Question regarding shared keys

[Grant Olson]

On 2/28/11 2:07 AM, Denise Schmid wrote:
>> It depends on what you mean by a "shared key".  There is just giving a
>> copy of the key to multiple people (in which case any one of them can use it),
>> or there are various key splitting algorithms where a key is broken into a
>> number of pieces, and a specified subset of those pieces can come
>> together, reconstruct the key, and do whatever they need to do.
> 
> It is the second.
> 
>>
>> The OpenPGP standard (which specifies how different implementations can
>> interoperate) does not really specify shared keys, beyond acknowledging that
>> they exist.  The PGP *implementation* of the standard, has a shared key
>> feature in the break-the-key-into-multiple-pieces sense.
> 
> This is what I meant. Does this mean that, if you want to encrypt a file, everybody has to use his/her key? The background of my question is that a company claims that one of their managers has forgotten the key and therefore, they can't decrypt some files. These files contain, of course, some evidence they should produce in a court case. Beside the fact that there seem to exist some ways to reconstruct keys, I ask myself if they didn't need the key to encrypt the files...
> 
> Best
> 

David's talking about an advanced scenario.  And maybe the company did
do this, but I've got a feeling you might be over-thinking things.

Normally, if you encrypt a file to four users, each user has their own
key that's completely independent of the other users.  If you're trying
to see if the file was encrypted to another manager, who hopefully
hasn't conveniently 'lost' his key, you can examine the encrypted file
and get a list of the keys it's encrypted to.

So for example, here you can see that even if I claim to have lost my
key, David is another person who could decrypt the contents...

johnmudhead:~ grant$ gpg -r kgo at grant-olson.net -r dshaw at jabberwocky.com
 --encrypt bar.txt
File `bar.txt.gpg' exists. Overwrite? (y/N) y
johnmudhead:~ grant$ gpg --list-packets bar.txt.gpg
:pubkey enc packet: version 3, algo 1, keyid 1458BCCB6A8F7CF6
	data: [2045 bits]
:pubkey enc packet: version 3, algo 16, keyid AE2827D11643B926
	data: [2047 bits]
	data: [2048 bits]
:encrypted data packet:
	length: 70
	mdc_method: 2
gpg: encrypted with 2048-bit ELG key, ID 1643B926, created 2002-01-28
      "David M. Shaw <dshaw at jabberwocky.com>"
gpg: encrypted with 2048-bit RSA key, ID 6A8F7CF6, created 2010-01-11
      "Grant T. Olson (Personal email) <kgo at grant-olson.net>"
:compressed packet: algo=2
:literal data packet:
	mode b (62), created 1298914148, name="bar.txt",
	raw data: 4 bytes


-- 
Grant

"I am gravely disappointed. Again you have made me unleash my dogs of war."

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