From: hfinney@shell.portal.com (Hal Finney)
Date: Sun, 28 Nov 93 20:04:40 PST
To: cypherpunks@toad.com
Subject: Re:  Cryptosplit 2.0
Message-ID: <9311290404.AA27229@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


Norm Hardy posted some code for Shamir secret sharing here about a month
ago, a nice short routine.  At around the same time, I created a program
to do the same thing and uploaded it to soda.  It is still in
/pub/cypherpunks/incoming as secsplit.zip.  It contains a DOS executable
and source for building under Unix or DOS.

I did the polynomial calculations a little differently from Norm and
Ray; their approaches may be more efficient.  But I did go to some
effort with the random-number generation on which the security of the
scheme depends.  My code uses the IDEA.C module from PGP for the
pseudo-random generator, seeding it with the time of day and an MD5
hash of the file being split.  So I think this should be pretty secure
in terms of the randomness involved.

The purpose of this program, as with Ray's and Norm's, is to split a file
into n pieces (all as big as the original file) such that any k of them
are sufficient to recover the original file, but k-1 pieces give you NO
information about the contents of the original file (other than its size).

One possible application is to split up your PGP secret key file this
way and distribute the pieces to trusted friends such that several of
them have to cooperate to recover your key.  Then if you accidentally lose
your key you can get the pieces back from your friends.

Hal