How to Give Error Message if Decrypt value change manually

Question

i have a password Encryptor and  Decrypt if any one person change Encrptor value mannualy example Actual value is(W18cJ8//VPFBwPsSo4kxjQ==) Modified Value Is(W18cJ8//VPFBwPsSo41234==) now Decryptor show error msg .so i need to show error msg in lable.Codes Are Below

--------------------------------------------------------------------------------------------------------------------

private string Decrypt(string cipherText)

{

string EncryptionKey = "Pothisam";

byte[] cipherBytes = Convert.FromBase64String(cipherText);

using (Aes encryptor = Aes.Create())

{

Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });

encryptor.Key = pdb.GetBytes(32);

encryptor.IV = pdb.GetBytes(16);

using (MemoryStream ms = new MemoryStream())

{

using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))

{

cs.Write(cipherBytes, 0, cipherBytes.Length);

cs.Close();

}

cipherText = Encoding.Unicode.GetString(ms.ToArray());

}

}

return cipherText;

}

 -------------------------------------------------------------------------------------------

private string Encrypt(string clearText)

{

string EncryptionKey = "Pothisam";

byte[] clearBytes = Encoding.Unicode.GetBytes(clearText);

using (Aes encryptor = Aes.Create())

{

Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });

encryptor.Key = pdb.GetBytes(32);

encryptor.IV = pdb.GetBytes(16);

using (MemoryStream ms = new MemoryStream())

{

using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))

{

cs.Write(clearBytes, 0, clearBytes.Length);

cs.Close();

}

clearText = Convert.ToBase64String(ms.ToArray());

}

}

return clearText;

}

 -------------------------------------------------------------------------

now showing error msg like this

 

Exception Details: System.Security.Cryptography.CryptographicException: Padding is invalid and cannot be removed.

Source Error:

Line 47:                     cs.Write(cipherBytes, 0, cipherBytes.Length);

 Line 48:                     cs.Close(); Line 49:               

                                } 

Line 50:                 cipherText = Encoding.Unicode.GetString(ms.ToArray()); 

Line 51:             }

 Please Any One Help Me