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Holographic Sonar Cryptography
Atomic Snarl writes: "New Scientist.com has this story on how to encrypt a underwater sonar message using multiple sound path timing.
By detecting and adapting for the current variations on underwater sound channels,
the transmitted message can be received intelligibly only at a single point.
This holographic approach suggests a method of web encryption using multiple
hop paths and ping times to create a message which can only be decoded when
received at a specific target node!"
subject sez all
Speed of sound through seawater isn't constant; it can vary dramatically with temperature and salinity.
I question the usefulness of this technique. As described in the article, then went between a land-based station and a hydrophone. The article claimed that for it to work, they needed to know the location of the hydrophone. It also seems that water conditions needed to remain static; if they change significantly, I don't think you have a link anymore.
And more importantly, when communicating with subs at sea, you don't know their precise location. Subs go out on patrol, and might be assigned to a given area, but they don't constantly update COMSUBLANT or whoever of their position and depth. Generally, they're alerted to incoming message traffic by ELF radio transmission, which tells them to go shallow and talk to a satellite for more detailed information.
A transmission system that requires the person sending the message to know exactly where the submarine is kind of defeats the entire purpose of having submarines in the first place.
It seems to me that the speed of sonar through water is a physical certainty; that's why we can accurately use it to detect the distance from an object.
The speed of sound through water varies with temperature, pressure, and salinity. Different frequencies are effected differently. The variance is generally small enough that it does not effect the usage of active sonars to determine range. (Passive sonars rely on mathematical processing.) However the variance is large enough to make communicating via sonar difficult, especially over longer ranges.
For range determination you are simply relying on the existence or absence of a return pulse during a given time gate. Previous sonar communications were analog, thus relying not only on the presence or absence of a pulse, but it's frequency, power and relationship to the pulses proceeding and following it. Enviromental changes that affected how the sound traveled could thus easily screw up the signal.