Subject: [Sip] draft-ietf-sip-certs-01, end-to-end securiy issue?

From: "Erik Ehrlund" <[email-masked]>
Date: Sat, 23 Sep 2006 16:05:21 +0200 (CEST)

Hello,

I have a question about how the routing of messages between the client and
server is done when the client downloads its credentials in
draft-ietf-sip-certs-01.

In Section "7.15.  Behavior of a Proxy Server" its said that the behavior
is the same as in Section "6.13" which says " There are no additional
requirements on a SIP Proxy, other than to transparently forward the
SUBSCRIBE and NOTIFY requests as required in SIP...."

My question comes from the fact that it's said earlier that "Bob's UA
(Bob2) does a TLS [11] handshake with the credential server to
authenticate that the UA is connected to the correct credential
server...." and later in Security Considerations "First it forms a TLS
connection to the Server, which provides integrity and privacy protection
and also authenticates the server to Bob's UA."

How can regular SIP routing with proxies be done with end to end transport
layer security? ( the proxies between the client and server won't be able
to read the SIP headers )

As I understand it end-to-end security between the SIP UA and Credential
Server is not easily accomplished when SIP Proxy routing is used. Thus, I
see two approaches, but none of them seems satisfactory:
* Require that there can be no SIP proxy between the SIP UA and the
  SIP proxy co-located with the Credential Server. This approach seems less
  attractive, since prohibiting SIP proxying for a specific SIP message
  can lead to problems in some environments.
* Allow the SIP proxy to route the messages to a backend Credential
  Server. Then the TLS session will be established between the SIP UA and
  the first SIP proxy, and some other security mechanism will be used to
  protect the communication between the SIP proxy and the Credential Server.
  The communication between the SIP UA and the Credential Server will be
  readable (clear-text or protected by weak password) at the SIP Proxy.
  Thus, we will not have any end-to-end protection of the data (private
  key, certificates) that is downloaded from the Credential Server. In cases
  when the user not trust the SIP proxy enough, this approach is not
  satisfying.

Best Regards
Erik Ehrlund

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