kernel.cpp

// Copyright (c) 2012-2013 The Peercoin developers
// Distributed under the MIT/X11 software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <boost/assign/list_of.hpp>

#include "kernel.h"
#include "db.h"
#include "init.h"

#include <math.h>       /* pow */

using namespace std;

// Protocol switch time of v0.3 kernel protocol
unsigned int nProtocolV03SwitchTime     = 1363800000;
unsigned int nProtocolV03TestSwitchTime = 1359781000;

// Modifier interval: time to elapse before new modifier is computed
// Set to 6-hour for production network and 20-minute for test network
unsigned int nModifierInterval = MODIFIER_INTERVAL;

// Hard checkpoints of stake modifiers to ensure they are deterministic
static std::map<int, unsigned int> mapStakeModifierCheckpoints =
  boost::assign::map_list_of
  ( 0, 0x00e00670b )
  ;

// Whether the given coinstake is subject to new v0.3 protocol
bool IsProtocolV03(unsigned int nTimeCoinStake)
{
  return (nTimeCoinStake >= (fTestNet? nProtocolV03TestSwitchTime : nProtocolV03SwitchTime));
}

// Get the last stake modifier and its generation time from a given block
static bool GetLastStakeModifier(const CBlockIndex* pindex, uint64& nStakeModifier, int64& nModifierTime)
{
  if(!pindex)
    return error("GetLastStakeModifier: null pindex");
  while(pindex && pindex->pprev && !pindex->GeneratedStakeModifier())
    pindex = pindex->pprev;
  if(!pindex->GeneratedStakeModifier())
    return error("GetLastStakeModifier: no generation at genesis block");
  nStakeModifier = pindex->nStakeModifier;
  nModifierTime = pindex->GetBlockTime();
  return true;
}

// Get selection interval section (in seconds)
static int64 GetStakeModifierSelectionIntervalSection(int nSection)
{
  assert (nSection >= 0 && nSection < 64);
  return (nModifierInterval * 63 / (63 + ((63 - nSection) * (MODIFIER_INTERVAL_RATIO - 1))));
}

// Get stake modifier selection interval (in seconds)
static int64 GetStakeModifierSelectionInterval()
{
  int64 nSelectionInterval = 0;
  for(int nSection=0; nSection<64; nSection++)
    nSelectionInterval += GetStakeModifierSelectionIntervalSection(nSection);
  return nSelectionInterval;
}

// select a block from the candidate blocks in vSortedByTimestamp, excluding
// already selected blocks in vSelectedBlocks, and with timestamp up to
// nSelectionIntervalStop.
static bool SelectBlockFromCandidates(vector<pair<int64, uint256> >& vSortedByTimestamp,
                                      map<uint256, const CBlockIndex*>& mapSelectedBlocks,
                                      int64 nSelectionIntervalStop, uint64 nStakeModifierPrev,
                                      const CBlockIndex** pindexSelected)
{
  bool fSelected = false;
  uint256 hashBest = 0;
  *pindexSelected = (const CBlockIndex*) 0;
  BOOST_FOREACH(const PAIRTYPE(int64, uint256)& item, vSortedByTimestamp)
  {
    if(!mapBlockIndex.count(item.second))
      return error("SelectBlockFromCandidates: failed to find block index for candidate block %s", item.second.ToString().c_str());
    const CBlockIndex* pindex = mapBlockIndex[item.second];
    if(fSelected && pindex->GetBlockTime() > nSelectionIntervalStop)
      break;
    if(mapSelectedBlocks.count(pindex->GetBlockHash()) > 0)
      continue;
    // compute the selection hash by hashing its proof-hash and the
    // previous proof-of-stake modifier
    uint256 hashProof = pindex->IsProofOfStake()? pindex->hashProofOfStake : pindex->GetBlockHash();
    CDataStream ss(SER_GETHASH, 0);
    ss << hashProof << nStakeModifierPrev;
    uint256 hashSelection = Hash(ss.begin(), ss.end());
    // the selection hash is divided by 2**32 so that proof-of-stake block
    // is always favored over proof-of-work block. this is to preserve
    // the energy efficiency property
    if(pindex->IsProofOfStake())
      hashSelection >>= 32;
    if(fSelected && hashSelection < hashBest)
    {
      hashBest = hashSelection;
      *pindexSelected = (const CBlockIndex*) pindex;
    }
    else if(!fSelected)
    {
      fSelected = true;
      hashBest = hashSelection;
      *pindexSelected = (const CBlockIndex*) pindex;
    }
  }
  if(fDebug && GetBoolArg("-printstakemodifier"))
    printf("SelectBlockFromCandidates: selection hash=%s\n", hashBest.ToString().c_str());
  return fSelected;
}

