"Safe handle has been closed"

I am getting and error that the "safe handle has been closed" when my code is running.
 
I have a code block in my function with a "using" statement and on some of the variables I get the exception that the variable has been disposed.
 
When I call the function the first time it runs fine but the second time I run the function it throws the "Safe handle has been closed" and the variable that I want to use has been disposed.
 

public virtual string Encrypt(string iString)
       {
           try
           {
               // TODO: Add Proper Exception Handlers
               StringBuilder stringBuilder;
               var pkCert = _CertFile.PublicKey.Key;
               using(var rsaCryptoServiceProvider = (RSACryptoServiceProvider) pkCert)
               {
                   int keySize = rsaCryptoServiceProvider.KeySize/8;
                   byte[] bytes = Encoding.UTF32.GetBytes(iString);
 
                   int maxLength = keySize - 42;
                   int dataLength = bytes.Length;
                   int iterations = dataLength/maxLength;
                   stringBuilder = new StringBuilder();
                   for (var i = 0; i <= iterations; i++)
                   {
                       var tempBytes = new byte[
                           (dataLength - maxLength*i > maxLength)
                               ? maxLength
                               : dataLength - maxLength*i];
                       Buffer.BlockCopy(bytes, maxLength*i, tempBytes, 0,
                                        tempBytes.Length);
                       byte[] encryptedBytes = rsaCryptoServiceProvider.Encrypt(tempBytes, _FOaep);
                       // Be aware the RSACryptoServiceProvider reverses the order of
                       // encrypted bytes. It does this after encryption and before
                       // decryption. If you do not require compatibility with Microsoft
                       // Cryptographic API (CAPI) and/or other vendors. Comment out the
                       // next line and the corresponding one in the DecryptString function.
                       Array.Reverse(encryptedBytes);
                       // Why convert to base 64?
                       // Because it is the largest power-of-two base printable using only
                       // ASCII characters
                       stringBuilder.Append(Convert.ToBase64String(encryptedBytes));
                   }
 
               }
               return stringBuilder.ToString();
           }
           catch (Exception ex)
           {
               throw new Exception(ex.Message);
           }
       }