Enable NRD kernel support (noop) feature flagged (#3303)

* wip

* commit

* add block level validation rule around NRD kernels and HF3 block height

* wip

* test coverage for kernel ser/deser for NRD kernels

* add some type safety around ser/deser of nrd relative height

* cleanup

* cleanup tx sig handling and add tests around NRD kernel sig

* test coverage for chain apply block containing NRD kernel

* verify kernel variants when adding tx to pool
NRD kernels will not be accepted until HF3

* add txpool test for NRD kernel handling

* fix merge

* fix api for NRD kernels

* commit

* wip - feature flag for NRD kernel support

* wip

* global config and thread local feature flag for NRD kernel support

* add feature flag support for NRD kernels
default to false when starting up node
allow it to be set to true in context of individual tests

* feature flag

* explicit testing of NRD kernel via feature flag

* add chain_type and feature flag logging on startup

* PR feedback and cleanup

* PR feedback

api/src/types.rs

@@ -509,6 +509,10 @@ impl TxKernelPrintable {
 			KernelFeatures::Plain { fee } => (fee, 0),
 			KernelFeatures::Coinbase => (0, 0),
 			KernelFeatures::HeightLocked { fee, lock_height } => (fee, lock_height),
+			KernelFeatures::NoRecentDuplicate {
+				fee,
+				relative_height,
+			} => (fee, relative_height.into()),
 		};
 		TxKernelPrintable {
 			features,

chain/tests/chain_test_helper.rs

@@ -50,7 +50,7 @@ pub fn init_chain(dir_name: &str, genesis: Block) -> Chain {
 }
 
 /// Build genesis block with reward (non-empty, like we have in mainnet).
-fn genesis_block<K>(keychain: &K) -> Block
+pub fn genesis_block<K>(keychain: &K) -> Block
 where
 	K: Keychain,
 {
@@ -69,6 +69,7 @@ where
 
 /// Mine a chain of specified length to assist with automated tests.
 /// Probably a good idea to call clean_output_dir at the beginning and end of each test.
+#[allow(dead_code)]
 pub fn mine_chain(dir_name: &str, chain_length: u64) -> Chain {
 	global::set_local_chain_type(ChainTypes::AutomatedTesting);
 	let keychain = keychain::ExtKeychain::from_random_seed(false).unwrap();
@@ -78,6 +79,7 @@ pub fn mine_chain(dir_name: &str, chain_length: u64) -> Chain {
 	chain
 }
 
+#[allow(dead_code)]
 fn mine_some_on_top<K>(chain: &mut Chain, chain_length: u64, keychain: &K)
 where
 	K: Keychain,

chain/tests/mine_nrd_kernel.rs

@@ -0,0 +1,151 @@
+// Copyright 2020 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+mod chain_test_helper;
+
+use grin_chain as chain;
+use grin_core as core;
+use grin_keychain as keychain;
+use grin_util as util;
+
+use self::chain_test_helper::{clean_output_dir, genesis_block, init_chain};
+use crate::chain::{Chain, Options};
+use crate::core::core::{Block, KernelFeatures, NRDRelativeHeight, Transaction};
+use crate::core::libtx::{build, reward, ProofBuilder};
+use crate::core::{consensus, global, pow};
+use crate::keychain::{ExtKeychain, ExtKeychainPath, Identifier, Keychain};
+use chrono::Duration;
+
+fn build_block<K>(chain: &Chain, keychain: &K, key_id: &Identifier, txs: Vec<Transaction>) -> Block
+where
+	K: Keychain,
+{
+	let prev = chain.head_header().unwrap();
+	let next_header_info = consensus::next_difficulty(1, chain.difficulty_iter().unwrap());
+	let fee = txs.iter().map(|x| x.fee()).sum();
+	let reward =
+		reward::output(keychain, &ProofBuilder::new(keychain), key_id, fee, false).unwrap();
+
+	let mut block = Block::new(&prev, txs, next_header_info.clone().difficulty, reward).unwrap();
+
+	block.header.timestamp = prev.timestamp + Duration::seconds(60);
+	block.header.pow.secondary_scaling = next_header_info.secondary_scaling;
+
+	chain.set_txhashset_roots(&mut block).unwrap();
+
+	let edge_bits = global::min_edge_bits();
+	block.header.pow.proof.edge_bits = edge_bits;
+	pow::pow_size(
+		&mut block.header,
+		next_header_info.difficulty,
+		global::proofsize(),
+		edge_bits,
+	)
+	.unwrap();
+
+	block
+}
+
+#[test]
+fn mine_block_with_nrd_kernel_and_nrd_feature_enabled() {
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+	global::set_local_nrd_enabled(true);
+
+	util::init_test_logger();
+
+	let chain_dir = ".grin.nrd_kernel";
+	clean_output_dir(chain_dir);
+
+	let keychain = ExtKeychain::from_random_seed(false).unwrap();
+	let pb = ProofBuilder::new(&keychain);
+	let genesis = genesis_block(&keychain);
+	let chain = init_chain(chain_dir, genesis.clone());
+
+	for n in 1..9 {
+		let key_id = ExtKeychainPath::new(1, n, 0, 0, 0).to_identifier();
+		let block = build_block(&chain, &keychain, &key_id, vec![]);
+		chain.process_block(block, Options::MINE).unwrap();
+	}
+
+	assert_eq!(chain.head().unwrap().height, 8);
+
+	let key_id1 = ExtKeychainPath::new(1, 1, 0, 0, 0).to_identifier();
+	let key_id2 = ExtKeychainPath::new(1, 2, 0, 0, 0).to_identifier();
+	let tx = build::transaction(
+		KernelFeatures::NoRecentDuplicate {
+			fee: 20000,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+		vec![
+			build::coinbase_input(consensus::REWARD, key_id1.clone()),
+			build::output(consensus::REWARD - 20000, key_id2.clone()),
+		],
+		&keychain,
+		&pb,
+	)
+	.unwrap();
+
+	let key_id9 = ExtKeychainPath::new(1, 9, 0, 0, 0).to_identifier();
+	let block = build_block(&chain, &keychain, &key_id9, vec![tx]);
+	chain.process_block(block, Options::MINE).unwrap();
+	chain.validate(false).unwrap();
+
+	clean_output_dir(chain_dir);
+}
+
+#[test]
+fn mine_invalid_block_with_nrd_kernel_and_nrd_feature_enabled_before_hf() {
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+	global::set_local_nrd_enabled(true);
+
+	util::init_test_logger();
+
+	let chain_dir = ".grin.invalid_nrd_kernel";
+	clean_output_dir(chain_dir);
+
+	let keychain = ExtKeychain::from_random_seed(false).unwrap();
+	let pb = ProofBuilder::new(&keychain);
+	let genesis = genesis_block(&keychain);
+	let chain = init_chain(chain_dir, genesis.clone());
+
+	for n in 1..8 {
+		let key_id = ExtKeychainPath::new(1, n, 0, 0, 0).to_identifier();
+		let block = build_block(&chain, &keychain, &key_id, vec![]);
+		chain.process_block(block, Options::MINE).unwrap();
+	}
+
+	assert_eq!(chain.head().unwrap().height, 7);
+
+	let key_id1 = ExtKeychainPath::new(1, 1, 0, 0, 0).to_identifier();
+	let key_id2 = ExtKeychainPath::new(1, 2, 0, 0, 0).to_identifier();
+	let tx = build::transaction(
+		KernelFeatures::NoRecentDuplicate {
+			fee: 20000,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+		vec![
+			build::coinbase_input(consensus::REWARD, key_id1.clone()),
+			build::output(consensus::REWARD - 20000, key_id2.clone()),
+		],
+		&keychain,
+		&pb,
+	)
+	.unwrap();
+
+	let key_id8 = ExtKeychainPath::new(1, 8, 0, 0, 0).to_identifier();
+	let block = build_block(&chain, &keychain, &key_id8, vec![tx]);
+	let res = chain.process_block(block, Options::MINE);
+	assert!(res.is_err());
+	clean_output_dir(chain_dir);
+}

chain/tests/mine_simple_chain.rs

@@ -81,7 +81,7 @@ fn mine_empty_chain() {
 
 #[test]
 fn mine_short_chain() {
-	let chain_dir = ".grin.genesis";
+	let chain_dir = ".grin.short";
 	clean_output_dir(chain_dir);
 	let chain = mine_chain(chain_dir, 4);
 	assert_eq!(chain.head().unwrap().height, 3);

core/src/consensus.rs

@@ -133,12 +133,15 @@ pub const FLOONET_FIRST_HARD_FORK: u64 = 185_040;
 /// Floonet second hard fork height, set to happen around 2019-12-19
 pub const FLOONET_SECOND_HARD_FORK: u64 = 298_080;
 
