Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!petrus!karn From: karn@petrus.UUCP (Phil R. Karn) Newsgroups: net.ham-radio Subject: Re: Ham Encryption Message-ID: <479@petrus.UUCP> Date: Sat, 24-Aug-85 16:28:27 EDT Article-I.D.: petrus.479 Posted: Sat Aug 24 16:28:27 1985 Date-Received: Sun, 25-Aug-85 12:59:23 EDT References: <860@brl-tgr.ARPA> Organization: Bell Communications Research, Inc Lines: 24 We are fortunate that the encryption prohibition is worded the way that it is, with the key point being whether there is intent to obscure the meaning of a message. This allows us complete freedom to experiment with any sophisticated, nonstandard encoding method we choose without worrying about difficult it might be to monitor. As long as it serves some valid communication purpose other than hiding the message, we're OK. A case in point is the 9600 baud FSK modem design by Steve Goode, K9NG. Steve's biggest problem was to get rid of the DC components in a data stream; this simplifies the design greatly, since you can use AC coupling and not worry about things like op-amp offsets, crystal tolerances, etc. One approach is Manchester encoding, but this effectively doubles the bandwidth required for the signal. Steve's solution was to add a pseudo-random stream to the data at the transmitter, removing it at the receiver. In the commercial world, this technique is commonly known as "bit scrambling", a term with unfortunate connotations. (The technique DOES resemble an encryption system, although one based on a linear feedback shift register would be very easy to break.) However, since Steve's intent is not to obscure the meaning of a message but rather to simplify the hardware design, it is acceptable. (Of course, it also helps that he published the algorithm and encoding/decoding circuit at the last ARRL packet radio conference.) Phil