[OTR-users] pidgin-otr: passphrase private key and sign public keys

[Brian Morrison]

On Tue, 8 Jul 2008 20:46:04 +0200
"Julian Dibbelt" <jdibbelt at gmail.com> wrote:

> > That's not the point of OTR, if someone else uses your identity to talk
> > to a friend of yours then what they say is even more plausibly deniable
> > than if you'd said it. What's the problem?
> 
> Using plaintext is also fairly plausably deniable (as in: the
> transcript is easily forgable). That cant be the point.

Of course it is. The encrypted stream can be decrypted by any
possible key, therefore all possible plain texts exist simultaneously.
Hence an eavesdropper cannot know what you've said and a faked
conversation is as believable as a real one.

The essential fact is that the session key is ephemeral, unrecoverable
and thus offers perfect forward secrecy. This is very important if you
live in an endemic surveillance state (like the UK) where having
encrypted information could lead to being served a notice to decrypt
with a 2 year jail sentence as a punishment for refusal. But, if you
never had access to the session key, you can't be forced to give it up.
The key on your disk is used to negotiate a session key using
Diffie-Helmann key exchange and to authenticate your contact by secrets
known to the pair of you, there is no advantage to keeping this key
secure because it doesn't affect the secrecy and deniability of past
conversations.

Understand now? It's a little like hiding in plain sight.

-- 

Brian Morrison

bdm at fenrir dot org dot uk

   "Arguing with an engineer is like wrestling with a pig in the mud;
    after a while you realize you are muddy and the pig is enjoying it."
    
GnuPG key ID DE32E5C5 - http://wwwkeys.uk.pgp.net/pgpnet/wwwkeys.html