[sdiy] Long LFSRs (Was Psych Tone)

[Ralph Moonen]

Indeed, where and how many taps is a big thing for pseudo randomness. Compare for instance the A5/1 crypto system that is used in GSM communication. It consists of 3 XOR’d LFSR’s and while the key length is way too short, not many other attacks are known on that cipher. On the other hand, the crypto1 algorithm (used in Mifare cards such as Oyster cards and Dutch OV-chipcards as well as many access control systems) has one LFSR, 48bit keys and the taps are set in a non-optimal way. it is very predictable to the extent that observing a length of bits can be completely calculated back to the begin state (i.e. crypto key).

cheerio!





> On 6 Jan 2019, at 18:16, Donald Tillman <don at till.com> wrote:
> 
> 
>> On Jan 6, 2019, at 2:42 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> 
>> 2 taps. Feedback from 126 and 127 as per recommended polynomials for maximum length sequences stated in this document:
>> 
>> http://www.xilinx.com/support/documentation/application_notes/xapp210.pdf
> 
> Ah...
> 
> For any given LFSR length, there are one, or more, or a lot more, choices for feedback taps for a maximum length sequence.
> 
> For a reasonable random generator you want to choose a set of taps that are distributed along the length, with one pretty close up front.
> 
> The setup you have there with two taps at the end effectively creates a transmission line, and that's what you're hearing.
> 
>  -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
> 
> 
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