Wiktionary
untappable
a. Unable to be tapped.
Usage examples of "untappable".
They were the untappable, unjammable, undetectable link which tied together angry men from Riga to Vladivostok.
The Darhel's AID, unlike those given to the humans so lavishly, was untappable, at least by any means available to the Bane Sidhe.
The untappable instant quantum devices were the tool of choice for communications security—at least between FTL hops.
She had designed the detector's interference attachment primarily to permit careful, unnoticeable manipulations of messages passing over supposedly untappable communication lines.