testnet: Add timewarp attack prevention for Testnet4

Author: fjahr

src/validation.cpp

@@ -4182,6 +4182,18 @@ static bool ContextualCheckBlockHeader(const CBlockHeader& block, BlockValidatio
     if (block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
         return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, "time-too-old", "block's timestamp is too early");
 
+    // Testnet4 only: Check timestamp against prev for difficulty-adjustment
+    // blocks to prevent timewarp attacks (see https://github.com/bitcoin/bitcoin/pull/15482).
+    if (consensusParams.enforce_BIP94) {
+        // Check timestamp for the first block of each difficulty adjustment
+        // interval, except the genesis block.
+        if (nHeight % consensusParams.DifficultyAdjustmentInterval() == 0) {
+            if (block.GetBlockTime() < pindexPrev->GetBlockTime() - 60 * 60 * 2) {
+                return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, "time-timewarp-attack", "block's timestamp is too early on diff adjustment block");
+            }
+        }
+    }
+
     // Check timestamp
     if (block.Time() > NodeClock::now() + std::chrono::seconds{MAX_FUTURE_BLOCK_TIME}) {
         return state.Invalid(BlockValidationResult::BLOCK_TIME_FUTURE, "time-too-new", "block timestamp too far in the future");