Implement native Notary contract

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 +1,71 @@
+// 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)
+        {
+            return tx.Signers.Any(p => p.Account.Equals(NativeContract.Notary.Hash));
+        }
+
+        /// <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 uses notary request service with <see cref="NotaryAssisted.NKeys" number of keys/>.
+        /// </summary>
+        [ReflectionCache(typeof(NotaryAssisted))]
+        NotaryAssisted = 0x22
     }
 }

src/Neo/SmartContract/Native/GasToken.cs

@@ -43,6 +43,14 @@ internal override async ContractTask OnPersist(ApplicationEngine engine)
             {
                 await Burn(engine, tx.Sender, tx.SystemFee + tx.NetworkFee);
                 totalNetworkFee += tx.NetworkFee;
+
+                // Reward for NotaryAssisted attribute will be minted to designated notary nodes
+                // by Notary contract.
+                var notaryAssisted = tx.GetAttribute<NotaryAssisted>();
+                if (notaryAssisted is not null)
+                {
+                    totalNetworkFee -= (notaryAssisted.NKeys + 1) * Policy.GetAttributeFee(engine.Snapshot, (byte)notaryAssisted.Type);
+                }
             }
             ECPoint[] validators = NEO.GetNextBlockValidators(engine.Snapshot, engine.ProtocolSettings.ValidatorsCount);
             UInt160 primary = Contract.CreateSignatureRedeemScript(validators[engine.PersistingBlock.PrimaryIndex]).ToScriptHash();

src/Neo/SmartContract/Native/NativeContract.cs

@@ -103,6 +103,11 @@ public CacheEntry GetAllowedMethods(NativeContract native, ApplicationEngine eng
         /// </summary>
         public static OracleContract Oracle { get; } = new();
 
+        /// <summary>
+        /// Gets the instance of the <see cref="Notary"/> class.
+        /// </summary>
+        public static Notary Notary { get; } = new();
+
         #endregion
 
         /// <summary>