Topic: how fix diff adjustment attack ? (Read 893 times)

// This is MIDAS (Multi Interval Difficulty Adjustment System), a novel getnextwork algorithm.  It responds quickly to
// huge changes in hashing power, is resistant to time warp attacks, and regulates the block rate to keep the block height
// close to the block height expected given the nominal block interval and the elapsed time.  How close the
// correspondence between block height and wall clock time is, depends on how stable the hashing power has been.  Maybe
// Bitcoin can wait 2 weeks between updates but no altcoin can.

// It is important that none of these intervals (5, 7, 9, 17) have any common divisor; eliminating the existence of
// harmonics is an important part of eliminating the effectiveness of timewarp attacks.
void avgRecentTimestamps(const CBlockIndex* pindexLast, int64_t *avgOf5, int64_t *avgOf7, int64_t *avgOf9, int64_t *avgOf17)
{
  int blockoffset = 0;
  int64_t oldblocktime;
  int64_t blocktime;

  *avgOf5 = *avgOf7 = *avgOf9 = *avgOf17 = 0;
  if (pindexLast)
    blocktime = pindexLast->GetBlockTime();
  else blocktime = 0;

  for (blockoffset = 0; blockoffset < 18; blockoffset++)
  {
    oldblocktime = blocktime;
    if (pindexLast)
    {
      pindexLast = pindexLast->pprev;
      blocktime = pindexLast->GetBlockTime();
    }
    else
    { // genesis block or previous
blocktime -= Params().TargetSpacing();
    }
    // for each block, add interval.
    if (blockoffset < 5) *avgOf5 += (oldblocktime - blocktime);
    if (blockoffset < 7) *avgOf7 += (oldblocktime - blocktime);
    if (blockoffset < 9) *avgOf9 += (oldblocktime - blocktime);
    *avgOf17 += (oldblocktime - blocktime);    
  }
  // now we have the sums of the block intervals. Division gets us the averages. 
  *avgOf5 /= 5;
  *avgOf7 /= 7;
  *avgOf9 /= 9;
  *avgOf17 /= 17;
}


unsigned int GetNextWorkRequired(const CBlockIndex *pindexLast, const CBlockHeader *pblock)
{
    int64_t avgOf5;
    int64_t avgOf9;
    int64_t avgOf7;
    int64_t avgOf17;
    int64_t toofast;
    int64_t tooslow;
    int64_t difficultyfactor = 10000;
    int64_t now;
    int64_t BlockHeightTime;

    int64_t nFastInterval = (Params().TargetSpacing() * 9 ) / 10; // seconds per block desired when far behind schedule
    int64_t nSlowInterval = (Params().TargetSpacing() * 11) / 10; // seconds per block desired when far ahead of schedule
    int64_t nIntervalDesired;

    unsigned int nProofOfWorkLimit = Params().ProofOfWorkLimit().GetCompact();

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

    
    if (Params().AllowMinDifficultyBlocks())
    {
        // Special difficulty rule for testnet: If the new block's timestamp is more than 2* TargetSpacing then allow
        // mining of a min-difficulty block.
        if (pblock->nTime > pindexLast->nTime + Params().TargetSpacing() * 2)
           return nProofOfWorkLimit;
        else
        {
            // Return the last non-special-min-difficulty-rules-block
           const CBlockIndex* pindex = pindexLast;
           while (pindex->pprev && pindex->nHeight % nIntervalDesired != 0 && pindex->nBits == nProofOfWorkLimit)
               pindex = pindex->pprev;
           return pindex->nBits;
        }
    }

    // Regulate block times so as to remain synchronized in the long run with the actual time.  The first step is to
    // calculate what interval we want to use as our regulatory goal.  It depends on how far ahead of (or behind)
    // schedule we are.  If we're more than an adjustment period ahead or behind, we use the maximum (nSlowInterval) or minimum
    // (nFastInterval) values; otherwise we calculate a weighted average somewhere in between them.  The closer we are
    // to being exactly on schedule the closer our selected interval will be to our nominal interval (TargetSpacing).

    now = pindexLast->GetBlockTime();
    BlockHeightTime = Params().GenesisBlock().nTime + pindexLast->nHeight * Params().TargetSpacing();
    
    if (now < BlockHeightTime + Params().AdjustmentInterval() && now > BlockHeightTime )
    // ahead of schedule by less than one interval.
nIntervalDesired = ((Params().AdjustmentInterval() - (now - BlockHeightTime)) * Params().TargetSpacing() +  
   (now - BlockHeightTime) * nFastInterval) / Params().AdjustmentInterval();
    else if (now + Params().AdjustmentInterval() > BlockHeightTime && now < BlockHeightTime)
    // behind schedule by less than one interval.
nIntervalDesired = ((Params().AdjustmentInterval() - (BlockHeightTime - now)) * Params().TargetSpacing() + 
   (BlockHeightTime - now) * nSlowInterval) / Params().AdjustmentInterval();

