为什么RijndaelManaged在加密后返回垃圾数据

我从http://msdn.microsoft.com/en-us/library/system.security.cryptography.cryptostream.aspx并将其修改如下。我有一个加密的例子，它在解码时工作得很好。但当使用加密函数时，它会返回带有奇怪符号的垃圾字符串。以下是加密/解密的功能。

加密字符串"嘿"的示例："？？？U？b？？z？Y？？"
再次解码时："ûc｛ÁpÅ`ñ""

我正在使用此代码将字节数组转换为字符串：

private string ByteArrayToString(byte[] input)
    {
        ASCIIEncoding dec = new ASCIIEncoding();
        return dec.GetString(input);
    }

以下是加密/解密功能。解密功能运行良好。

private string DecryptStringFromBytesAes(byte[] cipherText, byte[] Key, byte[] IV)
    {
        // Check arguments. 
        if (cipherText == null || cipherText.Length <= 0)
            throw new ArgumentNullException("cipherText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        // Declare the string used to hold 
        // the decrypted text.
        string plaintext = null;
        // Create an RijndaelManaged object 
        // with the specified key and IV. 
        using (RijndaelManaged aesAlg = new RijndaelManaged())
        {
            aesAlg.Key = Key;
            aesAlg.Padding = PaddingMode.Zeros;
            aesAlg.Mode = CipherMode.ECB;
            // Create a decrytor to perform the stream transform.
            ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
            // Create the streams used for decryption. 
            using (MemoryStream msDecrypt = new MemoryStream(cipherText))
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                    {
                        // Read the decrypted bytes from the decrypting stream 
                        // and place them in a string.
                        plaintext = srDecrypt.ReadToEnd();
                    }
                }
            }
        }
        return plaintext;
    }


private byte[] EncryptStringToBytesAes(string plainText, byte[] Key, byte[] IV)
    {
        // Check arguments. 
        if (plainText == null || plainText.Length <= 0)
            throw new ArgumentNullException("plainText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        byte[] encrypted;
        // Create an RijndaelManaged object 
        // with the specified key and IV. 
        using (RijndaelManaged aesAlg = new RijndaelManaged())
        {
            aesAlg.Key = Key;
            aesAlg.Padding = PaddingMode.Zeros;
            aesAlg.Mode = CipherMode.ECB;
            // Create a decrytor to perform the stream transform.
            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);
            // Create the streams used for encryption. 
            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
                        //Write all data to the stream.
                        swEncrypt.Write(plainText);
                    }
                    encrypted = msEncrypt.ToArray();
                }
            }
        }
        return encrypted;
    }

private string ByteArrayToHexString(byte[] data)
{
    return String.Concat(data.Select(b => b.ToString("x2")));
}

    public static string ToHexString(this byte[] bytes)
    {
        return bytes == null ? string.Empty : BitConverter.ToString(bytes).Replace("-", string.Empty);
    }
    public static byte[] FromHexString(this string hexString)
    {
        if (hexString == null)
        {
            return new byte[0];
        }
        var numberChars = hexString.Length;
        var bytes = new byte[numberChars / 2];
        for (var i = 0; i < numberChars; i += 2)
        {
            bytes[i / 2] = Convert.ToByte(hexString.Substring(i, 2), 16);
        }
        return bytes;
    }