As computing power continues to increase, previously unbreakable forms of encryption have crumbled. Now, though, we appear to be on the verge of what may be truly unbreakable quantum encryption. It’s possible in the not too distant future no one will be able to spy on a message secured with these advanced methods no matter how long they hammer at it. Researchers are now looking to take things one step further and completely camouflage a message so no one even knows that a message was sent in the first place.

If you can’t even risk an eavesdropper knowing that a message has been sent, let alone what it says, you need a covert communication system. That’s the idea at the heart of a new experiment conducted at the University of Massachusetts. Researchers there have developed a method of using photons to make a message invisible to everyone but the intended recipient.

The covert system relies on a technique called pulse position modulation, which is actually much more simple than you’d expect. It involves dividing a second, minute, or other unit of time into discrete bands, each of which correspond to a different letter or symbol. This code would have to be shared with the intended recipient ahead of time, which is perhaps the most notable flaw with the whole scheme. Once that’s done, through, a series of pulses could be delivered like optical Morse code to convey a message.

Covert laser

So how are carefully timed pulses of light in any way covert? The technique assumes very short pulses of photons are being used, and the environment (like a laser or fiber optic cable) contains a large number of photons that are completely unrelated to the message. They also assume that optical sensors are not perfect, and will always have unavoidable dark counts — when a detector registers a photon that didn’t actually exist. That means an eavesdropper (that’s “Willie” in the diagram above) would be unable to distinguish the coded message from background noise — like it never happened. By spreading the message out over longer time intervals, the effective bitrate is lowered, allowing it to sink even more deeply into the background.

The researchers were able to experimentally demonstrate that the technique works with lasers [arXiv:1404.7347], but they didn’t speculate on who might want to use such a system. For most people the sheer knowledge that communication took place isn’t a problem — it’s the content of the message that counts. Still, there are a number of situations that covert communication could be important. For instance, dissidents operating under the authority of an oppressive regime could face significant danger from sending any communications at all. Those working covert intelligence assignments might also be interested in hiding their messages from view. There are likely nefarious uses as well, but isn’t that the case with any technology?