Question regarding shared keys

[David Shaw]

On Feb 28, 2011, at 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?

No. Encryption is always possible by anyone.  The shared key only needs to be rejoined for decryption or to change the details of the shared key (adding or removing people who have a share, or changing the minimum number of people needed to restore the key), or other secret-key only operations.

> 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...

Interesting!  They have no backups of the key, no key sharing set up, and didn't use the ADK (Additional Decryption Key) feature of PGP?

David