-/// AutomatedTesting and UserTesting first hard fork height.
+/// AutomatedTesting and UserTesting HF1 height.
 pub const TESTING_FIRST_HARD_FORK: u64 = 3;
 
-/// AutomatedTesting and UserTesting second hard fork height.
+/// AutomatedTesting and UserTesting HF2 height.
 pub const TESTING_SECOND_HARD_FORK: u64 = 6;
 
+/// AutomatedTesting and UserTesting HF3 height.
+pub const TESTING_THIRD_HARD_FORK: u64 = 9;
+
 /// Compute possible block version at a given height, implements
 /// 6 months interval scheduled hard forks for the first 2 years.
 pub fn header_version(height: u64) -> HeaderVersion {
@@ -162,10 +165,12 @@ pub fn header_version(height: u64) -> HeaderVersion {
 				HeaderVersion(1)
 			} else if height < TESTING_SECOND_HARD_FORK {
 				HeaderVersion(2)
-			} else if height < 3 * HARD_FORK_INTERVAL {
+			} else if height < TESTING_THIRD_HARD_FORK {
 				HeaderVersion(3)
+			} else if height < 4 * HARD_FORK_INTERVAL {
+				HeaderVersion(4)
 			} else {
-				HeaderVersion(hf_interval)
+				HeaderVersion(5)
 			}
 		}
 	}

core/src/core/block.rs

@@ -63,6 +63,10 @@ pub enum Error {
 	InvalidPow,
 	/// Kernel not valid due to lock_height exceeding block header height
 	KernelLockHeight(u64),
+	/// NRD kernels are not valid prior to HF3.
+	NRDKernelPreHF3,
+	/// NRD kernels are not valid if disabled locally via "feature flag".
+	NRDKernelNotEnabled,
 	/// Underlying tx related error
 	Transaction(transaction::Error),
 	/// Underlying Secp256k1 error (signature validation or invalid public key
@@ -753,6 +757,7 @@ impl Block {
 		self.body.validate(Weighting::AsBlock, verifier)?;
 
 		self.verify_kernel_lock_heights()?;
+		self.verify_nrd_kernels_for_header_version()?;
 		self.verify_coinbase()?;
 
 		// take the kernel offset for this block (block offset minus previous) and
@@ -802,6 +807,7 @@ impl Block {
 		Ok(())
 	}
 
+	// Verify any absolute kernel lock heights.
 	fn verify_kernel_lock_heights(&self) -> Result<(), Error> {
 		for k in &self.body.kernels {
 			// check we have no kernels with lock_heights greater than current height
@@ -814,6 +820,21 @@ impl Block {
 		}
 		Ok(())
 	}
+
+	// NRD kernels are not valid if the global feature flag is disabled.
+	// NRD kernels were introduced in HF3 and are not valid for block version < 4.
+	// Blocks prior to HF3 containing any NRD kernel(s) are invalid.
+	fn verify_nrd_kernels_for_header_version(&self) -> Result<(), Error> {
+		if self.body.kernels.iter().any(|k| k.is_nrd()) {
+			if !global::is_nrd_enabled() {
+				return Err(Error::NRDKernelNotEnabled);
+			}
+			if self.header.version < HeaderVersion(4) {
+				return Err(Error::NRDKernelPreHF3);
+			}
+		}
+		Ok(())
+	}
 }
 
 impl From<UntrustedBlock> for Block {

core/src/core/transaction.rs

@@ -17,7 +17,7 @@
 use crate::core::hash::{DefaultHashable, Hashed};
 use crate::core::verifier_cache::VerifierCache;
 use crate::core::{committed, Committed};
-use crate::libtx::secp_ser;
+use crate::libtx::{aggsig, secp_ser};
 use crate::ser::{
 	self, read_multi, PMMRable, ProtocolVersion, Readable, Reader, VerifySortedAndUnique,
 	Writeable, Writer,
@@ -27,7 +27,7 @@ use enum_primitive::FromPrimitive;
 use keychain::{self, BlindingFactor};
 use std::cmp::Ordering;
 use std::cmp::{max, min};
-use std::convert::TryInto;
+use std::convert::{TryFrom, TryInto};
 use std::sync::Arc;
 use std::{error, fmt};
 use util::secp;
@@ -36,6 +36,70 @@ use util::static_secp_instance;
 use util::RwLock;
 use util::ToHex;
 
+/// Relative height field on NRD kernel variant.
+/// u16 representing a height between 1 and MAX (consensus::WEEK_HEIGHT).
+#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
+pub struct NRDRelativeHeight(u16);
+
+impl DefaultHashable for NRDRelativeHeight {}
+
+impl Writeable for NRDRelativeHeight {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		writer.write_u16(self.0)
+	}
+}
+
+impl Readable for NRDRelativeHeight {
+	fn read<R: Reader>(reader: &mut R) -> Result<Self, ser::Error> {
+		let x = reader.read_u16()?;
+		NRDRelativeHeight::try_from(x).map_err(|_| ser::Error::CorruptedData)
+	}
+}
+
+/// Conversion from a u16 to a valid NRDRelativeHeight.
+/// Valid height is between 1 and WEEK_HEIGHT inclusive.
+impl TryFrom<u16> for NRDRelativeHeight {
+	type Error = Error;
+
+	fn try_from(height: u16) -> Result<Self, Self::Error> {
+		if height == 0 {
+			Err(Error::InvalidNRDRelativeHeight)
+		} else if height
+			> NRDRelativeHeight::MAX
+				.try_into()
+				.expect("WEEK_HEIGHT const should fit in u16")
+		{
+			Err(Error::InvalidNRDRelativeHeight)
+		} else {
+			Ok(Self(height))
+		}
+	}
+}
+
+impl TryFrom<u64> for NRDRelativeHeight {
+	type Error = Error;
+
+	fn try_from(height: u64) -> Result<Self, Self::Error> {
+		Self::try_from(u16::try_from(height).map_err(|_| Error::InvalidNRDRelativeHeight)?)
+	}
+}
+
+impl From<NRDRelativeHeight> for u64 {
+	fn from(height: NRDRelativeHeight) -> Self {
+		height.0 as u64
+	}
+}
+
+impl NRDRelativeHeight {
+	const MAX: u64 = consensus::WEEK_HEIGHT;
+
+	/// Create a new NRDRelativeHeight from the provided height.
+	/// Checks height is valid (between 1 and WEEK_HEIGHT inclusive).
+	pub fn new(height: u64) -> Result<Self, Error> {
+		NRDRelativeHeight::try_from(height)
+	}
+}
+
 /// Various tx kernel variants.
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
 pub enum KernelFeatures {
@@ -53,12 +117,20 @@ pub enum KernelFeatures {
 		/// Height locked kernels have lock heights.
 		lock_height: u64,
 	},
+	/// "No Recent Duplicate" (NRD) kernels enforcing relative lock height between instances.
+	NoRecentDuplicate {
+		/// These have fees.
+		fee: u64,
+		/// Relative lock height.
+		relative_height: NRDRelativeHeight,
+	},
 }
 
 impl KernelFeatures {
 	const PLAIN_U8: u8 = 0;
 	const COINBASE_U8: u8 = 1;
 	const HEIGHT_LOCKED_U8: u8 = 2;
+	const NO_RECENT_DUPLICATE_U8: u8 = 3;
 