// Stake Modifier (hash modifier of proof-of-stake):
// The purpose of stake modifier is to prevent a txout (coin) owner from
// computing future proof-of-stake generated by this txout at the time
// of transaction confirmation. To meet kernel protocol, the txout
// must hash with a future stake modifier to generate the proof.
// Stake modifier consists of bits each of which is contributed from a
// selected block of a given block group in the past.
// The selection of a block is based on a hash of the block's proof-hash and
// the previous stake modifier.
// Stake modifier is recomputed at a fixed time interval instead of every 
// block. This is to make it difficult for an attacker to gain control of
// additional bits in the stake modifier, even after generating a chain of
// blocks.
bool ComputeNextStakeModifier(const CBlockIndex *pindexPrev, uint64 &nStakeModifier, bool &fGeneratedStakeModifier)
{
  nStakeModifier = 0;
  fGeneratedStakeModifier = false;
  if(!pindexPrev)
  {
    fGeneratedStakeModifier = true;
    return true;  // genesis block's modifier is 0
  }
  // First find current stake modifier and its generation block time
  // if it's not old enough, return the same stake modifier
  int64 nModifierTime = 0;
  if(!GetLastStakeModifier(pindexPrev, nStakeModifier, nModifierTime))
    return error("ComputeNextStakeModifier: unable to get last modifier");
  if(fDebug)
  {
    printf("ComputeNextStakeModifier: prev modifier=0x%016"PRI64x" time=%s\n", 
           nStakeModifier, DateTimeStrFormat(nModifierTime).c_str());
  }
  if(nModifierTime / nModifierInterval >= pindexPrev->GetBlockTime() / nModifierInterval)
    return true;

  // Sort candidate blocks by timestamp
  vector<pair<int64, uint256> > vSortedByTimestamp;
  vSortedByTimestamp.reserve(64 * nModifierInterval / STAKE_TARGET_SPACING);
  int64 nSelectionInterval = GetStakeModifierSelectionInterval();
  int64 nSelectionIntervalStart = (pindexPrev->GetBlockTime() / nModifierInterval) * nModifierInterval - nSelectionInterval;
  const CBlockIndex* pindex = pindexPrev;
  while(pindex && pindex->GetBlockTime() >= nSelectionIntervalStart)
  {
    vSortedByTimestamp.push_back(make_pair(pindex->GetBlockTime(), pindex->GetBlockHash()));
    pindex = pindex->pprev;
  }
  int nHeightFirstCandidate = pindex ? (pindex->nHeight + 1) : 0;
  reverse(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
  sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end());

  // Select 64 blocks from candidate blocks to generate stake modifier
  uint64 nStakeModifierNew = 0;
  int64 nSelectionIntervalStop = nSelectionIntervalStart;
  map<uint256, const CBlockIndex*> mapSelectedBlocks;
  for(int nRound=0; nRound<min(64, (int)vSortedByTimestamp.size()); nRound++)
  {
    // add an interval section to the current selection round
    nSelectionIntervalStop += GetStakeModifierSelectionIntervalSection(nRound);
    // select a block from the candidates of current round
    if(!SelectBlockFromCandidates(vSortedByTimestamp, mapSelectedBlocks, nSelectionIntervalStop, nStakeModifier, &pindex))
      return error("ComputeNextStakeModifier: unable to select block at round %d", nRound);
    // write the entropy bit of the selected block
    nStakeModifierNew |= (((uint64)pindex->GetStakeEntropyBit()) << nRound);
    // add the selected block from candidates to selected list
    mapSelectedBlocks.insert(make_pair(pindex->GetBlockHash(), pindex));
    if(fDebug && GetBoolArg("-printstakemodifier"))
      printf("ComputeNextStakeModifier: selected round %d stop=%s height=%d bit=%d\n",
             nRound, DateTimeStrFormat(nSelectionIntervalStop).c_str(), pindex->nHeight, pindex->GetStakeEntropyBit());
  }

