Support changing password for nep6 wallet

src/neo/Wallets/NEP6/NEP6Account.cs

@@ -6,7 +6,7 @@ namespace Neo.Wallets.NEP6
     internal class NEP6Account : WalletAccount
     {
         private readonly NEP6Wallet wallet;
-        private readonly string nep2key;
+        private string nep2key;
         private KeyPair key;
         public JObject Extra;
 
@@ -83,5 +83,30 @@ public bool VerifyPassword(string password)
                 return false;
             }
         }
+
+        public bool ChangePassword(string password_old, string password_new)
+        {
+            if (WatchOnly) return true;
+            if (nep2key == null)
+            {
+                if (key == null)
+                {
+                    return true;
+                }
+            }
+            else
+            {
+                if (!VerifyPassword(password_old))
+                {
+                    return false;
+                }
+                else if (key == null)
+                {
+                    key = wallet.DecryptKey(nep2key);
+                }
+            }
+            nep2key = key.Export(password_new, wallet.Scrypt.N, wallet.Scrypt.R, wallet.Scrypt.P);
+            return true;
+        }
     }
 }

src/neo/Wallets/NEP6/NEP6Wallet.cs

@@ -300,5 +300,21 @@ public override bool VerifyPassword(string password)
                 }
             }
         }
+
+        public bool ChangePassword(string password_old, string password_new)
+        {
+            lock (accounts)
+            {
+                foreach (NEP6Account account in accounts.Values)
+                {
+                    if (!account.ChangePassword(password_old, password_new)) return false;
+                }
+            }
+            if (password != null)
+            {
+                password = password_new;
+            }
+            return true;
+        }
     }
 }

tests/neo.UnitTests/Wallets/NEP6/UT_NEP6Account.cs

@@ -35,6 +35,20 @@ public void TestSetup()
             _account = new NEP6Account(_wallet, _hash);
         }
 
+        [TestMethod]
+        public void TestChangePassword()
+        {
+            _account = new NEP6Account(_wallet, _hash, _nep2);
+            _account.ChangePassword("b", "Satoshi").Should().BeTrue();
+            _account.Contract = new Contract();
+            _wallet.Unlock("Satoshi");
+            _account.ChangePassword("b", "Satoshi").Should().BeFalse();
+            _account.ChangePassword("Satoshi", "b").Should().BeTrue();
+            _account.VerifyPassword("b").Should().BeTrue();
+            _account.ChangePassword("b", "Satoshi").Should().BeTrue();
+            _wallet.Lock();
+        }
+
         [TestMethod]
         public void TestConstructorWithNep2Key()
         {

tests/neo.UnitTests/Wallets/NEP6/UT_NEP6Wallet.cs

@@ -70,6 +70,18 @@ public void TestCleanUp()
             if (Directory.Exists(rootPath)) Directory.Delete(rootPath);
         }
 
+        [TestMethod]
+        public void TestChangePassword()
+        {
+            uut.Unlock("123");
+            uut.CreateAccount(keyPair.PrivateKey);
+            uut.ChangePassword("456", "123").Should().BeFalse();
+            uut.ChangePassword("123", "456").Should().BeTrue();
+            uut.VerifyPassword("456").Should().BeTrue();
+            uut.ChangePassword("456", "123").Should().BeTrue();
+            uut.Lock();
+        }
+
         [TestMethod]
         public void TestConstructorWithPathAndName()
         {