Re: [tram] Eric Rescorla's Discuss on draft-ietf-tram-stunbis-16: (with DISCUSS and COMMENT)

[Eric Rescorla <[e-mail]>]

Not really. I have not received any response to my mail of Oct 23. As
before, I'm happy to have a call, but I believe we're reaching the limits
of what can be accomplished by email.

-Ekr


On Mon, Nov 19, 2018 at 6:35 AM Gonzalo Camarillo <
[e-mail]> wrote:

> Hi Spencer,
>
> what is the status of this? Are the authors and the document shepherd
> working with the relevant ADs on the discusses?
>
> https://datatracker.ietf.org/doc/draft-ietf-tram-stun-pmtud/ballot/
>
> Cheers,
>
> Gonzalo
>
> On 24-Oct-18 16:40, Spencer Dawkins at IETF wrote:
> > Hi, Marc,
> >
> > On Wed, Oct 24, 2018 at 3:44 AM Marc Petit-Huguenin <[e-mail]
> > <mailto:[e-mail]>> wrote:
> >
> >     Hi Spencer,
> >
> >     I sent my answers to Eric Rescorla questions on 2018-10-07 following
> >     an in-person meeting with my co-author, but never got a response
> >     back.  Because there was no change proposed by Eric I went ahead and
> >     published -19 a couple of weeks after that, with the text agreed in
> >     response to Adam's and Benjamin's comments.
> >
> >     https://www.ietf.org/mail-archive/web/tram/current/msg02635.html
> >
> >
> > Ah - I wonder if that was what had happened.
> >
> > It sounds like you did the right thing, and that Eric has now responded,
> > which is also the right thing to do.
> >
> > Thanks for helping me understand.
> >
> > Spencer
> >
> >
> >     On 10/23/18 7:28 PM, Spencer Dawkins at IETF wrote:
> >     > Hi, Marc,
> >     >
> >     > I see that a -19 has been submitted, but didn't see a reply from
> >     Eric in
> >     > this thread. Do you think that you've converged?
> >     >
> >     > (I saw an offer of a conference call, so thought an out-of-band
> >     > conversation might have happened)
> >     >
> >     > Thanks,
> >     >
> >     > Spencer
> >     >
> >     > On Sun, Oct 7, 2018 at 9:35 AM Marc Petit-Huguenin
> >     <[e-mail] <mailto:[e-mail]>> wrote:
> >     >
> >     >> Hi Eric,
> >     >>
> >     >> Please see inline.
> >     >>
> >     >> On 09/10/2018 03:25 AM, Eric Rescorla wrote:
> >     >>> On Sat, Sep 8, 2018 at 2:31 PM, Marc Petit-Huguenin
> >     <[e-mail] <mailto:[e-mail]>>
> >     >>> wrote:
> >     >>>
> >     >>>> Hi Eric,
> >     >>>>
> >     >>>> Apologies for the delay in getting back to that.
> >     >>>>
> >     >>>> I think that there is some misunderstanding in what STUNbis is
> >     trying to
> >     >>>> do, so please see my comments inline.
> >     >>>>
> >     >>>> On 06/18/2018 10:43 AM, Eric Rescorla wrote:
> >     >>>>> Hi folks,
> >     >>>>>
> >     >>>>> I've reviewed the new version, but I don't think that the
> biddown
> >     >>>>> discussion makes much sense.
> >     >>>>>
> >     >>>>> To recap, there are two hashes here:
> >     >>>>>
> >     >>>>> - The hash which you use to store the password with (currently
> >     mostly
> >     >>>> MD5)
> >     >>>>> - The hash you use to compute the MAC (currently SHA-1).
> >     >>>>>
> >     >>>>> First, let's stipulate that MD5 isn't a great choice here,
> >     though SHA-1
> >     >>>>> isn't a great choice
> >     >>>>> either for pwd hashing You want Argon or the like. With that
> said,
> >     >>>> there's
> >     >>>>> no sensible
> >     >>>>> biddown attack on that hash because it's a per-server feature,
> >     not a
> >     >>>>> per-transaction
> >     >>>>> feature. So, as long as the server has MD5-hashed passwords,
> the
> >     >>>> situation
> >     >>>>> is bad.
> >     >>>>
> >     >>>> In no place in STUNbis we are proposing to use SHA-1 for
> password
> >     >>>> encryption, so I am not sure where that come from.  What we
> >     propose is:
> >     >>>>
> >     >>>
> >     >>> You're right, it's SHA-256, but my criticisms apply equally
> there.
> >     >>
> >     >> SHA-256 was what the WG adopted.  Our attempts to add other
> passwords
> >     >> encryption mechanisms were denied.  It is true that Argon2 was
> >     not in that
> >     >> list (in our defense Argon2 was not known before July 2015), but
> >     I do not
> >     >> see why the WG would have accepted that one over the others.
> >     Anyway it is
> >     >> too late to fix this, as it is my understanding that the WG does
> >     not have
> >     >> enough energy to reach consensus on a new password algorithm.
> >     Someone can
> >     >> just write a draft adding Argon2 as password encryption, as we
> >     will have a
> >     >> IANA registry for that.
> >     >>
> >     >>>
> >     >>>
> >     >>>> - Establish a registry for new password algorithms (section
> >     18.5), so
> >     >>>> algorithms like Argon2 could be added later (but note that our
> own
> >     >>>> proposals to add more password algorithms were rejected by the
> >     working
> >     >>>> group).
> >     >>>> - Add a new password algorithm to that registry, namely SHA-256.
> >     >>>> - Register MD5 as an initial password algorithm for backward
