XSPLIT now own ftp.wimsey.bc.ca
I just uploaded a short program I wrote called XSPLIT to ftp.wimsey.bc.ca. this is a simple crypto tool that splits a file into many files for secure distribution. A simple way of escrowing your data to the parties >YOU< only trust and who don't know each other. Whenever Gibson's idea of swiss like databanks will occur, you could encrypt your file, split it with XSPLIT and upload it to several such databanks. XSPLIT takes a file, generates n-1 random numbers, writes those random numbers out to each of the n-1 files, and for the nth file it takes the XOR of all N random numbers, and XORs it with the plaintext. This way, you get to split your file in such a way as to require all pieces of it in order to rebuild it. This is certainly not earth shattering, and is similar to what clipper uses, only its something you have available to you and something I wrote on a whim. Enjoy. (Source is included as usual and should be unix portable.)
How about doing this with n of m? Anyone have code?
What do you mean? The sources are included with XSPLIT. The algorithm is very very simple. For all chars in input file: I read a character from the infile called C. I pick a number MOD the number of files. I call this X SUM=0 FOR FILES=0 TO N IF FILES!=X Pick Random Number Q Write Randome Number Q to File #FILES SUM=SUM XOR Q NEXT FILES WRITE C XOR Q to File #X What exactly did you mean by n of m? Since at each byte the numbers are picked randomly, the stream which holds the XOR'ed sum is random for each character. So none of the parties holds any more information than any of the others and it is all useless unless all of the files are joined together.
| 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.
Very cool. I wasn't aware that such a splitting program already existed, although XSPLIT is different than shade in that you need all the parts to put the file back together and if you miss a part, you don't have anything. Also, XSPLIT will produce N files of the same size as the original file you feed it. W What exactly is SHADE useful for? Distributing a file where some of it can get damaged? Some software RAID implementation? Can it be used for encryption?
Also, XSPLIT will produce N files of the same size as the original file you feed it.
I just glanced at the .doc and ran it once last night on my PC -- haven't looked at the source -- but a possible application of this occurred to me this morning. The N files are binary, but it should be easy to restrict them to ASCII using a command-line switch or a file for PRNG input, right? Then they would be suitable for Internet (re)mailing. (Concerns about cryptographic integrity are irrelevant for my purposes.) A remailer could receive, say, a 5k message, which might be ~4.5k after peeling off that remailer's layer of encryption. XSPLIT could then be invoked to produce several ASCII files of identical size. These bogus files could be mailed to various remailers at the same time as the "real" file, with a prepended instruction to send 'em to the bit bucket. Of course, latency would then have to be added before processing the "real" file to defeat traffic analyis. I'm probably missing something, but it's a thought anyway... Alan Westrope <awestrop@nyx.cs.du.edu> __________/|-, <adwestro@ouray.denver.colorado.edu> (_) \|-' finger for pgp 2.6 public key "Silent, We the Empire Await, Trystero!" -- Pynchon (sorta...)
Re: forcing text only... Welp, you can simply modify the sources for it yourself to do this. First, check to see if the character you've read is a control. If it is leave it alone and don't encrypt it. Then if it's an alphanumeric character, take the random number you've just read and do a binary AND operation on it with a number less than 31. Say 31 should suffice. This should do the trick for what you want... But you should know that it will kill the fuck out any security you might have ever had. The best solution is to first ZIP up the file you want to XSPLIT, then use XSPLIT on it, and feed the outputs of XSPLIT to something like UUENCODE...
participants (4)
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Adam Shostack -
adwestro＠ouray.Denver.Colorado.EDU -
Berzerk -
rarachel＠prism.poly.edu