Reformatted code example, see http://stackoverflow.com/questions/18851715/encrypting-and-decrypting-query-string-causes-unknown-text-to-be-appended.

gistfile1.txt

public string DecryptQueryString(string inputText, string key, string salt)
{
    byte[] encryptedData = Convert.FromBase64String(inputText);
    var secretKey = new PasswordDeriveBytes(Encoding.ASCII.GetBytes(key), Encoding.ASCII.GetBytes(salt));

    using (var rijndaelCipher = new RijndaelManaged())
    using (var decryptor = rijndaelCipher.CreateDecryptor(secretKey.GetBytes(32), secretKey.GetBytes(16)))
    using (var memoryStream = new MemoryStream(encryptedData))
    using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
    {
        byte[] plainText = new byte[encryptedData.Length];
        cryptoStream.Read(plainText, 0, plainText.Length);

        return Encoding.UTF8.GetString(plainText);
    }
}