[ask] how to encrypt the url?

Add another VARCHAR field to your table, named "secret_id".
If you insert a record, store a random string into that field. You can use CI's string helper.
In your url's, don't use the record id, but instead the secret_id.
If you want to be really safe, check if the random string already exists in your table before the record is inserted. If so, generate a new random string.

Hope this will help.