Subject: RE: [Simple] IMDN Issue 5: Changing an end-to-end signed message/cpim IM

From: "Burger, Eric" <[email-masked]>
Date: Thu, 3 Aug 2006 12:38:42 -0400

I would offer that end-to-end signed messages work: they prove the B2BUA
changed the message in the middle.

What if the message is end-to-end encrypted?  Then the whole thing blows
up.

Is this a bug or feature of CPIM?

-----Original Message-----
From: Hisham Khartabil [mailto:[email-masked]] 
Sent: Wednesday, August 02, 2006 4:50 AM
To: [email-masked]' WG
Cc: Miguel Garcia
Subject: [Simple] IMDN Issue 5: Changing an end-to-end signed
message/cpim IM

"   An intermediary that forwards an IM MAY change the recipient address
    in the CPIM To header field when forwarding (for example, a URI-List
    server changes the IM Recipient address from its own to the address
    of the final recipient of that IM for every copy it makes to be sent
    to the list members).  In this case, the intermediary MUST add an
    Original-To header field to the IM "

The problem is how to we handle message/cpim messages that are signed 
end to end?

I remember in earlier conversation with Miguel, I suggested to him 
covering the message/cpim case in his draft in sipping 
(draft-ietf-sipping-uri-list-message-07.txt). I believe this issue 
should be handled in that draft. Of course the same issue applies here 
since we are adding the Original-To header to the message/cpim part of 
the message.

Any comments?

Regards,
Hisham

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---

From: Hisham Khartabil <[email-masked]>
Date: Fri, 4 Aug 2006 08:33:30 +0200

Now that I think about it, if I send an IM to a URI-List server that 
will re-populate the To header, the original IM is actually sent to the 
URI-List server (the To header carries the URI-List server address). 
The final endpoint recipients are not known yet and therefore if the IM 
is signed, it is signed with the key of the URI-List server.

Some users do know who the final recipients are and can sign the 
message for those recipients, but then there has to be multiple bodies 
in that IM, one signed for each final recipients. This hardly makes 
sense. Why would u use the URI-List server in the first place if you do 
that.

Having said that, I am considering this issue closed.

Hisham

---

From: Aki Niemi <[email-masked]>
Date: Mon, 07 Aug 2006 09:49:04 +0300

> sense. Why would u use the URI-List server in the first place if you do 

No. Public-key crypto works so that only if you *encrypt*, you need to
use the recipient's public key, and do it individually per recipient.
You sign messages using your own private key. The recipients use your
public key to verify the signature.

Cheers,
Aki

---

From: Hisham Khartabil <[email-masked]>
Date: Mon, 7 Aug 2006 10:58:08 +0200

yes of course. I don't know what I was thinking :)

I think the only solution for this is to have the intermediary 
"consume" the message and create a new one that it signs itself (a 
B2BUA behaviour).

Hisham