/// <summary>
/// Checks that the nBits field of a block follow protocol rules.
/// </summary>
internal static bool IsNBitsValid(StoredBlock block, Subchain parentChain)
{
//todo: add test for this method
StoredBlock parentBlock = parentChain.GetBlockByOffset(0);
BigInteger blockDifficultyTarget = DifficultyUtils.NBitsToTarget(block.Header.NBits);
if (block.Height%DifficultyUtils.DifficultyAdjustmentIntervalInBlocks != 0)
{
BigInteger parentBlockDifficultyTarget = DifficultyUtils.NBitsToTarget(parentBlock.Header.NBits);
if (blockDifficultyTarget != parentBlockDifficultyTarget)
{
return false;
}
}
else
{
StoredBlock baseBlock = parentChain.GetBlockByHeight(block.Height - DifficultyUtils.DifficultyAdjustmentIntervalInBlocks);
// todo: check against non-bugged value and allow some percent-based difference?
// Note: There is a known bug here. It known as "off-by-one" or "Time Warp Bug".
// Bug Description: http://bitcoin.stackexchange.com/questions/20597/where-exactly-is-the-off-by-one-difficulty-bug
// Related Attack Description: https://bitcointalk.org/index.php?topic=43692.msg521772#msg521772
uint timeSpent = parentBlock.Header.Timestamp - baseBlock.Header.Timestamp;
BigInteger oldTarget = DifficultyUtils.NBitsToTarget(parentBlock.Header.NBits);
var newTarget = DifficultyUtils.CalculateNewTarget(timeSpent, oldTarget);
// this rounds newTarget value to a value that would fit into NBits
uint newNBits = DifficultyUtils.TargetToNBits(newTarget);
newTarget = DifficultyUtils.NBitsToTarget(newNBits);
if (blockDifficultyTarget != newTarget)
{
return false;
}
}
return true;
}
