Donate for the Cryptome archive of files from June 1996 to the present

31 January 2013. Add A2 comment.

30 January 2013

Inadvertent Via Cavity Emanations

This NSA paper raises the question of more general inadvertent via cavity emanations:

Based on the research of cavity resonation* -- which transmits non-electronically inadvertent emanations requiring TEMPEST or other protection -- is there research on similar threats of cavity emanations of other computer, network or communication components which might bypass electronic or other shielding protection built-in to systems?

Answers to cryptome[at]

* One of the most famous cavity resonators was installed in a Russian gift to the US Moscow embasssy.

NSA has published this undated one-page paper:

Technology Profile Fact Sheet

Title: Via Construction to Dampen Cavity Modes in Printed Circuit Boards

Aliases: None

Technical Challenge: Vias are used extensively in printed circuit board (PCB) construction to connect signals from the surface to internal layers and to provide connectivity between layers in the PCB. However, high speed signals that transit through these vias can launch propagating modes in cavities that are created within the PCB. There is very little damping present within the PCB laminate stack to attenuate these modes once they are launched, resulting in noise coupling with other signals that propagate along the vias. Although this drawback can be reduced by using buried vias or by increasing the number of layers for routing, either approach adds considerable expense to the PCB manufacturing process.

Description: This invention constitutes a significant addition to known technology. It modifies the construction of existing vias by addition of a radio frequency (RF) absorbing layer to the copper metallurgy that is currently used to connect different layers. By suitable choice of material thickness and resistivity, these modified vias will absorb RF energy rather than reflecting it, thereby reducing the level of noise coupling within the PCB.

In conventional PCB processing, the PCB laminates are stacked and cured, followed by mechanical drilling to define the through vias. The entire assembly is then immersed into a seed bath, and a uniform layer of copper is plated everywhere. In the proposed modification, the laminate is stacked and drilled as before. However, two additional steps are inserted. The first is a selective deposition of plating of a resistive material, which does not occur on the copper, but does coat all other surfaces. Second, the insulating layer is fortified by exposing the material to oxidation. (If the copper oxidizes also, the copper oxide is easily removed without affecting the insulating layer.) The process flow then continues as in the traditional method.

Demonstration Capability: Not at this time.

Potential Commercial Application(s): This invention is potentially applicable to any manufacturer of digital or analog systems that operate at high rates or frequencies.

Patent Status: A patent application filed has been filed with the USPTO.

Reference Number: 1493

Thanks to A, this appears to be the patent, not by NSA but by two individuals, Kwark; Young Hoon (Chappaqua, NY), Schuster; Christian (Buchholz, Niedersachsen, DE):

A2 sends:

I think the embassy bug was a mechanical box that would flex from sound and could be read via microwaves and the doppler effect on them from the moving box walls. The description of RF absorbing layers on a PCB would mostly be of benefit *inside* the closed shielded grounded conductive enclosure you'd find sensitive electronics in. Ie, for the benefit of other electronics in the enclosure, NOT for the reduction of ability of eavesdroppers. Sure, PCB RF-absorbtion might reduce the amount of signal (noise :-) you have to stop getting out, but its mostly of use for preventing interference within the system --the enclosure. Its a lot easier to generate whatever you want inside a chassis and filter the chassis than to do what is described. What is described is mostly useful for keeping in-chassis electronic noise down. Just my two yuan.