you want secret transmissions?...(they have it) could we transmit "Utilizing home audio tweeter piezoelectric elements and an 8bit microcontroller data was encoded into an acoustic wave which was transmitted through water and detected by another microcontroller using an identical piezoelectric element as a microphone" ? do you want it? TOR network?

   Inexpensive Underwater Data Communication.pdf by Elsa Cristina David on Scribd

Applications, among others... Send/Receive data...piezo air transducer!....here we have an external drive (!!) 25 p~p (peak to peak voltage waveform)

https://www.steminc.com/PZT/en/piezo-air-transducer-112-khz

http://www.communica.co.za/catalog/Details/P4066208523

For the countries interessested ...on transfering data and packets crossing the faraday cage ...where their servers are protected

Data Transfer and Wireless Changing Over Electro Magnetic Field

 

   Data Transfer and Wireless Changing Over Electro Magnetic Field by Elsa Cristina David on Scribd

well...last resource for their protection is...Transmitting data and power through a Faraday Cage boundary

While the Faraday Cage effect is essential in many contexts – preventing leakage from microwave ovens, lightning strike protection, screening data cables – TTP’s Fluxor technology will have major benefits for power and data transfer through metal shielding. TTP is already using the technology for monitoring fluid levels in steel pipes, taking readings from medical implants and measuring data from inside high-performance F1 engines.

The Fluxor method creates a ‘window’ for electromagnetic transmission of power and data by applying a strong DC magnetic field, which lines up the magnetic dipoles in the material to ‘saturate’ a small area of the metal screen. This reduces the permeability and increases penetration to make it possible to transfer electromagnetic power and signals. Experiments conducted by TTP using steels from 5-15mm thick show that the optimum operating frequency range is in the region of 400-500Hz.

In a typical operating scenario, a portable interrogator unit with a permanent magnet or electromagnet could be placed adjacent to a fixed sensor, through a metal wall. A Fluxor window is opened to transmit power to energise the sensor and transmit a signal back – all without the need for physical openings in the enclosure.

http://www.eedesignnewseurope.com/news/transmitting-data-and-power-through-faraday-cage-boundary

Arduino ESP8266 IoT : Relay Control Internet .....drone defender...

http://anodas.lt/elektronika/ENC28J60-Ethernet-Module

UFED Ultimate / PA Extract and decode every ounce of data within digital devices. Delve deep using leading-edge forensic examination capabilities and thoroughly review logical, file system and physically extracted data to discover critical evidence

https://www.cellebrite.com/en/product/solutions/extract-decode/