Implement NotaryAssisted transaction attribute

src/Neo.CLI/CLI/MainService.Blockchain.cs

@@ -212,6 +212,10 @@ public void OnShowTransactionCommand(UInt256 hash)
                                 ConsoleHelper.Info("", "    Type: ", $"{n.Type}");
                                 ConsoleHelper.Info("", "  Height: ", $"{n.Height}");
                                 break;
+                            case NotaryAssisted n:
+                                ConsoleHelper.Info("", "    Type: ", $"{n.Type}");
+                                ConsoleHelper.Info("", "   NKeys: ", $"{n.NKeys}");
+                                break;
                             default:
                                 ConsoleHelper.Info("", "  Type: ", $"{attribute.Type}");
                                 ConsoleHelper.Info("", "  Size: ", $"{attribute.Size} Byte(s)");

src/Neo/Network/P2P/Payloads/NotaryAssisted.cs

@@ -0,0 +1,73 @@
+// Copyright (C) 2015-2024 The Neo Project.
+//
+// NotaryAssisted.cs file belongs to the neo project and is free
+// software distributed under the MIT software license, see the
+// accompanying file LICENSE in the main directory of the
+// repository or http://www.opensource.org/licenses/mit-license.php
+// for more details.
+//
+// Redistribution and use in source and binary forms with or without
+// modifications are permitted.
+
+using Neo.IO;
+using Neo.Json;
+using Neo.Persistence;
+using Neo.SmartContract.Native;
+using System.IO;
+using System.Linq;
+
+namespace Neo.Network.P2P.Payloads
+{
+    public class NotaryAssisted : TransactionAttribute
+    {
+        /// <summary>
+        /// Indicates the number of keys participating in the transaction (main or fallback) signing process.
+        /// </summary>
+        public byte NKeys;
+
+        public override TransactionAttributeType Type => TransactionAttributeType.NotaryAssisted;
+
+        public override bool AllowMultiple => false;
+
+        public override int Size => base.Size + sizeof(byte);
+
+        protected override void DeserializeWithoutType(ref MemoryReader reader)
+        {
+            NKeys = reader.ReadByte();
+        }
+
+        protected override void SerializeWithoutType(BinaryWriter writer)
+        {
+            writer.Write(NKeys);
+        }
+
+        public override JObject ToJson()
+        {
+            JObject json = base.ToJson();
+            json["nkeys"] = NKeys;
+            return json;
+        }
+
+        public override bool Verify(DataCache snapshot, Transaction tx)
+        {
+            // Stub native Notary contract related check until the contract is implemented.
+            UInt160 notaryH = new UInt160();
+            return tx.Signers.Any(p => p.Account.Equals(notaryH));
+        }
+
+        /// <summary>
+        /// Calculates the network fee needed to pay for NotaryAssisted attribute. According to the
+        /// https://github.com/neo-project/neo/issues/1573#issuecomment-704874472, network fee consists of
+        /// the base Notary service fee per key multiplied by the expected number of transactions that should
+        /// be collected by the service to complete Notary request increased by one (for Notary node witness
+        /// itself).
+        /// </summary>
+        /// <param name="snapshot">The snapshot used to read data.</param>
+        /// <param name="tx">The transaction to calculate.</param>
+        /// <returns>The network fee of the NotaryAssisted attribute.</returns>
+        public override long CalculateNetworkFee(DataCache snapshot, Transaction tx)
+        {
+            return (NKeys + 1) * base.CalculateNetworkFee(snapshot, tx);
+        }
+    }
+}

src/Neo/Network/P2P/Payloads/TransactionAttributeType.cs

@@ -40,6 +40,12 @@
         /// Indicates that the transaction conflicts with <see cref="Conflicts.Hash"/>.
         /// </summary>
         [ReflectionCache(typeof(Conflicts))]
-        Conflicts = 0x21
+        Conflicts = 0x21,
+
+        /// <summary>
+        /// Indicates that the transaction is aimed to service notary request with <see cref="NotaryAssisted.NKeys" number of keys/>.
+        /// </summary>
+        [ReflectionCache(typeof(NotaryAssisted))]
+        NotaryAssisted = 0x22
     }
 }

src/Neo/SmartContract/Native/PolicyContract.cs

@@ -43,6 +43,11 @@ public sealed class PolicyContract : NativeContract
         /// </summary>
         public const uint DefaultAttributeFee = 0;
 
