Subject: [voice_encryption_and_scrambling] Re: What kind of rolling code?
From:    "pilot_rg" <pilot_rg@yahoo.com>
Date:    Sun, February 10, 2008 12:10 am
To:   	 voice_encryption_and_scrambling@yahoogroups.com


The main weakness of rolling code scramblers is they all use frequency
inversion as their means for providing security. If you can find a way
to attack inversion then the scrambler can be broken. Some ways to do
this include : Random Frequency Inversion Attacks and Signal
Processing attacks using software and voice spectrum graphs to
determine the approximate inversion frequency used as it changes
(rolling code). The number of possible inversion frequencies is small
(typically 2600hz-4000hz). That means at any given time an inversion
frequency in that range is in use. An exception to this is the
Transcrypt DES scrambler. It first digitizes the audio before
scrambling it with DSP software instead of analog components. This
allows this scrambler to use a much wider range of inversion
frequencies (1947hz-4800hz) and avoid overlapping or aliasing. It also
allows the inversion frequency to change much more rapidly (apprx.
2000+ times per second). The Transcrypt DES represents the highest
security of all frequency domain (inversion based) scramblers.