An indictment is a charge of commiting a crime. It is the first step towards being out in trial for a crime. No secret trial has occurred in the Assange case.

Ladar Levison was not served with a national security letter by the FBI. National security letters can not legally compel either the turning over of encryption keys or installation of a device to intercept communications. In fact it can't compel turning over communications period but only any existing records regarding such communications. Existing being a key word because a NSL also can't compel keeping of additional records beyond what has already been collected by the entity receiving the NSL. Lidar Levison received a wiretap order issued by a federal court. Wiretaps require the FBI to prove to a judge that there is probable cause that a felony has been committed. I can't think of a country in the world that doesn't allow wiretaps and require them to be kept secret. Requiring a judge to issue a wiretap order is in fact more stringent control over wiretaps than exists in most of the world including many European countries where police can implement wiretaps on their own accord.