Re: [xmpp] Gen-ART LC review of draft-ietf-uta-xmpp-05

[Peter Saint-Andre - &yet <[e-mail]>]

Hi Roni, thanks for the review.

On 4/3/15 3:59 AM, Roni Even wrote:
> I am the assigned Gen-ART reviewer for this draft. For background on
> Gen-ART, please see the FAQ at
> <http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.
>
> Please resolve these comments along with any other Last Call comments
> you may receive.
>
> Document: draft-ietf-uta-xmpp-05
>
> Reviewer: Roni Even
>
> Review Date:2015–4-3
>
> IETF LC End Date: 2015–4-13
>
> IESG Telechat date:
>
> Summary: This draft is not ready for publication as an Standard Track  RFC.
>
> Major issues:
>
> I am wondering why this document is a standard track and not
> Informational, reading it I get the impression that it repeats text from
> RFC6120 and does not provide new normative information.

Ah, I see the confusion.

Earlier versions of this document repeated the recommendations from 
draft-ietf-uta-tls-bcp and explicitly applied them to XMPP. At some 
point we refactored the document so that it wasn't repeating the text 
(because that's usually a bad idea - things can get out of sync etc.), 
but in the process we lost those normative statements.

To fix the problem, I suggest that we change the following paragraph and 
move it to be the first paragraph of Section 3:

OLD
       NOTE: Unless explicitly noted otherwise, all of the
       recommendations specified in [I-D.ietf-uta-tls-bcp] apply to XMPP.
       In the main, this document merely provides supplementary
       information; those who implement and deploy XMPP technologies are
       expected to follow the recommendations of [I-D.ietf-uta-tls-bcp].

NEW
       XMPP implementations and deployments MUST follow the
       recommendations provided in [I-D.ietf-uta-tls-bcp] unless
       explicitly noted otherwise herein.  Instead of repeating those
       recommendations here, this document mostly provides supplementary
       regarding implementation and deployment of XMPP technologies.

> Section 3.1 talks about TLS support and say that it SHOULD be tried  but
> since it is a SHOULD I assume that failure may happen and non TLS
> connections may be used ( I am not sure what RFC6120 say about it.

Section 3.1 attempts to say this (but clearly we did not phrase things 
very well): the lack of an indication that a connection endpoint 
supports TLS SHOULD NOT prevent a connecting entity from attempting TLS 
negotiation. In fact, I think we could say MUST NOT (the "SHOULD" comes 
from local policy).

Therefore I suggest the following rewording:

OLD
    The server (i.e., the XMPP receiving entity) to which a client or
    peer server (i.e., the XMPP initiating entity) connects might not
    offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
    :xmpp-tls'/>.  Although in general this stream feature indicates that
    the server supports XMPP 1.0 and therefore supports TLS, it is
    possible that this stream feature might be stripped out by an
    attacker (see Section 2.1 of [I-D.ietf-uta-tls-attacks]).  Therefore,
    the initiating entity SHOULD proceed with the stream negotiation even
    if the receiving entity does not advertise support for TLS.
    Similarly, although a receiving entity SHOULD include the <required/>
    child element to indicate that negotiation of TLS is mandatory, an
    initiating entity MUST NOT depend on receiving the <required/> flag
    in determining whether TLS will be enforced for the stream.

NEW
    The server (i.e., the XMPP receiving entity) to which a client or
    peer server (i.e., the XMPP initiating entity) connects might not
    offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
    :xmpp-tls'/>.  Although in general this stream feature indicates that
    the server supports XMPP 1.0 and therefore supports TLS, it is
    possible that this stream feature might be stripped out by an
    attacker (see Section 2.1 of [I-D.ietf-uta-tls-attacks]).
    Similarly, the <required/> child element of the <starttls/> stream
    feature is used to indicate that negotiation of TLS is mandatory,
    but could also be stripped out by an attacker. Therefore, the
    initiating entity MUST NOT be deterred from attempting TLS
    negotiation even if the receiving entity does not advertise support
    for TLS.  Instead, the initiating entity SHOULD (based on local
    policy) proceed with the stream negotiation and attempt to negotiate
    TLS.

> Section 3.4 may look like authentication is a MUST but section 3.5 talks
> about  unauthenticated connections

Well, there are client-to-server connections and server-to-server 
connections in XMPP. The former need to be authenticated and the latter 
do not (although we are working on ways to make authentication easier 
and more pervasive for server-to-server connections). I thought we had 
captured that in the text, but perhaps not.

> On section 3.7 I assume that providing e2e information is based on the
> XMPP architecture that may have only one server to server hop.  Are
> there other cases?

In Section 3.7 we weren't trying to say that information can be provided 
about the end-to-end encryption status of all the hops along a 
communications path because that's impossible (at least in a way that 
can be trusted). We might adjust the text to avoid any confusion, as 
follows:

OLD
    o  Determine if a client-to-server or server-to-server connection is
       encrypted and authenticated.
    o  Determine the version of TLS used for a client-to-server or
       server-to-server connection.

NEW
    o  Determine if a given incoming or outgoing XML stream is
       encrypted and authenticated using TLS.
    o  Determine the version of TLS used for a given stream.

Peter

-- 
Peter Saint-Andre
https://andyet.com/