 	/// Underlying (u8) value representing this kernel variant.
 	/// This is the first byte when we serialize/deserialize the kernel features.
@@ -67,6 +139,7 @@ impl KernelFeatures {
 			KernelFeatures::Plain { .. } => KernelFeatures::PLAIN_U8,
 			KernelFeatures::Coinbase => KernelFeatures::COINBASE_U8,
 			KernelFeatures::HeightLocked { .. } => KernelFeatures::HEIGHT_LOCKED_U8,
+			KernelFeatures::NoRecentDuplicate { .. } => KernelFeatures::NO_RECENT_DUPLICATE_U8,
 		}
 	}
 
@@ -76,18 +149,24 @@ impl KernelFeatures {
 			KernelFeatures::Plain { .. } => String::from("Plain"),
 			KernelFeatures::Coinbase => String::from("Coinbase"),
 			KernelFeatures::HeightLocked { .. } => String::from("HeightLocked"),
+			KernelFeatures::NoRecentDuplicate { .. } => String::from("NoRecentDuplicate"),
 		}
 	}
 
-	/// msg = hash(features)                       for coinbase kernels
-	///       hash(features || fee)                for plain kernels
-	///       hash(features || fee || lock_height) for height locked kernels
+	/// msg = hash(features)                           for coinbase kernels
+	///       hash(features || fee)                    for plain kernels
+	///       hash(features || fee || lock_height)     for height locked kernels
+	///       hash(features || fee || relative_height) for NRD kernels
 	pub fn kernel_sig_msg(&self) -> Result<secp::Message, Error> {
 		let x = self.as_u8();
 		let hash = match self {
 			KernelFeatures::Plain { fee } => (x, fee).hash(),
-			KernelFeatures::Coinbase => (x).hash(),
+			KernelFeatures::Coinbase => x.hash(),
 			KernelFeatures::HeightLocked { fee, lock_height } => (x, fee, lock_height).hash(),
+			KernelFeatures::NoRecentDuplicate {
+				fee,
+				relative_height,
+			} => (x, fee, relative_height).hash(),
 		};
 
 		let msg = secp::Message::from_slice(&hash.as_bytes())?;
@@ -97,14 +176,36 @@ impl KernelFeatures {
 	/// Write tx kernel features out in v1 protocol format.
 	/// Always include the fee and lock_height, writing 0 value if unused.
 	fn write_v1<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
-		let (fee, lock_height) = match self {
-			KernelFeatures::Plain { fee } => (*fee, 0),
-			KernelFeatures::Coinbase => (0, 0),
-			KernelFeatures::HeightLocked { fee, lock_height } => (*fee, *lock_height),
-		};
 		writer.write_u8(self.as_u8())?;
-		writer.write_u64(fee)?;
-		writer.write_u64(lock_height)?;
+		match self {
+			KernelFeatures::Plain { fee } => {
+				writer.write_u64(*fee)?;
+				// Write "empty" bytes for feature specific data (8 bytes).
+				writer.write_empty_bytes(8)?;
+			}
+			KernelFeatures::Coinbase => {
+				// Write "empty" bytes for fee (8 bytes) and feature specific data (8 bytes).
+				writer.write_empty_bytes(16)?;
+			}
+			KernelFeatures::HeightLocked { fee, lock_height } => {
+				writer.write_u64(*fee)?;
+				// 8 bytes of feature specific data containing the lock height as big-endian u64.
+				writer.write_u64(*lock_height)?;
+			}
+			KernelFeatures::NoRecentDuplicate {
+				fee,
+				relative_height,
+			} => {
+				writer.write_u64(*fee)?;
+
+				// 8 bytes of feature specific data. First 6 bytes are empty.
+				// Last 2 bytes contain the relative lock height as big-endian u16.
+				// Note: This is effectively the same as big-endian u64.
+				// We write "empty" bytes explicitly rather than quietly casting the u16 -> u64.
+				writer.write_empty_bytes(6)?;
+				relative_height.write(writer)?;
+			}
+		};
 		Ok(())
 	}
 
@@ -113,48 +214,75 @@ impl KernelFeatures {
 	/// Only write fee out for feature variants that support it.
 	/// Only write lock_height out for feature variants that support it.
 	fn write_v2<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		writer.write_u8(self.as_u8())?;
 		match self {
 			KernelFeatures::Plain { fee } => {
-				writer.write_u8(self.as_u8())?;
+				// Fee only, no additional data on plain kernels.
 				writer.write_u64(*fee)?;
 			}
 			KernelFeatures::Coinbase => {
-				writer.write_u8(self.as_u8())?;
+				// No additional data.
 			}
 			KernelFeatures::HeightLocked { fee, lock_height } => {
-				writer.write_u8(self.as_u8())?;
 				writer.write_u64(*fee)?;
+				// V2 height locked kernels use 8 bytes for the lock height.
 				writer.write_u64(*lock_height)?;
 			}
+			KernelFeatures::NoRecentDuplicate {
+				fee,
+				relative_height,
+			} => {
+				writer.write_u64(*fee)?;
+				// V2 NRD kernels use 2 bytes for the relative lock height.
+				relative_height.write(writer)?;
+			}
 		}
 		Ok(())
 	}
 
-	// Always read feature byte, 8 bytes for fee and 8 bytes for lock height.
-	// Fee and lock height may be unused for some kernel variants but we need
+	// Always read feature byte, 8 bytes for fee and 8 bytes for additional data
+	// representing lock height or relative height.
+	// Fee and additional data may be unused for some kernel variants but we need
 	// to read these bytes and verify they are 0 if unused.
 	fn read_v1<R: Reader>(reader: &mut R) -> Result<KernelFeatures, ser::Error> {
 		let feature_byte = reader.read_u8()?;
-		let fee = reader.read_u64()?;
-		let lock_height = reader.read_u64()?;
-
 		let features = match feature_byte {
 			KernelFeatures::PLAIN_U8 => {
-				if lock_height != 0 {
-					return Err(ser::Error::CorruptedData);
-				}
+				let fee = reader.read_u64()?;
+				// 8 "empty" bytes as additional data is not used.
+				reader.read_empty_bytes(8)?;
 				KernelFeatures::Plain { fee }
 			}
 			KernelFeatures::COINBASE_U8 => {
-				if fee != 0 {
-					return Err(ser::Error::CorruptedData);
-				}
-				if lock_height != 0 {
-					return Err(ser::Error::CorruptedData);
-				}
+				// 8 "empty" bytes as fee is not used.
+				// 8 "empty" bytes as additional data is not used.
+				reader.read_empty_bytes(16)?;
 				KernelFeatures::Coinbase
 			}
-			KernelFeatures::HEIGHT_LOCKED_U8 => KernelFeatures::HeightLocked { fee, lock_height },
+			KernelFeatures::HEIGHT_LOCKED_U8 => {
+				let fee = reader.read_u64()?;
+				// 8 bytes of feature specific data, lock height as big-endian u64.
+				let lock_height = reader.read_u64()?;
+				KernelFeatures::HeightLocked { fee, lock_height }
+			}
+			KernelFeatures::NO_RECENT_DUPLICATE_U8 => {
+				// NRD kernels are invalid if NRD feature flag is not enabled.
+				if !global::is_nrd_enabled() {
+					return Err(ser::Error::CorruptedData);
+				}
+
+				let fee = reader.read_u64()?;
+
+				// 8 bytes of feature specific data.
+				// The first 6 bytes must be "empty".
+				// The last 2 bytes is the relative height as big-endian u16.
+				reader.read_empty_bytes(6)?;
+				let relative_height = NRDRelativeHeight::read(reader)?;
+				KernelFeatures::NoRecentDuplicate {
+					fee,
+					relative_height,
+				}
+			}
 			_ => {
 				return Err(ser::Error::CorruptedData);
 			}
@@ -176,6 +304,19 @@ impl KernelFeatures {
 				let lock_height = reader.read_u64()?;
 				KernelFeatures::HeightLocked { fee, lock_height }
 			}
+			KernelFeatures::NO_RECENT_DUPLICATE_U8 => {
+				// NRD kernels are invalid if NRD feature flag is not enabled.
+				if !global::is_nrd_enabled() {
+					return Err(ser::Error::CorruptedData);
+				}
+
+				let fee = reader.read_u64()?;
+				let relative_height = NRDRelativeHeight::read(reader)?;
+				KernelFeatures::NoRecentDuplicate {
+					fee,
+					relative_height,
+				}
+			}
 			_ => {
 				return Err(ser::Error::CorruptedData);
 			}
@@ -246,6 +387,8 @@ pub enum Error {
 	/// Validation error relating to kernel features.
 	/// It is invalid for a transaction to contain a coinbase kernel, for example.
 	InvalidKernelFeatures,
+	/// NRD kernel relative height is limited to 1 week duration and must be greater than 0.
+	InvalidNRDRelativeHeight,
 	/// Signature verification error.
 	IncorrectSignature,
 	/// Underlying serialization error.
@@ -383,6 +526,14 @@ impl KernelFeatures {
 			_ => false,
 		}
 	}
+
+	/// Is this an NRD kernel?
+	pub fn is_nrd(&self) -> bool {
+		match self {
+			KernelFeatures::NoRecentDuplicate { .. } => true,
+			_ => false,
+		}
+	}
 }
 