+        /// <summary>
+        /// The default fee for NotaryAssisted attribute.
+        /// </summary>
+        public const uint DefaultNotaryAssistedAttributeFee = 1000_0000;
+
         /// <summary>
         /// The maximum execution fee factor that the committee can set.
         /// </summary>
@@ -73,6 +78,7 @@ internal override ContractTask Initialize(ApplicationEngine engine, Hardfork? ha
                 engine.Snapshot.Add(CreateStorageKey(Prefix_FeePerByte), new StorageItem(DefaultFeePerByte));
                 engine.Snapshot.Add(CreateStorageKey(Prefix_ExecFeeFactor), new StorageItem(DefaultExecFeeFactor));
                 engine.Snapshot.Add(CreateStorageKey(Prefix_StoragePrice), new StorageItem(DefaultStoragePrice));
+                engine.Snapshot.Add(CreateStorageKey(Prefix_AttributeFee).Add((byte)TransactionAttributeType.NotaryAssisted), new StorageItem(DefaultNotaryAssistedAttributeFee));
             }
             return ContractTask.CompletedTask;
         }

tests/Neo.UnitTests/Network/P2P/Payloads/UT_NotaryAssisted.cs

@@ -0,0 +1,88 @@
+// Copyright (C) 2015-2024 The Neo Project.
+//
+// UT_NotaryAssisted.cs file belongs to the neo project and is free
+// software distributed under the MIT software license, see the
+// accompanying file LICENSE in the main directory of the
+// repository or http://www.opensource.org/licenses/mit-license.php
+// for more details.
+//
+// Redistribution and use in source and binary forms with or without
+// modifications are permitted.
+
+using FluentAssertions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Neo.IO;
+using Neo.Network.P2P.Payloads;
+using Neo.SmartContract;
+using Neo.SmartContract.Native;
+using Neo.VM;
+using System;
+
+namespace Neo.UnitTests.Network.P2P.Payloads
+{
+    [TestClass]
+    public class UT_NotaryAssisted
+    {
+        [TestMethod]
+        public void Size_Get()
+        {
+            var attr = new NotaryAssisted() { NKeys = 4 };
+            attr.Size.Should().Be(1 + 1);
+        }
+
+        [TestMethod]
+        public void ToJson()
+        {
+            var attr = new NotaryAssisted() { NKeys = 4 };
+            var json = attr.ToJson().ToString();
+            Assert.AreEqual(@"{""type"":""NotaryAssisted"",""nkeys"":4}", json);
+        }
+
+        [TestMethod]
+        public void DeserializeAndSerialize()
+        {
+            var attr = new NotaryAssisted() { NKeys = 4 };
+
+            var clone = attr.ToArray().AsSerializable<NotaryAssisted>();
+            Assert.AreEqual(clone.Type, attr.Type);
+
+            // As transactionAttribute
+            byte[] buffer = attr.ToArray();
+            var reader = new MemoryReader(buffer);
+            clone = TransactionAttribute.DeserializeFrom(ref reader) as NotaryAssisted;
+            Assert.AreEqual(clone.Type, attr.Type);
+
+            // Wrong type
+            buffer[0] = 0xff;
+            Assert.ThrowsException<FormatException>(() =>
+            {
+                var reader = new MemoryReader(buffer);
+                TransactionAttribute.DeserializeFrom(ref reader);
+            });
+        }
+
+        [TestMethod]
+        public void Verify()
+        {
+            var attr = new NotaryAssisted() { NKeys = 4 };
+
+            // Temporary use Notary contract hash stub for valid transaction.
+            var txGood = new Transaction { Signers = new Signer[] { new Signer() { Account = UInt160.Zero } } };
+            var txBad = new Transaction { Signers = new Signer[] { new Signer() { Account = UInt160.Parse("0xa400ff00ff00ff00ff00ff00ff00ff00ff00ff01") } } };
+            var snapshot = TestBlockchain.GetTestSnapshot();
+
+            Assert.IsTrue(attr.Verify(snapshot, txGood));
+            Assert.IsFalse(attr.Verify(snapshot, txBad));
+        }
+
+        [TestMethod]
+        public void CalculateNetworkFee()
+        {
+            var snapshot = TestBlockchain.GetTestSnapshot();
+            var attr = new NotaryAssisted() { NKeys = 4 };
+            var tx = new Transaction { Signers = new Signer[] { new Signer() { Account = UInt160.Zero } } };
+
+            Assert.AreEqual((4 + 1) * 1000_0000, attr.CalculateNetworkFee(snapshot, tx));
+        }
+    }
+}