Bugtraq mailing list archives
Re: Joint encryption?
From: John Richard Moser <nigelenki () comcast net>
Date: Sat, 19 Feb 2005 07:17:10 -0500
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Casper.Dik () Sun COM wrote:
The case where N = 1 is simple authentication; the case where N = M is an easily solvable problem in the scope I'm looking at. I'm interested in the case where N > M and the data is encrypted. - Key is fragmented - Fragments are indpendently encrypted - Each user who can authenticate can decrypt PART of the key, but not all of it - M of the N users are needed to decrypt enough of the key to access the key in totalWhen you fragment the key as you propose, there's a danger of making the remaining fragment bruteforcable by "M-1" users as they're left to guess only 1/Mth of the key. I'd argue the best way is to give each of the M users a bit vector the length of the key and XOR the M vectors to get the key vector. This way, the security of the key is as strong whether you have 1 , 2 .. upto M-1 fragments. (Of course, you should also require each of the users to decrypt their key vectors)
Yeah I noticed that on the wikipedia :) http://en.wikipedia.org/wiki/Secret_sharing
Effectively, none of the users know any key bits by themselves.The problem is that I need a guaranteed way to create data for any valid N and M where N >= 3 > M >= 2 in which access to M fragments of the key (each fragment is encrypted) can be used to gain access to the rest of the fragments, which in turn allows any selection of M users to authenticate and gain physical access to the key.Exponentional might not be bad if you know that the numbers will be small; in O(N^M) space a solution is trivial.
I actually was looking around and found secret sharing and shamir's scheme, which look interesting. :) I understand the math behind it vaguely, which is good; how exactly to do finite fields and generate polynomials I can regress of any arbitrary degree 1..+inf uhh. Bearing in mind I have no practical use for this, it's still interesting.
Casper
- -- All content of all messages exchanged herein are left in the Public Domain, unless otherwise explicitly stated. Creative brains are a valuable, limited resource. They shouldn't be wasted on re-inventing the wheel when there are so many fascinating new problems waiting out there. -- Eric Steven Raymond -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org iD8DBQFCFy5FhDd4aOud5P8RAlz2AJ0aitdjpF6vNAFIWqiNTphiabd6MQCeIPhQ XBKiKVWK6B0+s56JVvD13eM= =hTx9 -----END PGP SIGNATURE-----
Current thread:
- Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? Damian Menscher (Feb 19)
- Re: Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? Casper . Dik (Feb 19)
- Re: Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? Robert C. Helling (Feb 21)
- Re: Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? devnull (Feb 19)
- Re: Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? peter zulu (Feb 21)
- Re: Joint encryption? John Richard Moser (Feb 19)
- Re: Joint encryption? Gandalf The White (Feb 21)
- RE: Joint encryption? David Schwartz (Feb 21)
- Re: Joint encryption? John Richard Moser (Feb 21)
- Re: Joint encryption? Valdis . Kletnieks (Feb 21)
- Re: Joint encryption? John Richard Moser (Feb 21)
- Re: Joint encryption? Ruud H.G. van Tol (Feb 21)
- Re: Joint encryption? Damian Menscher (Feb 19)