 impl TxKernel {
@@ -401,6 +552,11 @@ impl TxKernel {
 		self.features.is_height_locked()
 	}
 
+	/// Is this an NRD kernel?
+	pub fn is_nrd(&self) -> bool {
+		self.features.is_nrd()
+	}
+
 	/// Return the excess commitment for this tx_kernel.
 	pub fn excess(&self) -> Commitment {
 		self.excess
@@ -422,14 +578,13 @@ impl TxKernel {
 		let sig = &self.excess_sig;
 		// Verify aggsig directly in libsecp
 		let pubkey = &self.excess.to_pubkey(&secp)?;
-		if !secp::aggsig::verify_single(
+		if !aggsig::verify_single(
 			&secp,
 			&sig,
 			&self.msg_to_sign()?,
 			None,
 			&pubkey,
 			Some(&pubkey),
-			None,
 			false,
 		) {
 			return Err(Error::IncorrectSignature);
@@ -440,9 +595,9 @@ impl TxKernel {
 	/// Batch signature verification.
 	pub fn batch_sig_verify(tx_kernels: &[TxKernel]) -> Result<(), Error> {
 		let len = tx_kernels.len();
-		let mut sigs: Vec<secp::Signature> = Vec::with_capacity(len);
-		let mut pubkeys: Vec<secp::key::PublicKey> = Vec::with_capacity(len);
-		let mut msgs: Vec<secp::Message> = Vec::with_capacity(len);
+		let mut sigs = Vec::with_capacity(len);
+		let mut pubkeys = Vec::with_capacity(len);
+		let mut msgs = Vec::with_capacity(len);
 
 		let secp = static_secp_instance();
 		let secp = secp.lock();
@@ -453,7 +608,7 @@ impl TxKernel {
 			msgs.push(tx_kernel.msg_to_sign()?);
 		}
 
-		if !secp::aggsig::verify_batch(&secp, &sigs, &msgs, &pubkeys) {
+		if !aggsig::verify_batch(&secp, &sigs, &msgs, &pubkeys) {
 			return Err(Error::IncorrectSignature);
 		}
 
@@ -668,9 +823,9 @@ impl TransactionBody {
 			.iter()
 			.filter_map(|k| match k.features {
 				KernelFeatures::Coinbase => None,
-				KernelFeatures::Plain { fee } | KernelFeatures::HeightLocked { fee, .. } => {
-					Some(fee)
-				}
+				KernelFeatures::Plain { fee } => Some(fee),
+				KernelFeatures::HeightLocked { fee, .. } => Some(fee),
+				KernelFeatures::NoRecentDuplicate { fee, .. } => Some(fee),
 			})
 			.fold(0, |acc, fee| acc.saturating_add(fee))
 	}
@@ -1577,10 +1732,9 @@ mod test {
 	use crate::core::hash::Hash;
 	use crate::core::id::{ShortId, ShortIdentifiable};
 	use keychain::{ExtKeychain, Keychain, SwitchCommitmentType};
-	use util::secp;
 
 	#[test]
-	fn test_kernel_ser_deser() {
+	fn test_plain_kernel_ser_deser() {
 		let keychain = ExtKeychain::from_random_seed(false).unwrap();
 		let key_id = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
 		let commit = keychain
@@ -1596,12 +1750,35 @@ mod test {
 			excess_sig: sig.clone(),
 		};
 
+		// Test explicit protocol version.
+		for version in vec![ProtocolVersion(1), ProtocolVersion(2)] {
+			let mut vec = vec![];
+			ser::serialize(&mut vec, version, &kernel).expect("serialized failed");
+			let kernel2: TxKernel = ser::deserialize(&mut &vec[..], version).unwrap();
+			assert_eq!(kernel2.features, KernelFeatures::Plain { fee: 10 });
+			assert_eq!(kernel2.excess, commit);
+			assert_eq!(kernel2.excess_sig, sig.clone());
+		}
+
+		// Test with "default" protocol version.
 		let mut vec = vec![];
 		ser::serialize_default(&mut vec, &kernel).expect("serialized failed");
 		let kernel2: TxKernel = ser::deserialize_default(&mut &vec[..]).unwrap();
 		assert_eq!(kernel2.features, KernelFeatures::Plain { fee: 10 });
 		assert_eq!(kernel2.excess, commit);
 		assert_eq!(kernel2.excess_sig, sig.clone());
+	}
+
+	#[test]
+	fn test_height_locked_kernel_ser_deser() {
+		let keychain = ExtKeychain::from_random_seed(false).unwrap();
+		let key_id = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+		let commit = keychain
+			.commit(5, &key_id, SwitchCommitmentType::Regular)
+			.unwrap();
+
+		// just some bytes for testing ser/deser
+		let sig = secp::Signature::from_raw_data(&[0; 64]).unwrap();
 
 		// now check a kernel with lock_height serialize/deserialize correctly
 		let kernel = TxKernel {
@@ -1613,20 +1790,123 @@ mod test {
 			excess_sig: sig.clone(),
 		};
 
+		// Test explicit protocol version.
+		for version in vec![ProtocolVersion(1), ProtocolVersion(2)] {
+			let mut vec = vec![];
+			ser::serialize(&mut vec, version, &kernel).expect("serialized failed");
+			let kernel2: TxKernel = ser::deserialize(&mut &vec[..], version).unwrap();
+			assert_eq!(kernel.features, kernel2.features);
+			assert_eq!(kernel2.excess, commit);
+			assert_eq!(kernel2.excess_sig, sig.clone());
+		}
+
+		// Test with "default" protocol version.
 		let mut vec = vec![];
 		ser::serialize_default(&mut vec, &kernel).expect("serialized failed");
 		let kernel2: TxKernel = ser::deserialize_default(&mut &vec[..]).unwrap();
-		assert_eq!(
-			kernel2.features,
-			KernelFeatures::HeightLocked {
-				fee: 10,
-				lock_height: 100
-			}
-		);
+		assert_eq!(kernel.features, kernel2.features);
 		assert_eq!(kernel2.excess, commit);
 		assert_eq!(kernel2.excess_sig, sig.clone());
 	}
 
