Re: Forward Secrecy

<vedaal@hush.com> Thu, 24 February 2005 19:36 UTC

Message-Id: <200502241901.j1OJ1WMn083642@mailserver3.hushmail.com>
Date: Thu, 24 Feb 2005 11:01:28 -0800
To: ietf-openpgp@imc.org
Subject: Re: Forward Secrecy
From: vedaal@hush.com
Sender: owner-ietf-openpgp@mail.imc.org
Precedence: bulk



On Thu, 24 Feb 2005 08:16:03 -0800 Ben Laurie <ben@algroup.co.uk> 
wrote:

>http://www.links.org/dnssec/draft-brown-pgp-pfs-04.html
>http://www.links.org/dnssec/draft-brown-pgp-pfs-04.txt
>
>Forward Secrecy Extensions for OpenPGP

>Comments, please!


the site lists the following statement:

"If expired keys are securely deleted, attackers will never be able 
to retrieve them to decrypt captured ciphertext. Therefore when a 
public encryption key expires, an OpenPGP client MUST securely wipe 
the corresponding private key"


it would also need the suggestion/requirement that the OpenPGP 
client
NOT be allowed to make a 'backup' of the private key,
something now routinely done by default

but even if it does so, and does not make any backups,
it is still not foolproof,
it just requires the adversary to do 'more work'

assuming the sender corresponds with 'n' different recipients,
and sends a new subkey packet to each of them for each encryption,

if the adversary can intercept each e-mail message, and stores 
them,
then the adversary now needs the 'n' long-term private keys of the 
recipients, and can then recover the subkeys and the messages

so, the security still depends on the recipient's long term
private keys not being compromised, as it did without the use of 
the subkeys

but if the sender doesn't encrypt to self, and encrypts only to the 
receiver,
how is the security improved by having different subkeys each time 
for that receiver?


vedaal




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