Re: [TLS] draft-ietf-tls-renegotation: next steps

[Eric Rescorla <[e-mail]>]

At Wed, 16 Dec 2009 10:53:37 +0100,
<[e-mail]> wrote:
> - The current draft doesn't clearly say what should be included in
> legacy (insecure) renegotiation ClientHellos. I am not sure if we have
> enough clear opinions to call consensus, but keeping it aligned with
> the initial ClientHello (either MSCV or extension) seems to be one
> simple approach (but I hope to see the actual text).

Attached is some candidate text that attempts to implement this. 

  It is possible that un-upgraded servers will request that the client
  renegotiate.  It is RECOMMENDED that clients refuse this
  renegotiation request.  Clients which do so MUST respond to such
  requests with a "no_renegotiation" alert [RFC 5246 requires this
  alert to be at the "warning" level.]  It is possible that the
  apparently un-upgraded server is in fact an attacker who is then
  allowing the client to renegotiate with a different, legitimate,
  upgraded server.  In order to detect this attack, clients which
  choose to renegotiate MUST provide either the
  TLS_RENEGO_PROTECTION_REQUEST SCSV or "renegotiation_info" in their
  ClientHello.  In a legitimate renegotiation with an un-upgraded
  server, either of these signals will be ignored by the server.
  However, if the server (incorrectly) fails to ignore extensions,
  sending the "renegotiation_info" extension may cause a handshake
  failure.  Thus, it is permitted, though NOT RECOMMENDED, for the
  client to simply send the SCSV.  This is the only situation in which
  clients are permitted to not send the "renegotiation_info" extension
  in a ClientHello which is used for renegotiation.

  Note that in the case of this downgrade attack attack above, if this
  is the initial handshake from the server's perspective, then use of
  the SCSV from the client precludes detection of this attack by the
  server.  However, the attack will be detected by the client when the
  server sends an empty "renegotiation_info" extension and the client
  is expecting one containing the previous verify data.  By contrast,
  if the client sends the "renegotiation_info" extension, then the
  server will immediately detect the attack.

After flip-flopping on this in my head a few times, however, my
personal view, is that I think this goes too far in the direction of
accomodating broken servers. Sending RI in this instance only creates
an interop problem when a server (1) is doing something we know to be
really unsafe and (2) can't even ignore extensions correctly. We've
seen a number of suggestions that we actually forbid renegotiation in
case (1) and while I suspect WG consensus doesn't go that far, it's not
clear to me that we need to not only allow it but also compensate for
servers which are broken in other respects. So, my preference would
be to simply mandate RI with the previous verify_data here as in
all other cases.


> I've asked the document editor to update the draft as soon as
> possible. The IESG will discuss this document this Thursday (December
> 17), and I hope we can have an approved specification before
> Christmas.

I'm working on revisions now.

-Ekr