Enforce BIP94 (TestNet4) difficult adjustment logic

NBitcoin.Tests/Testnet4PowTests.cs

@@ -0,0 +1,80 @@
+using Xunit;
+
+namespace NBitcoin.Tests
+{
+	/// <summary>
+	/// Tests for BIP 94 (Testnet4) difficulty adjustment rules.
+	/// BIP 94 uses the FIRST block of the previous period as the difficulty anchor, not the LAST.
+	/// </summary>
+	public class Testnet4PowTests
+	{
+		// Real testnet4 headers at difficulty boundary where BIP 94 matters:
+		// Block 28224 (first of period): nBits=0x1a0082a5, Block 30239 (last): nBits=0x1d00ffff (min diff!)
+		// Block 30240's difficulty 0x1973b070 was calculated from 28224, NOT from 30239's min-difficulty.
+		const string Header28224 = "0060992d545f75503d65d04310b1904921b09b6f98c0674c20a84f2d6e00000000000000fe7f1811f7694b3cff4ed7a8ef0447fb59139d763e1a58c6ffbd6198eae38e83c1cf5c66a582001a2c76f01d";
+		const string Header30239 = "0000002092e737868048420bb08385b9937cec6b2f77784509a0995507000000000000002e4de945b5edd021ad3420e46a7ebe5dbd5d9bc5d15b319dec8f9afe3fde9334de276d66ffff001db5b35b90";
+		const string Header30240 = "00805222bbde84820b42d69d70d6b4c45d5ec570ed2a891436833d25eae0f8ea0000000026a7ae7a6e174ca8bb25e9d3d3063b2c54dba4fb16012108d7c5ffff27957e43752a6d6670b073194c48554f";
+
+		[Fact]
+		[Trait("UnitTest", "UnitTest")]
+		public void CanCalculateTestnet4DifficultyWithBIP94()
+		{
+			var network = Network.TestNet4;
+			Assert.True(network.Consensus.EnforceBIP94);
+
+			var header28224 = BlockHeader.Parse(Header28224, network);
+			var header30239 = BlockHeader.Parse(Header30239, network);
+			var header30240 = BlockHeader.Parse(Header30240, network);
+
+			Assert.Equal(0x1a0082a5u, header28224.Bits.ToCompact());
+			Assert.Equal(0x1d00ffffu, header30239.Bits.ToCompact());
+			Assert.Equal(0x1973b070u, header30240.Bits.ToCompact());
+
+			var block30240 = BuildTestChain(header28224, header30239, header30240, network.Consensus);
+			Assert.Equal(header30240.Bits, block30240.GetWorkRequired(network.Consensus));
+		}
+
+		[Fact]
+		[Trait("UnitTest", "UnitTest")]
+		public void BIP94PreventsMinDifficultyFromDistortingRetarget()
+		{
+			var network = Network.TestNet4;
+			var header28224 = BlockHeader.Parse(Header28224, network);
+			var header30239 = BlockHeader.Parse(Header30239, network);
+			var header30240 = BlockHeader.Parse(Header30240, network);
+
+			var block30240 = BuildTestChain(header28224, header30239, header30240, network.Consensus);
+
+			var targetWithBIP94 = block30240.GetWorkRequired(network.Consensus);
+
+			var consensusWithoutBIP94 = network.Consensus.Clone();
+			consensusWithoutBIP94.EnforceBIP94 = false;
+			var targetWithoutBIP94 = block30240.GetWorkRequired(consensusWithoutBIP94);
+
+			Assert.Equal(new Target(0x1973b070), targetWithBIP94);
+			Assert.True(targetWithoutBIP94 > targetWithBIP94);
+		}
+
+		private static ChainedBlock BuildTestChain(BlockHeader first, BlockHeader last, BlockHeader next, Consensus consensus)
+		{
+			const int firstHeight = 28224;
+			const int lastHeight = 30239;
+
+			var current = new ChainedBlock(first, firstHeight);
+			for (int height = firstHeight + 1; height <= lastHeight; height++)
+			{
+				var header = consensus.ConsensusFactory.CreateBlockHeader();
+				header.HashPrevBlock = current.HashBlock;
+				header.Bits = (height == lastHeight) ? last.Bits : first.Bits;
+				header.BlockTime = (height == lastHeight) ? last.BlockTime : first.BlockTime;
+				current = new ChainedBlock(header, header.GetHash(), current);
+			}
+
+			var nextHeader = consensus.ConsensusFactory.CreateBlockHeader();
+			nextHeader.HashPrevBlock = current.HashBlock;
+			nextHeader.BlockTime = next.BlockTime;
+			nextHeader.Bits = next.Bits;
+			return new ChainedBlock(nextHeader, nextHeader.GetHash(), current);
+		}
+	}
+}

NBitcoin/Bitcoin.Testnet4.cs

@@ -65,6 +65,7 @@ private Network CreateTestnet4()
 				SupportTaproot = true,
 				SupportSegwit = true,
 				CoinType = 1,
+				EnforceBIP94 = true, // Testnet4 a different difficulty adjustment rule
 			};
 
 			// Modify the testnet genesis block so the timestamp is valid for a later start.

NBitcoin/ChainedBlock.cs

@@ -1,4 +1,4 @@
-#if NO_NATIVE_BIGNUM
+#if NO_NATIVE_BIGNUM
 using NBitcoin.BouncyCastle.Math;
 #else
 using System.Numerics;
@@ -431,7 +431,10 @@ public Target GetWorkRequired(Consensus consensus)
 				nActualTimespan = TimeSpan.FromTicks(consensus.PowTargetTimespan.Ticks * 4);
 
 			// Retarget
-			var bnNew = pindexLast.Header.Bits.ToBigInteger();
+			// TestNet4 (BIP 94) uses the first block of the previous period instead of the last block.
+			var baseBlock = consensus.EnforceBIP94 ? pindexFirst : pindexLast;
+			var bnNew = baseBlock.Header.Bits.ToBigInteger();
+
 #if NO_NATIVE_BIGNUM
 			bnNew = bnNew.Multiply(BigInteger.ValueOf((long)nActualTimespan.TotalSeconds));
 			bnNew = bnNew.Divide(BigInteger.ValueOf((long)consensus.PowTargetTimespan.TotalSeconds));

NBitcoin/Network.cs

@@ -674,6 +674,20 @@ public bool NeverNeedPreviousTxForSigning
 			}
 		}
 
+		bool _EnforceBIP94;
+		public bool EnforceBIP94
+		{
+			get
+			{
+				return _EnforceBIP94;
+			}
+			set
+			{
+				EnsureNotFrozen();
+				_EnforceBIP94 = value;
+			}
+		}
+
 		public virtual Consensus Clone()
 		{
 			var consensus = new Consensus();
@@ -715,6 +729,7 @@ protected void Fill(Consensus consensus)
 			consensus._SupportSegwit = _SupportSegwit;
 			consensus._SupportTaproot = _SupportTaproot;
 			consensus._NeverNeedPreviousTxForSigning = _NeverNeedPreviousTxForSigning;
+			consensus._EnforceBIP94 = _EnforceBIP94;
 		}
 	}
 	public partial class Network