Transact SQL :: Encrypt Server Column Using Trigger

We have to encrypt one of the SQL Server column data at the database level, we don’t want to modify the application programs, because we use ERP application and we don’t want to reapply our customization very few months, so we are looking for an option  to encrypt and decrypt the column level data value using trigger/procedure/function,  I used  instead of trigger on that table to encrypt and decrypt the values but it is not working, we want only one user(Application Service account) have an ability to encrypt and decrypt the data, not for any other id.. I read somewhere we cannot automatically encrypt/decrypt the column value using trigger, is that true...