How possible fix adjustment atack ?
the sample of pow code used on altcoin ( scrypt coin based on old sources of litecoin)
main.cpp
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());
}