+	#[test]
+	fn test_nrd_kernel_ser_deser() {
+		global::set_local_nrd_enabled(true);
+
+		let keychain = ExtKeychain::from_random_seed(false).unwrap();
+		let key_id = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+		let commit = keychain
+			.commit(5, &key_id, SwitchCommitmentType::Regular)
+			.unwrap();
+
+		// just some bytes for testing ser/deser
+		let sig = secp::Signature::from_raw_data(&[0; 64]).unwrap();
+
+		// now check an NRD kernel will serialize/deserialize correctly
+		let kernel = TxKernel {
+			features: KernelFeatures::NoRecentDuplicate {
+				fee: 10,
+				relative_height: NRDRelativeHeight(100),
+			},
+			excess: commit,
+			excess_sig: sig.clone(),
+		};
+
+		// Test explicit protocol version.
+		for version in vec![ProtocolVersion(1), ProtocolVersion(2)] {
+			let mut vec = vec![];
+			ser::serialize(&mut vec, version, &kernel).expect("serialized failed");
+			let kernel2: TxKernel = ser::deserialize(&mut &vec[..], version).unwrap();
+			assert_eq!(kernel.features, kernel2.features);
+			assert_eq!(kernel2.excess, commit);
+			assert_eq!(kernel2.excess_sig, sig.clone());
+		}
+
+		// Test with "default" protocol version.
+		let mut vec = vec![];
+		ser::serialize_default(&mut vec, &kernel).expect("serialized failed");
+		let kernel2: TxKernel = ser::deserialize_default(&mut &vec[..]).unwrap();
+		assert_eq!(kernel.features, kernel2.features);
+		assert_eq!(kernel2.excess, commit);
+		assert_eq!(kernel2.excess_sig, sig.clone());
+	}
+
+	#[test]
+	fn nrd_kernel_verify_sig() {
+		let keychain = ExtKeychain::from_random_seed(false).unwrap();
+		let key_id = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+
+		let mut kernel = TxKernel::with_features(KernelFeatures::NoRecentDuplicate {
+			fee: 10,
+			relative_height: NRDRelativeHeight(100),
+		});
+
+		// Construct the message to be signed.
+		let msg = kernel.msg_to_sign().unwrap();
+
+		let excess = keychain
+			.commit(0, &key_id, SwitchCommitmentType::Regular)
+			.unwrap();
+		let skey = keychain
+			.derive_key(0, &key_id, SwitchCommitmentType::Regular)
+			.unwrap();
+		let pubkey = excess.to_pubkey(&keychain.secp()).unwrap();
+
+		let excess_sig =
+			aggsig::sign_single(&keychain.secp(), &msg, &skey, None, Some(&pubkey)).unwrap();
+
+		kernel.excess = excess;
+		kernel.excess_sig = excess_sig;
+
+		// Check the signature verifies.
+		assert_eq!(kernel.verify(), Ok(()));
+
+		// Modify the fee and check signature no longer verifies.
+		kernel.features = KernelFeatures::NoRecentDuplicate {
+			fee: 9,
+			relative_height: NRDRelativeHeight(100),
+		};
+		assert_eq!(kernel.verify(), Err(Error::IncorrectSignature));
+
+		// Modify the relative_height and check signature no longer verifies.
+		kernel.features = KernelFeatures::NoRecentDuplicate {
+			fee: 10,
+			relative_height: NRDRelativeHeight(101),
+		};
+		assert_eq!(kernel.verify(), Err(Error::IncorrectSignature));
+
+		// Swap the features out for something different and check signature no longer verifies.
+		kernel.features = KernelFeatures::Plain { fee: 10 };
+		assert_eq!(kernel.verify(), Err(Error::IncorrectSignature));
+
+		// Check signature verifies if we use the original features.
+		kernel.features = KernelFeatures::NoRecentDuplicate {
+			fee: 10,
+			relative_height: NRDRelativeHeight(100),
+		};
+		assert_eq!(kernel.verify(), Ok(()));
+	}
+
 	#[test]
 	fn commit_consistency() {
 		let keychain = ExtKeychain::from_seed(&[0; 32], false).unwrap();
@@ -1678,20 +1958,20 @@ mod test {
 	}
 
 	#[test]
-	fn kernel_features_serialization() {
+	fn kernel_features_serialization() -> Result<(), Error> {
 		let mut vec = vec![];
-		ser::serialize_default(&mut vec, &(0u8, 10u64, 0u64)).expect("serialized failed");
-		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..]).unwrap();
+		ser::serialize_default(&mut vec, &(0u8, 10u64, 0u64))?;
+		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..])?;
 		assert_eq!(features, KernelFeatures::Plain { fee: 10 });
 
 		let mut vec = vec![];
-		ser::serialize_default(&mut vec, &(1u8, 0u64, 0u64)).expect("serialized failed");
-		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..]).unwrap();
+		ser::serialize_default(&mut vec, &(1u8, 0u64, 0u64))?;
+		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..])?;
 		assert_eq!(features, KernelFeatures::Coinbase);
 
 		let mut vec = vec![];
-		ser::serialize_default(&mut vec, &(2u8, 10u64, 100u64)).expect("serialized failed");
-		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..]).unwrap();
+		ser::serialize_default(&mut vec, &(2u8, 10u64, 100u64))?;
+		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..])?;
 		assert_eq!(
 			features,
 			KernelFeatures::HeightLocked {
@@ -1700,9 +1980,55 @@ mod test {
 			}
 		);
 
+		// NRD kernel support not enabled by default.
 		let mut vec = vec![];
-		ser::serialize_default(&mut vec, &(3u8, 0u64, 0u64)).expect("serialized failed");
+		ser::serialize_default(&mut vec, &(3u8, 10u64, 100u16)).expect("serialized failed");
 		let res: Result<KernelFeatures, _> = ser::deserialize_default(&mut &vec[..]);
 		assert_eq!(res.err(), Some(ser::Error::CorruptedData));
+
+		// Additional kernel features unsupported.
+		let mut vec = vec![];
+		ser::serialize_default(&mut vec, &(4u8)).expect("serialized failed");
+		let res: Result<KernelFeatures, _> = ser::deserialize_default(&mut &vec[..]);
+		assert_eq!(res.err(), Some(ser::Error::CorruptedData));
+
+		Ok(())
+	}
+
+	#[test]
+	fn kernel_features_serialization_nrd_enabled() -> Result<(), Error> {
+		global::set_local_nrd_enabled(true);
+
+		let mut vec = vec![];
+		ser::serialize_default(&mut vec, &(3u8, 10u64, 100u16))?;
+		let features: KernelFeatures = ser::deserialize_default(&mut &vec[..])?;
+		assert_eq!(
+			features,
+			KernelFeatures::NoRecentDuplicate {
+				fee: 10,
+				relative_height: NRDRelativeHeight(100)
+			}
+		);
+
+		// NRD with relative height 0 is invalid.
+		vec.clear();
+		ser::serialize_default(&mut vec, &(3u8, 10u64, 0u16))?;
+		let res: Result<KernelFeatures, _> = ser::deserialize_default(&mut &vec[..]);
+		assert_eq!(res.err(), Some(ser::Error::CorruptedData));
+
+		// NRD with relative height WEEK_HEIGHT+1 is invalid.
+		vec.clear();
+		let invalid_height = consensus::WEEK_HEIGHT + 1;
+		ser::serialize_default(&mut vec, &(3u8, 10u64, invalid_height as u16))?;
+		let res: Result<KernelFeatures, _> = ser::deserialize_default(&mut &vec[..]);
+		assert_eq!(res.err(), Some(ser::Error::CorruptedData));
+
+		// Kernel variant 4 (and above) is invalid.
+		let mut vec = vec![];
+		ser::serialize_default(&mut vec, &(4u8))?;
+		let res: Result<KernelFeatures, _> = ser::deserialize_default(&mut &vec[..]);
+		assert_eq!(res.err(), Some(ser::Error::CorruptedData));
+
+		Ok(())
 	}
 }

