bugtraq - Re: Joint encryption?

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Message-ID: <4218D053.80003@comcast.net>
Date: Sun, 20 Feb 2005 13:00:51 -0500
From: John Richard Moser <nigelenki@...cast.net>
To: Valdis.Kletnieks@...edu
Cc: bugtraq@...urityfocus.com
Subject: Re: Joint encryption?

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Valdis.Kletnieks@...edu wrote:
> On Fri, 18 Feb 2005 02:42:44 EST, John Richard Moser said:
> 
> 
>>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.
> 
> 
> Schneier's 'Applied Cryptography' discusses "secret sharing" schemes, and
> chasing the references from there should be sufficient.
> 
> 
>>Reminder that the idea here is to use a physical method, not bare access
>>control that can be evaded by loading a modified kernel.
> 
> 
> Not sure what you mean by "a physical method", unless you go to something like
> the old "2 keys must be inserted at the same time in slots more than an arm's
> reach apart" type of scheme.

In essence, if you encrypt something, and destroy the key, you destroy
the data.  At least, the point of encrypting something is so that it
"can't" (reasonably) be accessed without the key (or a 4096 qubit
computer) (in the life of the universe).  There's no point to cyphering
something if it can be retrieved without the key in some interval which
ends within the period that the data is supposed to remain secret.

Because of this, I consider encryption to be a physical barrier.  You
can get the disk, you can use another OS on it, you can hardware hack
the machine, you can take it apart, you can do a MFM examination, splice
it in a chemical vat, spectral analyze it, put it in a tractor beam and
try to extract the data that way, fire phasers at it, whatever, you
don't get to the data without the key.

(note too that a lock is a physical barrier; but a good sledgehammer
will shatter the mechanism if it's bigger than the lock)

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