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GSM cellphones use a codec to compress analog voice data onto a low bit rate datastream.
This codec is just another bit of embedded software; it would be fairly easy to have an encryption stage on the output of the coded that scrambled the codec output based on a keyword or cipher key.
PDAs with built in modems have more than enough power in the processor to do this.
It would not be perfect - but adding (say) 256 bit encryption based on a one-time code phrase would make it ruinously diffcult and expensive to tap people's conversations. Plus you'd have to know it wasn't just a point-to-point data call. Most videophones have a DSP capable of this sort of encryption; if you don't want video, you can use the spare MIPS to get pretty secure 3.6 KHz audio.
One up for civil liberties !
Even the UK govt realise they might be going to far... [mymus, Jun 17 2002, last modified Oct 21 2004]
A5: The GSM Encryption Algorithm
"The GSM encryption algorithm, A5, is not much good." [pottedstu, Jun 18 2002, last modified Oct 21 2004]
GSM Security and Encryption
A good, detailed description. [pottedstu, Jun 18 2002, last modified Oct 21 2004]
||a secure pasty for you!
though of course this would never be allowed to happen.
||The only problem I see is: How do you syncronize the code words or phases in an un-interceptable manner? Some sort of public key encryption might be called for.
||GSM already incorporates an encryption algorithm. However it is not exactly state of the art technology, and it is decoded at the base station, rather than data being transmitted in an encrypted form all the way from phone to phone, so it will prevent casual eavesdropping with a radio receiver but the telcos could still monitor your call, or a determined cracker could break it easily.
||Pottedstu: Hence the encryption codec - the speech is still scrambled handset to handset, even when it's on the spine. A LIG wouldn't be any immediate use to a tapper; you'd have to record the data stream and try to crack it offline. Putting it though an FFT would give you the signature of the algorithm but wouldn't give you the decoded speech. I agree that the GSM ciphering is pretty weak and anyone with a CMU200 can listen in fairly easily. This way, neither the Telcos or Big Brother can easily listen in.
||You could add the same sort of encryption to GPRS or any IP-based transport fairly easily. Throw another Pentiscum at the job, they're cheap (and nasty) enough.
||<aside purpose="historical note" >I'm not sure if it's still the case but at one time it was illegal in the UK to use a scrambler device on public telephone networks.</aside>