Re: Pseudorandom Hashing

From: John Larkin (jjlarkin_at_highSNIPlandTHIStechPLEASEnology.com)
Date: 09/23/04 

Date: Thu, 23 Sep 2004 15:39:01 -0700

On Thu, 23 Sep 2004 14:40:28 -0700, Tim Wescott
<tim@wescottnospamdesign.com> wrote:

>I am having trouble coming up with the right keywords to do a web
>search, so help me out here:
>
>There is a technique where, to significantly reduce the probability of
>getting a long string of zeros, a message is run through a CRC
>generator, and the output bits are taken off. The transmitted message
>is thoroughly hashed, yet it is a simple matter of a shift register and
>some XOR gates to decode the message on the other end.
>
>I thought I knew how to do this, yet in trying to actually make it work
>I find that over half of my brain cells appear to be attending a
>management seminar.
>
>So, know where I can find out how to do this right? "pseudorandom" and
>"hash" get me tons of cryptography, but not what I'm looking for.
>
>Thanks.

It's called data scrambling. Modems sometimes use it to make sure
there are enough data transitions to keep the receive end in sync.

Try "modem data scrambling" or something.

There are also n/m codes, where each group of n data bits is replaced
with a mapped group of m bits, where m = n+1, often. This is also a
variation on RLL (run length limiting) coding, I think.

John