core/src/global.rs

@@ -140,11 +140,19 @@ lazy_static! {
 	/// This is accessed via get_chain_type() which allows the global value
 	/// to be overridden on a per-thread basis (for testing).
 	pub static ref GLOBAL_CHAIN_TYPE: OneTime<ChainTypes> = OneTime::new();
+
+	/// Global feature flag for NRD kernel support.
+	/// If enabled NRD kernels are treated as valid after HF3 (based on header version).
+	/// If disabled NRD kernels are invalid regardless of header version or block height.
+	pub static ref GLOBAL_NRD_FEATURE_ENABLED: OneTime<bool> = OneTime::new();
 }
 
 thread_local! {
 	/// Mainnet|Floonet|UserTesting|AutomatedTesting
 	pub static CHAIN_TYPE: Cell<Option<ChainTypes>> = Cell::new(None);
+
+	/// Local feature flag for NRD kernel support.
+	pub static NRD_FEATURE_ENABLED: Cell<Option<bool>> = Cell::new(None);
 }
 
 /// Set the chain type on a per-thread basis via thread_local storage.
@@ -174,6 +182,36 @@ pub fn init_global_chain_type(new_type: ChainTypes) {
 	GLOBAL_CHAIN_TYPE.init(new_type)
 }
 
+/// One time initialization of the global chain_type.
+/// Will panic if we attempt to re-initialize this (via OneTime).
+pub fn init_global_nrd_enabled(enabled: bool) {
+	GLOBAL_NRD_FEATURE_ENABLED.init(enabled)
+}
+
+/// Explicitly enable the NRD global feature flag.
+pub fn set_local_nrd_enabled(enabled: bool) {
+	NRD_FEATURE_ENABLED.with(|flag| flag.set(Some(enabled)))
+}
+
+/// Is the NRD feature flag enabled?
+/// Look at thread local config first. If not set fallback to global config.
+/// Default to false if global config unset.
+pub fn is_nrd_enabled() -> bool {
+	NRD_FEATURE_ENABLED.with(|flag| match flag.get() {
+		None => {
+			if GLOBAL_NRD_FEATURE_ENABLED.is_init() {
+				let global_flag = GLOBAL_NRD_FEATURE_ENABLED.borrow();
+				flag.set(Some(global_flag));
+				global_flag
+			} else {
+				// Global config unset, default to false.
+				false
+			}
+		}
+		Some(flag) => flag,
+	})
+}
+
 /// Return either a cuckoo context or a cuckatoo context
 /// Single change point
 pub fn create_pow_context<T>(

core/src/libtx/aggsig.rs

@@ -441,6 +441,16 @@ pub fn verify_single(
 	)
 }
 
+/// Verify a batch of signatures.
+pub fn verify_batch(
+	secp: &Secp256k1,
+	sigs: &Vec<Signature>,
+	msgs: &Vec<Message>,
+	pubkeys: &Vec<PublicKey>,
+) -> bool {
+	aggsig::verify_batch(secp, sigs, msgs, pubkeys)
+}
+
 /// Just a simple sig, creates its own nonce, etc
 pub fn sign_with_blinding(
 	secp: &Secp256k1,
@@ -449,7 +459,6 @@ pub fn sign_with_blinding(
 	pubkey_sum: Option<&PublicKey>,
 ) -> Result<Signature, Error> {
 	let skey = &blinding.secret_key(&secp)?;
-	//let pubkey_sum = PublicKey::from_secret_key(&secp, &skey)?;
 	let sig = aggsig::sign_single(secp, &msg, skey, None, None, None, pubkey_sum, None)?;
 	Ok(sig)
 }

core/src/ser.rs

@@ -186,6 +186,11 @@ pub trait Writer {
 
 	/// Writes a fixed number of bytes. The reader is expected to know the actual length on read.
 	fn write_fixed_bytes<T: AsRef<[u8]>>(&mut self, bytes: T) -> Result<(), Error>;
+
+	/// Writes a fixed length of "empty" bytes.
+	fn write_empty_bytes(&mut self, length: usize) -> Result<(), Error> {
+		self.write_fixed_bytes(vec![0u8; length])
+	}
 }
 
 /// Implementations defined how different numbers and binary structures are
@@ -213,6 +218,17 @@ pub trait Reader {
 	/// Access to underlying protocol version to support
 	/// version specific deserialization logic.
 	fn protocol_version(&self) -> ProtocolVersion;
+
+	/// Read a fixed number of "empty" bytes from the underlying reader.
+	/// It is an error if any non-empty bytes encountered.
+	fn read_empty_bytes(&mut self, length: usize) -> Result<(), Error> {
+		for _ in 0..length {
+			if self.read_u8()? != 0u8 {
+				return Err(Error::CorruptedData);
+			}
+		}
+		Ok(())
+	}
 }
 
 /// Trait that every type that can be serialized as binary must implement.

core/tests/block.rs

@@ -14,16 +14,15 @@
 
 mod common;
 use crate::common::{new_block, tx1i2o, tx2i1o, txspend1i1o};
-use crate::core::consensus::BLOCK_OUTPUT_WEIGHT;
-use crate::core::core::block::Error;
+use crate::core::consensus::{self, BLOCK_OUTPUT_WEIGHT, TESTING_THIRD_HARD_FORK};
+use crate::core::core::block::{Block, BlockHeader, Error, HeaderVersion};
 use crate::core::core::hash::Hashed;
 use crate::core::core::id::ShortIdentifiable;
-use crate::core::core::transaction::{self, Transaction};
-use crate::core::core::verifier_cache::{LruVerifierCache, VerifierCache};
-use crate::core::core::Committed;
-use crate::core::core::{
-	Block, BlockHeader, CompactBlock, HeaderVersion, KernelFeatures, OutputFeatures,
+use crate::core::core::transaction::{
+	self, KernelFeatures, NRDRelativeHeight, OutputFeatures, Transaction,
 };
+use crate::core::core::verifier_cache::{LruVerifierCache, VerifierCache};
+use crate::core::core::{Committed, CompactBlock};
 use crate::core::libtx::build::{self, input, output};
 use crate::core::libtx::ProofBuilder;
 use crate::core::{global, ser};
@@ -84,6 +83,178 @@ fn very_empty_block() {
 	);
 }
 
