Bugtraq mailing list archives
Re: Joint encryption?
From: Casper.Dik () Sun COM
Date: Sat, 19 Feb 2005 11:24:20 +0100
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 total
When 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) 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. Casper
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)
(Thread continues...)
- Re: Joint encryption? Damian Menscher (Feb 19)