Invalid length for a Base-64 char array or string error in C#

When I am trying to decrypt the encrypted string using C#, I am getting an error as below

Here is the HTML Razor anchor link code clicking on which, I am trying to decrypt it

<li>
<a class="ajax" href="/NexSignsPlugin/TemplateConfiguration?key=@crypto.EncryptStringAES(mTemplateConn.LicenseKey)&TemplateGUID=@mTemplateConn.TemplateGUID">
<i class="zmdi zmdi-edit"></i> Edit Template</a>
</li>

C# code of controller:

 public ActionResult TemplateConfiguration(string key, Guid TemplateGUID, bool pv = false)
        {
            string licenseKey = utility.security.crypto.DecryptStringAES(key);

            Areas.NexSigns2.ViewModels.TemplateConfigurationViewModel vm = new ViewModels.TemplateConfigurationViewModel();
            vm.mTemplateLicenseKey = DBentity.TemplateLicenseKeyConnections.Where(x => x.LicenseKey == licenseKey && x.TemplateGUID == TemplateGUID).First();

            vm.TemplateController = new AttributeController(vm.mTemplateLicenseKey.AttributeXML);

            if (pv == true)
                return PartialView(vm);
            else
                return View(vm);
        }

C# class which decrypts the string:

public static string DecryptStringAES(string cipherText)
        {

           
            // Declare the string used to hold
            // the decrypted text.
            string plaintext = null;

            try
            {
                // generate the key from the shared secret and the salt
                Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);

                // Create the streams used for decryption.
                //getting error here
                byte[] bytes = Convert.FromBase64String(cipherText);

                //code removed for clarity
            }
      }

So what's the error and how can I resolve it? any help is appreciated

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2 Answers