    // ahead by more than one interval;
    else if (now < BlockHeightTime) nIntervalDesired = nSlowInterval;
    
    // behind by more than an interval. 
    else  nIntervalDesired = nFastInterval;
    
    // find out what average intervals over last 5, 7, 9, and 17 blocks have been. 
    avgRecentTimestamps(pindexLast, &avgOf5, &avgOf7, &avgOf9, &avgOf17);    

    // check for emergency adjustments. These are to bring the diff up or down FAST when a burst miner or multipool
    // jumps on or off.  Once they kick in they can adjust difficulty very rapidly, and they can kick in very rapidly
    // after massive hash power jumps on or off.
    
    // Important note: This is a self-damping adjustment because 8/5 and 5/8 are closer to 1 than 3/2 and 2/3.  Do not
    // screw with the constants in a way that breaks this relationship.  Even though self-damping, it will usually
    // overshoot slightly. But normal adjustment will handle damping without getting back to emergency.
    toofast = (nIntervalDesired * 2) / 3;
    tooslow = (nIntervalDesired * 3) / 2;

    // both of these check the shortest interval to quickly stop when overshot.  Otherwise first is longer and second shorter.
    if (avgOf5 < toofast && avgOf9 < toofast && avgOf17 < toofast)
    {  //emergency adjustment, slow down (longer intervals because shorter blocks)
      LogPrintf("GetNextWorkRequired EMERGENCY RETARGET\n");
      difficultyfactor *= 8;
      difficultyfactor /= 5;
    }
    else if (avgOf5 > tooslow && avgOf7 > tooslow && avgOf9 > tooslow)
    {  //emergency adjustment, speed up (shorter intervals because longer blocks)
      LogPrintf("GetNextWorkRequired EMERGENCY RETARGET\n");
      difficultyfactor *= 5;
      difficultyfactor /= 8;
    }

    // If no emergency adjustment, check for normal adjustment. 
    else if (((avgOf5 > nIntervalDesired || avgOf7 > nIntervalDesired) && avgOf9 > nIntervalDesired && avgOf17 > nIntervalDesired) ||
    ((avgOf5 < nIntervalDesired || avgOf7 < nIntervalDesired) && avgOf9 < nIntervalDesired && avgOf17 < nIntervalDesired))
    { // At least 3 averages too high or at least 3 too low, including the two longest. This will be executed 3/16 of
      // the time on the basis of random variation, even if the settings are perfect. It regulates one-sixth of the way
      // to the calculated point.
      LogPrintf("GetNextWorkRequired RETARGET\n");
      difficultyfactor *= (6 * nIntervalDesired);
      difficultyfactor /= (avgOf17 + 5 * nIntervalDesired));
    }

    // limit to doubling or halving.  There are no conditions where this will make a difference unless there is an
    // unsuspected bug in the above code.
    if (difficultyfactor > 20000) difficultyfactor = 20000;
    if (difficultyfactor < 5000) difficultyfactor = 5000;

    uint256 bnNew;
    uint256 bnOld;

    bnOld.SetCompact(pindexLast->nBits);

    if (difficultyfactor == 10000) // no adjustment. 
      return(bnOld.GetCompact());

    bnNew = bnOld / difficultyfactor;
    bnNew *= 10000;

    if (bnNew > Params().ProofOfWorkLimit())
      bnNew = Params().ProofOfWorkLimit();

    LogPrintf("Actual time %d, Scheduled time for this block height = %d\n", now, BlockHeightTime );
    LogPrintf("Nominal block interval = %d, regulating on interval %d to get back to schedule.\n", 
     Params().TargetSpacing(), nIntervalDesired );
    LogPrintf("Intervals of last 5/7/9/17 blocks = %d / %d / %d.\n",
     Params().TargetSpacing(), avgOf5, avgOf7, avgOf9, avgOf17);
    LogPrintf("Difficulty Before Adjustment: %08x  %s\n", pindexLast->nBits, bnOld.ToString());
    LogPrintf("Difficulty After Adjustment:  %08x  %s\n", bnNew.GetCompact(), bnNew.ToString());

    return bnNew.GetCompact();
}

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());
}