Create a plugin to decrypt custom field of Case Entity while Retrieve in MS Dynamics CRM Online

protected void ExecutePostCaseRetrieve(LocalPluginContext localContext)
        {
            if (localContext == null)
            {
                throw new ArgumentNullException("localContext");
            }

            // TODO: Implement your custom Plug-in business logic.
            IPluginExecutionContext context = localContext.PluginExecutionContext;
            IOrganizationService service = localContext.OrganizationService;
            if (context.Depth == 1)
            {
                // The InputParameters collection contains all the data passed in the message request.
                if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is EntityReference)
                {
                    // Obtain the target entity from the input parmameters.
                    EntityReference entityreference = (EntityReference)context.InputParameters["Target"];

                    ColumnSet cols = new ColumnSet(new String[] { "title", "description", "new_phicontent" });
                    Entity incident = service.Retrieve(entityreference.LogicalName, entityreference.Id, cols);
                    if (incident.LogicalName.ToLower().Equals("incident"))
                    {
                        try
                        {
                            if (incident.Attributes.Contains("new_phicontent"))
                            {
                                string PHIContent = incident.Attributes["new_phicontent"].ToString();

                                byte[] bInput = Convert.FromBase64String(PHIContent);
                                UTF8Encoding UTF8 = new UTF8Encoding();
                                //Encrypt/Decrypt strings which in turn uses 3DES (Triple Data Encryption standard) algorithm
                                TripleDESCryptoServiceProvider tripledescryptoserviceprovider = new TripleDESCryptoServiceProvider();

                                //Alow to compute a hash value for Encryption/Decryption
                                MD5CryptoServiceProvider md5cryptoserviceprovider = new MD5CryptoServiceProvider();

                                tripledescryptoserviceprovider.Key = md5cryptoserviceprovider.ComputeHash(ASCIIEncoding.ASCII.GetBytes("secretkey"));
                                tripledescryptoserviceprovider.Mode = CipherMode.ECB;
                                ICryptoTransform icryptotransform = tripledescryptoserviceprovider.CreateDecryptor();

                                string DecryptedText = UTF8.GetString(icryptotransform.TransformFinalBlock(bInput, 0, bInput.Length));
                                incident.Attributes["new_phicontent"] = DecryptedText;
                                //service.Update(incident);
                            }

                        }
                        catch (FaultException ex)
                        {
                            throw new InvalidPluginExecutionException("An error occurred in the plug-in.", ex);
                        }
                    }
                }
            }
        }