  // Print selection map for visualization of the selected blocks
  if(fDebug && GetBoolArg("-printstakemodifier"))
  {
    string strSelectionMap = "";
    // '-' indicates proof-of-work blocks not selected
    strSelectionMap.insert(0, pindexPrev->nHeight - nHeightFirstCandidate + 1, '-');
    pindex = pindexPrev;
    while(pindex && pindex->nHeight >= nHeightFirstCandidate)
    {
      // '=' indicates proof-of-stake blocks not selected
      if(pindex->IsProofOfStake())
        strSelectionMap.replace(pindex->nHeight - nHeightFirstCandidate, 1, "=");
      pindex = pindex->pprev;
    }
    BOOST_FOREACH(const PAIRTYPE(uint256, const CBlockIndex*)& item, mapSelectedBlocks)
    {
      // 'S' indicates selected proof-of-stake blocks
      // 'W' indicates selected proof-of-work blocks
      strSelectionMap.replace(item.second->nHeight - nHeightFirstCandidate, 1, item.second->IsProofOfStake()? "S" : "W");
    }
    printf("ComputeNextStakeModifier: selection height [%d, %d] map %s\n", 
           nHeightFirstCandidate, pindexPrev->nHeight, strSelectionMap.c_str());
  }
  if(fDebug)
  {
    printf("ComputeNextStakeModifier: new modifier=0x%016"PRI64x" time=%s\n", 
           nStakeModifierNew, DateTimeStrFormat(pindexPrev->GetBlockTime()).c_str());
  }

  nStakeModifier = nStakeModifierNew;
  fGeneratedStakeModifier = true;
  return true;
}

// The stake modifier used to hash for a stake kernel is chosen as the stake
// modifier about a selection interval later than the coin generating the kernel
static bool GetKernelStakeModifier(const CBlockIndex *pindexFrom, uint64 &nStakeModifier, 
                                   int &nStakeModifierHeight, int64 &nStakeModifierTime, 
                                   bool fPrintProofOfStake)
{
  nStakeModifier = 0;
  if(!pindexFrom)
    return error("GetKernelStakeModifier() : pindexFrom is NULL");

  nStakeModifierHeight = pindexFrom->nHeight;
  nStakeModifierTime = pindexFrom->GetBlockTime();
  int64 nStakeModifierSelectionInterval = GetStakeModifierSelectionInterval();
  const CBlockIndex* pindex = pindexFrom;

  // loop to find the stake modifier later by a selection interval
  while(nStakeModifierTime < pindexFrom->GetBlockTime() + nStakeModifierSelectionInterval)
  {
    if(!pindex->pnext)
    { 
      // reached best block; may happen if node is behind on block chain
      if(fPrintProofOfStake || 
         (pindex->GetBlockTime() + nStakeMinAge - nStakeModifierSelectionInterval > GetAdjustedTime()))
        return error("GetKernelStakeModifier() : reached best block %s at height %d from block %s",
                     pindex->GetBlockHash().ToString().c_str(), pindex->nHeight, 
                     pindexFrom->GetBlockHash().ToString().c_str());
      else
        return false;
    }
    pindex = pindex->pnext;
    if(pindex->GeneratedStakeModifier())
    {
      nStakeModifierHeight = pindex->nHeight;
      nStakeModifierTime = pindex->GetBlockTime();
    }
  }
  nStakeModifier = pindex->nStakeModifier;
  return true;
}

