构建没有'='的加密密钥字符

我偶然发现了这段旧的c#代码，我想知道。net Framework 4.5是否有更优雅和紧凑的东西来做同样的事情:加密文本，避免结果中的'='字符。

谢谢。

EDIT:另外，数字40来自哪里，为什么不需要处理较长的文本?

    public static string BuildAutoLoginUrl(string username)
    {
        // build a plain text string as username#AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
        if (username.Length < 40)
        {
            //cycle to avoid '=' character at the end of the encrypted string
            int len = username.Length;
            do
            {
                if (len == username.Length)
                {
                    username += "#";
                }
                username += "A";
                len++;
            } while (len < 41);
        }
        return @"http://www.domain.com/Account/AutoLogin?key=" + EncryptStringAES(username, sharedKey);
    }
    public static string EncryptStringAES(string plainText, string sharedSecret)
    {
        if (string.IsNullOrEmpty(plainText))
            throw new ArgumentNullException("plainText");
        if (string.IsNullOrEmpty(sharedSecret))
            throw new ArgumentNullException("sharedSecret");
        string outStr = null; // Encrypted string to return
        RijndaelManaged aesAlg = null; // RijndaelManaged object used to encrypt the data.
        try
        {
            // generate the key from the shared secret and the salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);
            // Create a RijndaelManaged object
            // with the specified key and IV.
            aesAlg = new RijndaelManaged();
            aesAlg.Key = key.GetBytes(aesAlg.KeySize/8);
            aesAlg.IV = key.GetBytes(aesAlg.BlockSize/8);
            // 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);
                    }
                }
                outStr = Convert.ToBase64String(msEncrypt.ToArray());
            }
        }
        finally
        {
            // Clear the RijndaelManaged object.
            if (aesAlg != null)
                aesAlg.Clear();
        }
        // Return the encrypted bytes from the memory stream.
        return outStr;
    }

谢谢。