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From: Matt Frazier (mfrazier, cryptofreak dot org) Date: 2001.03.08 - 23.34 MST
X-Mailer: Microsoft Outlook Express 5.60.2296.0000 2PSK is common as pointed out in my previous link, in fact, there's some Mathematica action for M-way PSK, but the most common types are 2 and Q (or 4) way PSK. 2 way would make sense here, except that I'm concerned about the 'tits' at the beginning -- can we safely assume that it's just the end of a previous signal? Although I can't find any other explanation, and that may be the only way to encode the first signal (since you would need to know the shift from previous). It still seems a little shot-in-the-dark-like. Google search for "Phase Shift Keying" results in most of my basis for this message. Matt (Benny's Margaritas Rule) Frazier ----- Original Message ----- From: "Michael Dwyer" <n0zap, yahoo dot com> To: <mod-chal, cryptofreak dot org> Sent: Thursday, March 08, 2001 11:04 PM Subject: Re: Or I'm on crack > Here is how I look at the signal: > 1. No changes in amplitude > 2. No changes in frequency > 3. Signal hits zero once every T. > 4. Signal is sine-wave looking > > A 300 baud modem uses Bell103/FSK -- It would show different > frequencies of sound. However, it is interesting in this: "Positive > sine wave at 1070Hz to send 0 and negative sine wave at 1270Hz to send > a 1" With the exception of the lack of obvious differences in > frequency, these phase shifts we are seeing could be described as a > sign wave being negated on 2T boundaries. > > 1200bps [Bell 212A] modems use a single frequency (1200Hz). It groups > the bits two at a time and sends one of the four possibly values by > selecting the coresponding phase of the sine wave. (PSK) -- This is > remarkably similar to what we are seeing, except they we are only > seeing 2 distinct phases instead of four. 600bps modem? > > 2400bps [V.22bis] modems combine phase and amplitude keying -- 3 > different amplitudes, and 12 phases. WAY too complex. We certainly > are not seeing different amplitudes and phases. > > v32, etc modems are likely even more complex than the 2400 modem, so it > is likely that we're not seeing any of these. > > Ethernet [802.11] modems use manchester encoding, where the signal > drops from 1 to 0 to signal a 0, and rises from 0 to 1 to signal a 1. > During the clock pulse, the signal jumps to the level required by the > next transition. We could be seeing this, but I doubt it > o Ethernet is usually shown as square waves, with a 0 and 1 component. > We're seeing sine waves with a +1/-1 component. > o Ethernet is typically not called a Modem, and the exercise calls it > "an unprotected modem" > o Real-world simple modems (like the 300/1200 ones above) don't use > manchester. > > So, I theorize that timing happens at "tits", and the bit is read on > the phase of the signal -- or more simply, wx it is rising or falling > midway between timing marks. > > Here is some further theorizing: > > We only see one stream. There is no duplex connection coming back, so > we are probably safe to assume that this connection is half duplex. > This is important because it means that error checking is pretty > simplistic. I'm not sure I can explain this, but without an obvious > reply, a CRC wouldn't make sense. There is no way for the host to > reply, and no obvious waiting for a reply. > > Lemme try this again: The paper says "ERROR CHECKING" and not "ERROR > CORRECTING". Which is to say, that the best we can hope for is parity > bits at this physical layer. The actual message may hold its own CRC, > but as far as the layer we're looking at goes, parity is the only > existing option. > > Encryption is also at a higher layer, and can be ingnored for the time > being. > > I propose we transcribe the signals as if it was a 2-phase PSK modem, > then try to parse it for parity. n81 is kind of standard, but the > paper intimates that some form of error checking must exist... > > I'll look at it some more... > > > > __________________________________________________ > Do You Yahoo!? > Get email at your own domain with Yahoo! Mail. > http://personal.mail.yahoo.com/ > -- > This is the mod-chal mailing list. To unsubscribe, email > majordomo, cryptofreak dot org with message body 'unsubscribe mod-chal'. > Or, for more information, visit http://www.cryptofreak.org/. > -- This is the mod-chal mailing list. To unsubscribe, email majordomo, cryptofreak dot org with message body 'unsubscribe mod-chal'. Or, for more information, visit http://www.cryptofreak.org/.
This archive was generated by hypermail 2b30 : 2001.09.26 - 14.03 MDT |
