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