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Those awkward silences during phone calls can communicate a lot. Especially if you're sending hidden messages during them. Computer scientists at the Warsaw University of Technology have come up with a way to secretly send nearly 2000 bits of encrypted data per second during a typical Skype conversation by exploiting the peculiarities of how Skype packages up voice data. They reported their findings this week.

Steganography, as Warsaw's Jozef Lubacz, Wojciech Mazurczyk, and Krzysztof Szczypiorksi explained in their excellent feature article in February 2010, differs from encryption in that its not just that the message is unreadable; the fact that it's being sent at all is hidden as well. Steganography's been in use for thousands of years and has evolved to include hiding data within compressed image files in such a way that the image isn't degraded so much that a human eye would notice. The problem with that sort of steganography, the researchers point out, is that it leaves a trace. The image file will wind up on some server somewhere with your message embedded in it.

A safer class of steganography, from the spy's point of view, is called network steganography. This gets around the traceability problem by hiding the message in the peculiarities of the manner in which a network works. Mazurczyk, Szczypiorski, and colleagues have already come up with several versions of this including LACK, which hides messages in data packet delays; HICCUPS, which makes the message seem like the expected noise or distortion; and the less-creatively-named Protocol Steganography, which slips messages into underutilized data fields.

The new scheme SkyDe (for Skype Hide) uses the peculiarities of how Skype works to hide the message. First the researchers noted that even when there's silence in a Skype call, the software is still generating and sending packets of audio data. After analyzing Skype calls, they found that they could reliably identify those silence packets, because they were only about half the size of packets containing voices. Sky-De encrypts your hidden message, grabs a certain portion of outgoing silence packets,, and stuffs the encrypted message into them. A person at the receiving end's SkyDe software would then grab the small packets and let the big ones through. Skype software interprets the lost small packets as silence anyway, so it doesn't miss them.

A video explains it all:

Mazurczyk, Szczypiorski, and colleagues are particularly pleased with SkyDe, because it hits a number of criteria you'd want for any good steganographic system. With such systems you're always trading off between the quality of the carrier (the Skype call in this case), the bandwidth available for hidden messages, and the ability to keep secret that you are passing a message (they call it "undetectability"). Without causing a suspicious change in voice quality, they could load up 30 percent of the silence packets for a bandwidth of 1.8 kilobits per second. They also found that even if someone in between the callers were intercepting the packets and analyzing them by how often certain byte values occur, SkyDe is pretty much undetectable. Warsaw's Network Security Group is planning to work on boosting SkyDe's bandwidth among other things.

So spy agencies have more to worry about than ever. First it was how easy it is to hack the Cisco IP phones in government offices, now its hidden messages in Skype calls. What's next? Messages encoded in genes?

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Metamaterials Could Solve One of 6G’s Big Problems

There’s plenty of bandwidth available if we use reconfigurable intelligent surfaces

12 min read
An illustration depicting cellphone users at street level in a city, with wireless signals reaching them via reflecting surfaces.

Ground level in a typical urban canyon, shielded by tall buildings, will be inaccessible to some 6G frequencies. Deft placement of reconfigurable intelligent surfaces [yellow] will enable the signals to pervade these areas.

Chris Philpot

For all the tumultuous revolution in wireless technology over the past several decades, there have been a couple of constants. One is the overcrowding of radio bands, and the other is the move to escape that congestion by exploiting higher and higher frequencies. And today, as engineers roll out 5G and plan for 6G wireless, they find themselves at a crossroads: After years of designing superefficient transmitters and receivers, and of compensating for the signal losses at the end points of a radio channel, they’re beginning to realize that they are approaching the practical limits of transmitter and receiver efficiency. From now on, to get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself. But how can we possibly engineer and control a wireless environment, which is determined by a host of factors, many of them random and therefore unpredictable?

Perhaps the most promising solution, right now, is to use reconfigurable intelligent surfaces. These are planar structures typically ranging in size from about 100 square centimeters to about 5 square meters or more, depending on the frequency and other factors. These surfaces use advanced substances called metamaterials to reflect and refract electromagnetic waves. Thin two-dimensional metamaterials, known as metasurfaces, can be designed to sense the local electromagnetic environment and tune the wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface. So as the waves fall on such a surface, it can alter the incident waves’ direction so as to strengthen the channel. In fact, these metasurfaces can be programmed to make these changes dynamically, reconfiguring the signal in real time in response to changes in the wireless channel. Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

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