The History of SSH (and Neurophone writeup: it works!)

Lots of neat stuff comes from Finland — including Linux, Abloy locks, and SSH. The link’s a concise perspective on the larger issues that forged SSH: both the Cold War paranoia that had the entire country thinking security, and the key-escrow / backdoor debate that inspired the software’s author to make it as secure as he could. Finland being one of the battlegrounds of the Cold War, you can see how constant attack is a key element of developing defenses.

In other news, the Neurophone works! At seven (!) gain stages, the most complex opamp-based audio thing I ever built, and certainly the lowest fidelity. I’ve attached a schematic. Keep in mind my version is built with whatever I had kicking around, so you could probably do better. Certainly the last three stages can be replaced with anything that has a gain of 300-500 or so. And there’s much room for improvement with respect to fidelity.

(Regarding the earlier question of modulation: exactly how it modulates the output seems to vary depending on the input signal level. Drive it to clipping and you get nearly straight-up PWM. Drive it with something weaker and things get weird; almost like PSK’ing an analog signal.)

The trick to getting it working turned out to be in the electrodes and in their placement. Instead of simple metal foil electrodes, the trick is to use a piezo disc: the kind with a circle of silver-backed piezo material on a brass disc substrate. You solder one wire to the piezo area on each disc, and skin conductivity between the two brass substrates connects the electrodes.

Depending on where you put the electrodes, the Neurophone seems to operate either as a Neurophone only, or as a combination Neurophone/bone conduction speaker.

For the purposes of testing, I used extremely high isolation earplugs: available here in drugstores as ‘luxury’ earplugs, they’re basically ~2g cylinders of specially formulated fiber-reinforced wax. With them in, you don’t hear anything at all. (noise from blood moving through your head and the like swamps whatever sound seeps past)

To get it to work as a Neurophone only, I had to play around quite a bit to find somewhere the electrodes would work. I presume this is related to the nature of mine being only a crude approximation of the original. In my case, I found they worked well all around the head/neck/shoulders area as well as the upper back, and also around the belt line. Hands, feet, and arms did not produce an audible signal if I was taking precautions against hearing it via normal means.

With the elecrodes down at the belt line, it’s highly unlikely there’s any bone conduction making it up to the ears, yet you can still hear tones in the middle of your head. (Using a sine-wave signal generator to make test tones, in addition to the 50kHz generator the design requires. In ‘Neurophone-only’ mode music and the like are pretty much unintelligible, with only very certain frequencies coming through.)

(Interestingly, signal tended to be stronger in the opposite side of my head from where the electrodes were placed: with them on the left side of my body, the signal seemed stronger in the right side of my head.)

As for combination Neurophone/bone conduction: the piezo discs put out quite enough volume to hear if you put them on your forehead, so I’m presuming in that case you’re hearing via bone conduction as well as via the ‘Neurophone effect’. In that case the fidelity is much higher — a bit better than that of the piezo transducer in free air — even if it doesn’t approach that of a regular speaker.

I did additionally try testing the rumor of wireless Neurophone effects: the idea that by running the Neurophone into an antenna or coil, the sound would be perceived without physical contact. In my case, it was a ~1mH coil of wire (either 3 or 0.7 ohms, don’t remember) with a 1.5KOhm resistor in series. Holding it near my head and sweeping the signal generator I heard nothing.

“Finland is a next-door neighbor to Russia, with which it had strained ties historically. One phenomenon of the Cold War situation of the ’70s and ’80s is that it was commonplace for Finnish inhabitants to build underground garages for their houses which also served as bomb shelters. This got Ylonen thinking about security.

Later in the ’90s, the U.S. government’s push to have industry vendors install a key-escrow system in every product using encryption — so the government could gain access to encrypted data — also had a big impact on Ylonen.

The idea, which came to a head during the Clinton Administration, was based on the notion that key-escow was necessary to fight terrorism and crime.

However, the idea that governments anywhere would find such an easy way to obtain an individual’s encrypted data troubled Ylonen. Consequently, “I wrote Secure Shell during the crypto debate,” said Ylonen.

The idea of key-escrow pushed by the National Security Agency and the Clinton Administration collapsed under the pressure of opposing forces, both from industry and civil-rights groups, by 2001. The Cold War limped to an end as East European states abandoned Communism in the late ’80 and the Soviet Union collapsed in 1991, giving rise to modern-day Russia.

SSH Communications Security Corp., however, lives on, with about 190 employees working for it today.”


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