// slimcoin kernel protocol
// coinstake must meet hash target according to the protocol:
// kernel (input 0) must meet the formula
//     hash(nStakeModifier + txPrev.block.nTime + txPrev.offset + txPrev.nTime + txPrev.vout.n + nTime) < bnTarget * nCoinDayWeight
// this ensures that the chance of getting a coinstake is proportional to the
// amount of coin age one owns.
// The reason this hash is chosen is the following:
//   nStakeModifier: 
//       (v0.3) scrambles computation to make it very difficult to precompute
//              future proof-of-stake at the time of the coin's confirmation
//       (v0.2) nBits (deprecated): encodes all past block timestamps
//   txPrev.block.nTime: prevent nodes from guessing a good timestamp to
//                       generate transaction for future advantage
//   txPrev.offset: offset of txPrev inside block, to reduce the chance of 
//                  nodes generating coinstake at the same time
//   txPrev.nTime: reduce the chance of nodes generating coinstake at the same
//                 time
//   txPrev.vout.n: output number of txPrev, to reduce the chance of nodes
//                  generating coinstake at the same time
//   block/tx hash should not be used here as they can be generated in vast
//   quantities so as to generate blocks faster, degrading the system back into
//   a proof-of-work situation.
//
bool CheckStakeKernelHash(unsigned int nBits, const CBlockIndex *pindexFrom, unsigned int nTxPrevOffset,
                          const CTransaction &txPrev, const COutPoint &prevout, unsigned int nTimeTx,
                          uint256 &hashProofOfStake, bool fPrintProofOfStake)
{
  if(nTimeTx < txPrev.nTime)  // Transaction timestamp violation
    return error("CheckStakeKernelHash() : nTime violation");

  unsigned int nTimeBlockFrom = pindexFrom->GetBlockTime();
  if(nTimeBlockFrom + nStakeMinAge > nTimeTx) // Min age requirement
    return error("CheckStakeKernelHash() : min age violation");

  CBigNum bnTargetPerCoinDay;
  bnTargetPerCoinDay.SetCompact(nBits);
  int64 nValueIn = txPrev.vout[prevout.n].nValue;

  // v0.3 protocol kernel hash weight starts from 0 at the 30-day min age
  // this change increases active coins participating the hash and helps
  // to secure the network when proof-of-stake difficulty is low
  int64 nTimeWeight = min((int64)nTimeTx - txPrev.nTime, (int64)STAKE_MAX_AGE) - 
    (IsProtocolV03(nTimeTx) ? nStakeMinAge : 0);

  CBigNum bnCoinDayWeight = CBigNum(nValueIn) * nTimeWeight / COIN / (24 * 60 * 60);

  // Calculate hash
  CDataStream ss(SER_GETHASH, 0);
  uint64 nStakeModifier = 0;
  int nStakeModifierHeight = 0;
  int64 nStakeModifierTime = 0;
  if(IsProtocolV03(nTimeTx))  // v0.3 protocol
  { 
    if(!GetKernelStakeModifier(pindexFrom, nStakeModifier, nStakeModifierHeight, 
                               nStakeModifierTime, fPrintProofOfStake))
      return false;
    ss << nStakeModifier;
  }else  // v0.2 protocol
    ss << nBits;

  ss << nTimeBlockFrom << nTxPrevOffset << txPrev.nTime << prevout.n << nTimeTx;
  hashProofOfStake = Hash(ss.begin(), ss.end());

  if(fPrintProofOfStake)
  {
    if(IsProtocolV03(nTimeTx))
      printf("CheckStakeKernelHash() : using modifier 0x%016"PRI64x" at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
             nStakeModifier, nStakeModifierHeight,
             DateTimeStrFormat(nStakeModifierTime).c_str(),
             mapBlockIndex[pindexFrom->GetBlockHash()]->nHeight,
             DateTimeStrFormat(pindexFrom->GetBlockTime()).c_str());
    printf("CheckStakeKernelHash() : check protocol=%s modifier=0x%016"PRI64x" nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
           IsProtocolV03(nTimeTx)? "0.3" : "0.2",
           IsProtocolV03(nTimeTx)? nStakeModifier : (uint64) nBits,
           nTimeBlockFrom, nTxPrevOffset, txPrev.nTime, prevout.n, nTimeTx,
           hashProofOfStake.ToString().c_str());
  }

  // Now check if proof-of-stake hash meets target protocol
  if(CBigNum(hashProofOfStake) > bnTargetPerCoinDay * bnCoinDayWeight)
    return false;

  if(fDebug && !fPrintProofOfStake)
  {
    if(IsProtocolV03(nTimeTx))
      printf("CheckStakeKernelHash() : using modifier 0x%016"PRI64x" at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
             nStakeModifier, nStakeModifierHeight, 
             DateTimeStrFormat(nStakeModifierTime).c_str(),
             mapBlockIndex[pindexFrom->GetBlockHash()]->nHeight,
             DateTimeStrFormat(pindexFrom->GetBlockTime()).c_str());
    printf("CheckStakeKernelHash() : pass protocol=%s modifier=0x%016"PRI64x" nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
           IsProtocolV03(nTimeTx)? "0.3" : "0.2",
           IsProtocolV03(nTimeTx)? nStakeModifier : (uint64) nBits,
           nTimeBlockFrom, nTxPrevOffset, txPrev.nTime, prevout.n, nTimeTx,
           hashProofOfStake.ToString().c_str());
  }

