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Re: Laptop Encryption Software
From: Gary Flynn <flynngn () JMU EDU>
Date: Mon, 5 Mar 2007 17:50:18 -0500

Valdis Kletnieks wrote:
On Mon, 05 Mar 2007 15:23:22 EST, Gary Flynn said:

The one area that could present a problem is that EFS uses a
unique symmetric key for each file and there is no mechanism
that I know of to export those keys. Nor would I want to
try to manage them if I could. I don't even think they're
handled by Microsoft's PKI.

Probably a total non-issue, as long as EFS keeps *one* copy of the symmetric
key in the file metadata (presumably encrypted in such a way that the key can
be decrypted by the user or recovery agent keys), for the exact same reason
that you don't need to escrow an SSL or PGP symmetric session key - it travels
with the data, and if you have the right public/private key pair, you can
recover it.

Did you have a use case in mind where exporting those keys would be useful
in any way?

I can think of a couple:

1. The copy on disk stored with a file gets corrupted ( affects
   only one file ).

2. The file encryption keys cannot be decrypted because
   the user/recovery agent account keys are unavailable
   due to loss, corruption, maliciousness, etc. ( affects
   all files whose symmetric encryption keys are protected
   by that user/recovery agent account key. )

I think the situation is different than SSL because the
SSL symmetric key and session are not persistent while
the file encryption key and file data are persistent.

There would be no reason to want to archive or access
the SSL symmetric key except for troubleshooting or
malicious purposes. On the other hand, there would be
value for an organization to be able to access a backup
of the file encryption keys should the originals
become unavailable. They'd need to do so to recover
the files.

When you refer to PGP, do you mean the file encryption
product or the mail encryption product?

If the latter email product, the situation is similar, though
not identical, to SSL. The symmetric session key is used
for a transient mail message.

If you're referring to PGP file encryption, I'd expect
key escrow issues to be similar to EFS. You'd certainly
want to escrow the user's private key. If symmetric
encryption keys are uniquely stored in the files then
there is no escrow in case of a problem without the
ability to export and manage those keys.

I guess its how deep one wants to go with key
escrow and recovery capabilities and that, in turn,
depends upon how valuable the data is.

Gary Flynn
Security Engineer
James Madison University

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