+#[test]
+fn block_with_nrd_kernel_pre_post_hf3() {
+	// automated testing - HF{1|2|3} at block heights {3, 6, 9}
+	// Enable the global NRD feature flag. NRD kernels valid at HF3 at height 9.
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+	global::set_local_nrd_enabled(true);
+
+	let keychain = ExtKeychain::from_random_seed(false).unwrap();
+	let builder = ProofBuilder::new(&keychain);
+	let key_id1 = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+	let key_id2 = ExtKeychain::derive_key_id(1, 2, 0, 0, 0);
+
+	let mut tx = build::transaction(
+		KernelFeatures::NoRecentDuplicate {
+			fee: 2,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+		vec![input(7, key_id1), output(5, key_id2)],
+		&keychain,
+		&builder,
+	)
+	.unwrap();
+
+	let prev_height = TESTING_THIRD_HARD_FORK - 2;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is invalid at header version 3 if it contains an NRD kernel.
+	assert_eq!(b.header.version, HeaderVersion(3));
+	assert_eq!(
+		b.validate(&BlindingFactor::zero(), verifier_cache()),
+		Err(Error::NRDKernelPreHF3)
+	);
+
+	let prev_height = TESTING_THIRD_HARD_FORK - 1;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is valid at header version 4 (at HF height) if it contains an NRD kernel.
+	assert_eq!(b.header.height, TESTING_THIRD_HARD_FORK);
+	assert_eq!(b.header.version, HeaderVersion(4));
+	assert!(b
+		.validate(&BlindingFactor::zero(), verifier_cache())
+		.is_ok());
+
+	let prev_height = TESTING_THIRD_HARD_FORK;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is valid at header version 4 if it contains an NRD kernel.
+	assert_eq!(b.header.version, HeaderVersion(4));
+	assert!(b
+		.validate(&BlindingFactor::zero(), verifier_cache())
+		.is_ok());
+}
+
+#[test]
+fn block_with_nrd_kernel_nrd_not_enabled() {
+	// automated testing - HF{1|2|3} at block heights {3, 6, 9}
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+
+	let keychain = ExtKeychain::from_random_seed(false).unwrap();
+	let builder = ProofBuilder::new(&keychain);
+	let key_id1 = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+	let key_id2 = ExtKeychain::derive_key_id(1, 2, 0, 0, 0);
+
+	let mut tx = build::transaction(
+		KernelFeatures::NoRecentDuplicate {
+			fee: 2,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+		vec![input(7, key_id1), output(5, key_id2)],
+		&keychain,
+		&builder,
+	)
+	.unwrap();
+
+	let prev_height = TESTING_THIRD_HARD_FORK - 2;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is invalid as NRD not enabled.
+	assert_eq!(b.header.version, HeaderVersion(3));
+	assert_eq!(
+		b.validate(&BlindingFactor::zero(), verifier_cache()),
+		Err(Error::NRDKernelNotEnabled)
+	);
+
+	let prev_height = TESTING_THIRD_HARD_FORK - 1;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is invalid as NRD not enabled.
+	assert_eq!(b.header.height, TESTING_THIRD_HARD_FORK);
+	assert_eq!(b.header.version, HeaderVersion(4));
+	assert_eq!(
+		b.validate(&BlindingFactor::zero(), verifier_cache()),
+		Err(Error::NRDKernelNotEnabled)
+	);
+
+	let prev_height = TESTING_THIRD_HARD_FORK;
+	let prev = BlockHeader {
+		height: prev_height,
+		version: consensus::header_version(prev_height),
+		..BlockHeader::default()
+	};
+	let b = new_block(
+		vec![&mut tx],
+		&keychain,
+		&builder,
+		&prev,
+		&ExtKeychain::derive_key_id(1, 1, 0, 0, 0),
+	);
+
+	// Block is invalid as NRD not enabled.
+	assert_eq!(b.header.version, HeaderVersion(4));
+	assert_eq!(
+		b.validate(&BlindingFactor::zero(), verifier_cache()),
+		Err(Error::NRDKernelNotEnabled)
+	);
+}
+
 #[test]
 // builds a block with a tx spending another and check that cut_through occurred
 fn block_with_cut_through() {

pool/src/transaction_pool.rs

@@ -20,7 +20,8 @@
 use self::core::core::hash::{Hash, Hashed};
 use self::core::core::id::ShortId;
 use self::core::core::verifier_cache::VerifierCache;
-use self::core::core::{transaction, Block, BlockHeader, Transaction, Weighting};
+use self::core::core::{transaction, Block, BlockHeader, HeaderVersion, Transaction, Weighting};
+use self::core::global;
 use self::util::RwLock;
 use crate::pool::Pool;
 use crate::types::{BlockChain, PoolAdapter, PoolConfig, PoolEntry, PoolError, TxSource};
@@ -132,6 +133,24 @@ where
 		Ok(())
 	}
 
+	/// Verify the tx kernel variants and ensure they can all be accepted to the txpool/stempool
+	/// with respect to current header version.
+	fn verify_kernel_variants(
+		&self,
+		tx: &Transaction,
+		header: &BlockHeader,
+	) -> Result<(), PoolError> {
+		if tx.kernels().iter().any(|k| k.is_nrd()) {
+			if !global::is_nrd_enabled() {
+				return Err(PoolError::NRDKernelNotEnabled);
+			}
+			if header.version < HeaderVersion(4) {
+				return Err(PoolError::NRDKernelPreHF3);
+			}
+		}
+		Ok(())
+	}
+
 	/// Add the given tx to the pool, directing it to either the stempool or
 	/// txpool based on stem flag provided.
 	pub fn add_to_pool(
@@ -147,6 +166,9 @@ where
 			return Err(PoolError::DuplicateTx);
 		}
 
+		// Check this tx is valid based on current header version.
+		self.verify_kernel_variants(&tx, header)?;
+
 		// Do we have the capacity to accept this transaction?
 		let acceptability = self.is_acceptable(&tx, stem);
 		let mut evict = false;

pool/src/types.rs

@@ -221,6 +221,12 @@ pub enum PoolError {
 	/// Attempt to add a duplicate tx to the pool.
 	#[fail(display = "Duplicate tx")]
 	DuplicateTx,
+	/// NRD kernels will not be accepted by the txpool/stempool pre-HF3.
+	#[fail(display = "NRD kernel pre-HF3")]
+	NRDKernelPreHF3,
+	/// NRD kernels are not valid if disabled locally via "feature flag".
+	#[fail(display = "NRD kernel not enabled")]
+	NRDKernelNotEnabled,
 	/// Other kinds of error (not yet pulled out into meaningful errors).
 	#[fail(display = "General pool error {}", _0)]
 	Other(String),

pool/tests/common.rs

@@ -216,10 +216,26 @@ where
 {
 	let input_sum = input_values.iter().sum::<u64>() as i64;
 	let output_sum = output_values.iter().sum::<u64>() as i64;
-
 	let fees: i64 = input_sum - output_sum;
 	assert!(fees >= 0);
 
+	test_transaction_with_kernel_features(
+		keychain,
+		input_values,
+		output_values,
+		KernelFeatures::Plain { fee: fees as u64 },
+	)
+}
+
+pub fn test_transaction_with_kernel_features<K>(
+	keychain: &K,
+	input_values: Vec<u64>,
+	output_values: Vec<u64>,
+	kernel_features: KernelFeatures,
+) -> Transaction
+where
+	K: Keychain,
+{
 	let mut tx_elements = Vec::new();
 
 	for input_value in input_values {
@@ -233,7 +249,7 @@ where
 	}
 
 	libtx::build::transaction(
-		KernelFeatures::Plain { fee: fees as u64 },
+		kernel_features,
 		tx_elements,
 		keychain,
 		&libtx::ProofBuilder::new(keychain),

