From: Adam Shostack <adam@bwh.harvard.edu>
Date: Tue, 26 Jul 94 20:11:25 PDT
To: berzerk@xmission.xmission.com (Berzerk)
Subject: XSplit & N/M alternatives
In-Reply-To: <Pine.3.89.9407262029.A6257-0100000@xmission>
Message-ID: <199407270310.XAA11583@duke.bwh.harvard.edu>
MIME-Version: 1.0
Content-Type: text/plain



| How about doing this with n of m?  Anyone have code?

You can also get shade from
ftp.dsi.unimi.it:/pub/security/crypt/code/shade.tar.gz


From: hebrais@mirkwood.CAM.ORG (Philippe Hebrais)
Newsgroups: alt.sources
Subject: shade -- split a file with shadows
Message-ID: <hebrais.0xu4@mirkwood.CAM.ORG>
Date: 13 Dec 92 07:57:05 GMT
Organization: Secte des adorateurs des semiconducteurs
Lines: 675
X-Newsreader: MeNews 2.8.0


                SSSSS   HH   HH    AAA    DDDD     EEEEEEE
               SS   SS  HH   HH   AA AA   DD DD    EE
              SSS      HH   HH  AA   AA  DD  DD   EE
               SSS     HH   HH  AA   AA  DD   DD  EE
               SSS    HHHHHHH  AAAAAAA  DD   DD  EEEE
                SSS   HH   HH  AA   AA  DD   DD  EE
                SSS  HH   HH  AA   AA  DD  DD   EE
            SS   SS  HH   HH  AA   AA  DD DD    EE
            SSSSS   HH   HH  AA   AA  DDDD     EEEEEEE

`shade' is a file splitting and merging utility.  It takes a large
file and splits it into uniformly sized blocks.  It can also output
extra blocks (called shadows).  These shadows can be used to recover
missing sections if they get corrupted or it they are lost.  With a
single shadow, `shade' can recover ANY single missing block.  As many
shadows are needed as there are blocks missing.  If too few blocks
and shadows are available, nothing can be recovered.

For example, foo.bar (259042 bytes) is split into 5 sections
of 45000 bytes, 1 section of 34042 bytes and 2 shadows of
45000 bytes.  Each of these 8 parts is sent through email.
Even if any two of these eight parts gets lost, the original
foo.bar can be reconstructed.

`shade' is a simple application of the chinese remainder theorem
for polynomials with coeficients modulo two.  For more information
see the comments at the beginning of project.c.



SAMPLE USAGE

Split "bar" (111042 bytes) into 20000 byte chunks and output 2
shadows.  All these parts will be uuencoded and output to
foo.uu.001, foo.uu.002, etc.

    % shade -u -k 2 -l 20000 -o foo bar
    [001] [002] [003] [004] [005] [006] [aaa] [aab] Done.

Merge these parts back together:

    % rm foo.uu.003 foo.uu.005
    % cat foo.uu.* | shade -m -u
    Merging bar (111042 bytes)
    Got section 4 (20000 bytes)
    Got section 1 (20000 bytes)
    Got section 2 (20000 bytes)
    Got section 6 (11042 bytes)
    Got shadow 1 (20000 bytes)
    Got shadow 2 (20000 bytes)

    Missing: [003] [005]

    Recovering 2 sections:
    [001] [002] [aaa] [004] [aab] [006]



DISTRIBUTION

Shade is copyright Philippe Hebrais 1992.  You have the permission
to use this code is anyway you feel appropriate as long as you give
credit where it is due.  There is no warranty of any kind.  I am not
responsible for any damage caused directly or indirectly by this
program.


AUTHOR
          ,
Philippe Hebrais <hebrais@mirkwood.cam.org>

--
      Philippe Hebrais   hebrais@mirkwood.cam.org
   Voix: (514)731-9146   uunet!philmtl!altitude!mirkwood!hebrais



-- 
Adam Shostack 				       adam@bwh.harvard.edu

Politics.  From the greek "poly," meaning many, and ticks, a small,
annoying bloodsucker.