  return true;
}

// Check kernel hash target and coinstake signature
bool CheckProofOfStake(const CTransaction &tx, unsigned int nBits, uint256 &hashProofOfStake)
{
  if(!tx.IsCoinStake())
    return error("CheckProofOfStake() : called on non-coinstake %s", tx.GetHash().ToString().c_str());

  // Kernel (input 0) must match the stake hash target per coin age (nBits)
  const CTxIn& txin = tx.vin[0];

  // First try finding the previous transaction in database
  CTxDB txdb("r");
  CTransaction txPrev;
  CTxIndex txindex;

  // previous transaction not in main chain, may occur during initial download
  if(!txPrev.ReadFromDisk(txdb, txin.prevout, txindex))
    return tx.DoS(1, error("CheckProofOfStake() : INFO: read txPrev failed"));

  txdb.Close();

  // Verify signature
  if(!VerifySignature(txPrev, tx, 0, true, 0))
    return tx.DoS(100, error("CheckProofOfStake() : VerifySignature failed on coinstake %s",
                             tx.GetHash().ToString().c_str()));

  // Read block header
  CBlock block;

  // unable to read block of previous transaction
  if(!block.ReadFromDisk(txindex.pos.nFile, txindex.pos.nBlockPos, false))
    return fDebug ? error("CheckProofOfStake() : read block failed") : false; 

  //There is no way to get the CBlockIndex from a CTransaction, the block containing the tx must be read
  // and then, using the block's hash, lookup the CBlockIndex
  uint256 blockHash = block.GetHash();
  if(!mapBlockIndex.count(blockHash))
    return fDebug ? error("CheckProofOfStake() : CBlockIndex lookup failed") : false;
  CBlockIndex *pindexFrom = mapBlockIndex[blockHash];

  // may occur during initial download or if behind on block chain sync
  if(!CheckStakeKernelHash(nBits, pindexFrom, txindex.pos.nTxPos - txindex.pos.nBlockPos, 
                           txPrev, txin.prevout, tx.nTime, hashProofOfStake, fDebug))
    return tx.DoS(1, error("CheckProofOfStake() : INFO: check kernel failed on coinstake %s, hashProof=%s", 
                           tx.GetHash().ToString().c_str(), hashProofOfStake.ToString().c_str()));

  return true;
}

// Check whether the coinstake timestamp meets protocol
bool CheckCoinStakeTimestamp(int64 nTimeBlock, int64 nTimeTx)
{
  if(IsProtocolV03(nTimeTx))  // v0.3 protocol
    return (nTimeBlock == nTimeTx);
  else // v0.2 protocol
    return ((nTimeTx <= nTimeBlock) && (nTimeBlock <= nTimeTx + nMaxClockDrift));
}

// Get stake modifier checksum
unsigned int GetStakeModifierChecksum(const CBlockIndex* pindex)
{
  assert(pindex->pprev || pindex->GetBlockHash() == hashGenesisBlock);

  // Hash previous checksum with flags, hashProofOfStake and nStakeModifier
  CDataStream ss(SER_GETHASH, 0);
  if(pindex->pprev)
    ss << pindex->pprev->nStakeModifierChecksum;

  ss << pindex->nFlags << pindex->hashProofOfStake << pindex->nStakeModifier;
  uint256 hashChecksum = Hash(ss.begin(), ss.end());
  hashChecksum >>= (256 - 32);
  return hashChecksum.Get64();
}

// Check stake modifier hard checkpoints
bool CheckStakeModifierCheckpoints(s32int nHeight, u32int nStakeModifierChecksum)
{
  // Testnet has no checkpoints
  if(fTestNet) 
    return true;

  if(mapStakeModifierCheckpoints.count(nHeight))
    return nStakeModifierChecksum == mapStakeModifierCheckpoints[nHeight];

  return true;
}