pool/tests/nrd_kernels.rs

@@ -0,0 +1,216 @@
+// Copyright 2020 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+pub mod common;
+
+use self::core::core::hash::Hashed;
+use self::core::core::verifier_cache::LruVerifierCache;
+use self::core::core::{
+	Block, BlockHeader, HeaderVersion, KernelFeatures, NRDRelativeHeight, Transaction,
+};
+use self::core::global;
+use self::core::pow::Difficulty;
+use self::core::{consensus, libtx};
+use self::keychain::{ExtKeychain, Keychain};
+use self::pool::types::PoolError;
+use self::util::RwLock;
+use crate::common::*;
+use grin_core as core;
+use grin_keychain as keychain;
+use grin_pool as pool;
+use grin_util as util;
+use std::sync::Arc;
+
+#[test]
+fn test_nrd_kernel_verification_block_version() {
+	util::init_test_logger();
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+	global::set_local_nrd_enabled(true);
+
+	let keychain: ExtKeychain = Keychain::from_random_seed(false).unwrap();
+
+	let db_root = ".grin_nrd_kernels";
+	clean_output_dir(db_root.into());
+
+	let mut chain = ChainAdapter::init(db_root.into()).unwrap();
+
+	let verifier_cache = Arc::new(RwLock::new(LruVerifierCache::new()));
+
+	// Initialize the chain/txhashset with an initial block
+	// so we have a non-empty UTXO set.
+	let add_block = |prev_header: BlockHeader, txs: Vec<Transaction>, chain: &mut ChainAdapter| {
+		let height = prev_header.height + 1;
+		let key_id = ExtKeychain::derive_key_id(1, height as u32, 0, 0, 0);
+		let fee = txs.iter().map(|x| x.fee()).sum();
+		let reward = libtx::reward::output(
+			&keychain,
+			&libtx::ProofBuilder::new(&keychain),
+			&key_id,
+			fee,
+			false,
+		)
+		.unwrap();
+		let mut block = Block::new(&prev_header, txs, Difficulty::min(), reward).unwrap();
+
+		// Set the prev_root to the prev hash for testing purposes (no MMR to obtain a root from).
+		block.header.prev_root = prev_header.hash();
+
+		chain.update_db_for_block(&block);
+		block
+	};
+
+	let block = add_block(BlockHeader::default(), vec![], &mut chain);
+	let header = block.header;
+
+	// Now create tx to spend that first coinbase (now matured).
+	// Provides us with some useful outputs to test with.
+	let initial_tx = test_transaction_spending_coinbase(&keychain, &header, vec![10, 20, 30, 40]);
+
+	// Mine that initial tx so we can spend it with multiple txs
+	let mut block = add_block(header, vec![initial_tx], &mut chain);
+	let mut header = block.header;
+
+	// Initialize a new pool with our chain adapter.
+	let mut pool = test_setup(Arc::new(chain.clone()), verifier_cache);
+
+	let tx_1 = test_transaction_with_kernel_features(
+		&keychain,
+		vec![10, 20],
+		vec![24],
+		KernelFeatures::NoRecentDuplicate {
+			fee: 6,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+	);
+
+	assert!(header.version < HeaderVersion(4));
+
+	assert_eq!(
+		pool.add_to_pool(test_source(), tx_1.clone(), false, &header),
+		Err(PoolError::NRDKernelPreHF3)
+	);
+
+	// Now mine several more blocks out to HF3
+	for _ in 0..7 {
+		block = add_block(header, vec![], &mut chain);
+		header = block.header;
+	}
+	assert_eq!(header.height, consensus::TESTING_THIRD_HARD_FORK);
+	assert_eq!(header.version, HeaderVersion(4));
+
+	// Now confirm we can successfully add transaction with NRD kernel to txpool.
+	assert_eq!(
+		pool.add_to_pool(test_source(), tx_1.clone(), false, &header),
+		Ok(()),
+	);
+
+	assert_eq!(pool.total_size(), 1);
+
+	let txs = pool.prepare_mineable_transactions().unwrap();
+	assert_eq!(txs.len(), 1);
+
+	// Cleanup db directory
+	clean_output_dir(db_root.into());
+}
+
+#[test]
+fn test_nrd_kernel_verification_nrd_disabled() {
+	util::init_test_logger();
+	global::set_local_chain_type(global::ChainTypes::AutomatedTesting);
+
+	let keychain: ExtKeychain = Keychain::from_random_seed(false).unwrap();
+
+	let db_root = ".grin_nrd_kernel_disabled";
+	clean_output_dir(db_root.into());
+
+	let mut chain = ChainAdapter::init(db_root.into()).unwrap();
+
+	let verifier_cache = Arc::new(RwLock::new(LruVerifierCache::new()));
+
+	// Initialize the chain/txhashset with an initial block
+	// so we have a non-empty UTXO set.
+	let add_block = |prev_header: BlockHeader, txs: Vec<Transaction>, chain: &mut ChainAdapter| {
+		let height = prev_header.height + 1;
+		let key_id = ExtKeychain::derive_key_id(1, height as u32, 0, 0, 0);
+		let fee = txs.iter().map(|x| x.fee()).sum();
+		let reward = libtx::reward::output(
+			&keychain,
+			&libtx::ProofBuilder::new(&keychain),
+			&key_id,
+			fee,
+			false,
+		)
+		.unwrap();
+		let mut block = Block::new(&prev_header, txs, Difficulty::min(), reward).unwrap();
+
+		// Set the prev_root to the prev hash for testing purposes (no MMR to obtain a root from).
+		block.header.prev_root = prev_header.hash();
+
+		chain.update_db_for_block(&block);
+		block
+	};
+
+	let block = add_block(BlockHeader::default(), vec![], &mut chain);
+	let header = block.header;
+
+	// Now create tx to spend that first coinbase (now matured).
+	// Provides us with some useful outputs to test with.
+	let initial_tx = test_transaction_spending_coinbase(&keychain, &header, vec![10, 20, 30, 40]);
+
+	// Mine that initial tx so we can spend it with multiple txs
+	let mut block = add_block(header, vec![initial_tx], &mut chain);
+	let mut header = block.header;
+
+	// Initialize a new pool with our chain adapter.
+	let mut pool = test_setup(Arc::new(chain.clone()), verifier_cache);
+
+	let tx_1 = test_transaction_with_kernel_features(
+		&keychain,
+		vec![10, 20],
+		vec![24],
+		KernelFeatures::NoRecentDuplicate {
+			fee: 6,
+			relative_height: NRDRelativeHeight::new(1440).unwrap(),
+		},
+	);
+
+	assert!(header.version < HeaderVersion(4));
+
+	assert_eq!(
+		pool.add_to_pool(test_source(), tx_1.clone(), false, &header),
+		Err(PoolError::NRDKernelNotEnabled)
+	);
+
+	// Now mine several more blocks out to HF3
+	for _ in 0..7 {
+		block = add_block(header, vec![], &mut chain);
+		header = block.header;
+	}
+	assert_eq!(header.height, consensus::TESTING_THIRD_HARD_FORK);
+	assert_eq!(header.version, HeaderVersion(4));
+
+	// NRD kernel support not enabled via feature flag, so not valid.
+	assert_eq!(
+		pool.add_to_pool(test_source(), tx_1.clone(), false, &header),
+		Err(PoolError::NRDKernelNotEnabled)
+	);
+
+	assert_eq!(pool.total_size(), 0);
+
+	let txs = pool.prepare_mineable_transactions().unwrap();
+	assert_eq!(txs.len(), 0);
+
+	// Cleanup db directory
+	clean_output_dir(db_root.into());
+}

src/bin/grin.rs

@@ -64,6 +64,10 @@ fn log_build_info() {
 	debug!("{}", detailed_info);
 }
 
+fn log_feature_flags() {
+	info!("Feature: NRD kernel enabled: {}", global::is_nrd_enabled());
+}
+
 fn main() {
 	let exit_code = real_main();
 	std::process::exit(exit_code);
@@ -140,9 +144,6 @@ fn real_main() -> i32 {
 	};
 	init_logger(Some(logging_config), logs_tx);
 
-	// One time initialization of the global chain_type.
-	global::init_global_chain_type(config.members.unwrap().server.chain_type);
-
 	if let Some(file_path) = &config.config_file_path {
 		info!(
 			"Using configuration file at {}",
@@ -154,6 +155,22 @@ fn real_main() -> i32 {
 
 	log_build_info();
 
+	// Initialize our global chain_type and feature flags (NRD kernel support currently).
+	// These are read via global and not read from config beyond this point.
+	global::init_global_chain_type(config.members.unwrap().server.chain_type);
+	info!("Chain: {:?}", global::get_chain_type());
+	match global::get_chain_type() {
+		global::ChainTypes::Mainnet => {
+			// Set various mainnet specific feature flags.
+			global::init_global_nrd_enabled(false);
+		}
+		_ => {
+			// Set various non-mainnet feature flags.
+			global::init_global_nrd_enabled(true);
+		}
+	}
+	log_feature_flags();
+
 	// Execute subcommand
 	match args.subcommand() {
 		// server commands and options