int64 nTargetSpacing = 5 * 60;
static const int64 nInterval = nTargetTimespan / nTargetSpacing;

//
// minimum amount of work that could possibly be required nTime after
// minimum work required was nBase
//
unsigned int ComputeMinWork(unsigned int nBase, int64 nTime)
{
    // Testnet has min-difficulty blocks
    // after nTargetSpacing*2 time between blocks:
    if (fTestNet && nTime > nTargetSpacing*2)
        return bnProofOfWorkLimit.GetCompact();

    CBigNum bnResult;
    bnResult.SetCompact(nBase);
    while (nTime > 0 && bnResult < bnProofOfWorkLimit)
    {
        // Maximum 400% adjustment...
        bnResult *= 4;
        // ... in best-case exactly 4-times-normal target time
        nTime -= nTargetTimespan*4;
    }
    if (bnResult > bnProofOfWorkLimit)
        bnResult = bnProofOfWorkLimit;
    return bnResult.GetCompact();
}

unsigned int static GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock)
{
    unsigned int nProofOfWorkLimit = bnProofOfWorkLimit.GetCompact();

    // Genesis block
    if (pindexLast == NULL)
        return nProofOfWorkLimit;

    // Only change once per interval
    if ((pindexLast->nHeight+1) % nInterval != 0)
    {
        // Special difficulty rule for testnet:
        if (fTestNet)
        {
            // If the new block's timestamp is more than 2* 10 minutes
            // then allow mining of a min-difficulty block.
            if (pblock->nTime > pindexLast->nTime + nTargetSpacing*2)
                return nProofOfWorkLimit;
            else
            {
                // Return the last non-special-min-difficulty-rules-block
                const CBlockIndex* pindex = pindexLast;
                while (pindex->pprev && pindex->nHeight % nInterval != 0 && pindex->nBits == nProofOfWorkLimit)
                    pindex = pindex->pprev;
                return pindex->nBits;
            }
        }

        return pindexLast->nBits;
    }

    // LiteBitcoin: This fixes an issue where a 51% attack can change difficulty at will.
    // Go back the full period unless it's the first retarget after genesis. Code courtesy of Art Forz
    int blockstogoback = nInterval-1;
    if ((pindexLast->nHeight+1) != nInterval)
        blockstogoback = nInterval;

    // Go back by what we want to be 14 days worth of blocks
    const CBlockIndex* pindexFirst = pindexLast;
    for (int i = 0; pindexFirst && i < blockstogoback; i++)
        pindexFirst = pindexFirst->pprev;
    assert(pindexFirst);

    // Limit adjustment step
    int64 nActualTimespan = pindexLast->GetBlockTime() - pindexFirst->GetBlockTime();
    printf("  nActualTimespan = %"PRI64d"  before bounds\n", nActualTimespan);
    if (nActualTimespan < nTargetTimespan/4)
        nActualTimespan = nTargetTimespan/4;
    if (nActualTimespan > nTargetTimespan*4)
        nActualTimespan = nTargetTimespan*4;

    // Retarget
    CBigNum bnNew;
    bnNew.SetCompact(pindexLast->nBits);
    bnNew *= nActualTimespan;
    bnNew /= nTargetTimespan;

    if (bnNew > bnProofOfWorkLimit)
        bnNew = bnProofOfWorkLimit;

    /// debug print
    printf("GetNextWorkRequired RETARGET\n");
    printf("nTargetTimespan = %"PRI64d"    nActualTimespan = %"PRI64d"\n", nTargetTimespan, nActualTimespan);
    printf("Before: %08x  %s\n", pindexLast->nBits, CBigNum().SetCompact(pindexLast->nBits).getuint256().ToString().c_str());
    printf("After:  %08x  %s\n", bnNew.GetCompact(), bnNew.getuint256().ToString().c_str());

    return bnNew.GetCompact();
}

bool CheckProofOfWork(uint256 hash, unsigned int nBits)
{
    CBigNum bnTarget;
    bnTarget.SetCompact(nBits);

    // Check range
    if (bnTarget <= 0 || bnTarget > bnProofOfWorkLimit)
        return error("CheckProofOfWork() : nBits below minimum work");

    // Check proof of work matches claimed amount
    if (hash > bnTarget.getuint256())
        return error("CheckProofOfWork() : hash doesn't match nBits");

    return true;
}

// Return maximum amount of blocks that other nodes claim to have
int GetNumBlocksOfPeers()
{
    return std::max(cPeerBlockCounts.median(), Checkpoints::GetTotalBlocksEstimate());
}