Rijndael 加密/解密

我有以下用于加密和解密的代码。问题是，在解密时，除了解密的文本之外，我在文本后面还有一些"aaaaa"。为什么？需要一些帮助。感谢！

public static byte[] Encrypt(byte[] PlainTextBytes, string key , string InitialVector)
        {
            try
            {
                System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
               Byte[] KeyBytes = encoding.GetBytes(key);
                byte[] InitialVectorBytes = encoding.GetBytes(InitialVector); 
                RijndaelManaged SymmetricKey = new RijndaelManaged();
                ICryptoTransform Encryptor = SymmetricKey.CreateEncryptor(KeyBytes, InitialVectorBytes);
                MemoryStream MemStream = new MemoryStream();
                CryptoStream CryptoStream = new CryptoStream(MemStream, Encryptor, CryptoStreamMode.Write);
                CryptoStream.Write(PlainTextBytes, 0, PlainTextBytes.Length);
                CryptoStream.FlushFinalBlock();
                byte[] CipherTextBytes = MemStream.ToArray();
               return CipherTextBytes;

/

/decrytion

public static string Decrypt(byte[] PlainTextBytes1, string key, string InitialVector)
    {
        System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
        Byte[] KeyBytes = encoding.GetBytes(key);
        RijndaelManaged SymmetricKey = new RijndaelManaged();
        byte[] InitialVectorBytes = Encoding.UTF8.GetBytes(InitialVector);
        ICryptoTransform Decryptor = SymmetricKey.CreateDecryptor(KeyBytes, InitialVectorBytes);
        MemoryStream MemStream1 = new MemoryStream(PlainTextBytes1);
        CryptoStream CryptoStream = new CryptoStream(MemStream1, Decryptor, CryptoStreamMode.Read);
        Byte[] pltxt = new byte[PlainTextBytes1.Length];
        CryptoStream.Read(pltxt, 0, pltxt.Length);
         ASCIIEncoding textConverter = new ASCIIEncoding();
         round = textConverter.GetString(pltxt);
          return round;
    }

我错在哪里？

Byte[] pltxt = new byte[PlainTextBytes1.Length];

 CryptoStream.Read(pltxt, 0, pltxt.Length);