> >     >> compatibility
> >     >>>> purpose.
> >     >>>>
> >     >>>> As for the biddown protection itself, it is my recollection
> that it
> >     >>>> happened more or less like that:
> >     >>>>
> >     >>>>
> >     >>>> INT. MEETING ROOM - DAY
> >     >>>>
> >     >>>> One of the co-editors of STUNBis stands at the microphone:
> >     >>>>
> >     >>>>                            CO-EDITOR
> >     >>>>                  We added SHA-256 protection for passwords
> >     >>>>                  in STUNBis.
> >     >>>>
> >     >>>>                            SOMEONE (V.O.)
> >     >>>>                  As MD5 still need to be supported, you need to
> add
> >     >>>>                  protection for bid-down attacks.
> >     >>>>
> >     >>>> CLOSE-UP on CO-EDITOR ROLLING HIS EYES
> >     >>>>
> >     >>>>                            CO-EDITOR
> >     >>>>                  OK, I'll work on that.
> >     >>>>
> >     >>>> Four to eight months has passed.
> >     >>>>
> >     >>>> INT. ANOTHER MEETING ROOM - DAY
> >     >>>>
> >     >>>> The same co-editor of STUNBis stands at the microphone:
> >     >>>>
> >     >>>>                            CO-EDITOR
> >     >>>>                  We added a nice mechanism to prevent bid-down
> >     >>>>                  attacks on passwords.  Any comments?
> >     >>>>
> >     >>>>                            THE ROOM
> >     >>>>                  (silence)
> >     >>>>
> >     >>>>                            CO-EDITOR
> >     >>>>                  Moving on...
> >     >>>>
> >     >>>
> >     >>> I don't see how any of this is relevant to my technical points
> >     above.
> >     >>
> >     >> My point was that we, the co-editors, did not decide on adding
> >     bid-down
> >     >> protection, someone asked us to do so and no-one in the WG saw
> >     any problem
> >     >> with that.  The reasons that person wanted that are not known to
> >     us but, as
> >     >> you insist, here's one reason I can think of:
> >     >>
> >     >> It is a fact that, for operational reasons, a password database
> >     cannot be
> >     >> re-encrypted at once.  Also for operational reasons, the MD5
> password
> >     >> cannot be immediately removed from the database as soon the user
> >     submitted
> >     >> a new one.  In fact, and for quite some time, both encrypted
> >     variants of
> >     >> the same password may have to be kept in that database, because a
> >     single
> >     >> user may use a mix of devices, some of them that use an RFC 5389
> >     client,
> >     >> some that use a STUNbis client.  It is up to the STUN server
> owner to
> >     >> decide how long the migration to STUNbis will take and when it
> >     will be
> >     >> acceptable to reject all RFC 5389 (i.e. MD5) clients (that
> >     migration time
> >     >> can be purposely reduced to 0 seconds but that's the choice and
> >     >> responsibility of the owner of the server).
> >     >>
> >     >> Meanwhile we still need to be sure that if the STUN client is
> >     implementing
> >     >> STUNbis it unconditionally gets the additional protection of the
> new
> >     >> password encryption algorithm.  That's where the biddown
> >     protection kicks
> >     >> in, by preventing an online attacker to have the server
> >     misidentifying a
> >     >> STUNbis client as an RFC 5389 client, by preventing an online
> >     attacker to
> >     >> have the client misidentifying a STUNbis server as a RFC 5389
> >     server, and
> >     >> having both them use the MD5 encrypted password instead of the
> >     SHA-256
> >     >> encrypted password, all of that easily done by stripping the
> >     unprotected
> >     >> 401 response of the new STUNbis PASSWORD-ALGORITHMS attribute.
> >     >>
> >     >>>
> >     >>>
> >     >>>
> >     >>>>> The second topic is the hash used to compute the MAC. However,
> >     I don't
> >     >>>> see
> >     >>>>> how
> >     >>>>> this gives you sensible biddown protection because that hash
> >     is also
> >     >> used
> >     >>>>> to compute
> >     >>>>> MAC over the negotiation: an attacker who has compromised a
> >     MAC which
> >     >> the
> >     >>>>> server
> >     >>>>> supports will quite likely be able to forge a MAC over the
> >     transcript
> >     >> as
> >     >>>>> well. This is,
> >     >>>>> I suppose, potentially useful as a defense against some other
> >     weakness
> >     >>>>> (e.g.,
> >     >>>>> version #), but I don't really see how the current design
> >     helps against
> >     >>>>> attacks on the
> >     >>>>> MAC.
> >     >>>>
> >     >>>> There is no biddown attack protection for the MAC, as stated in
> >     Section
> >     >>>> 16.3:
> >     >>>>
> >     >>>> "The bid-down protection mechanism described in this document
> >     is new,
> >     >>>>  and thus cannot currently protect against a bid-down attack
> that
> >     >>>>  lowers the strength of the hash algorithm to HMAC-SHA1."
> >     >>>>
> >     >>>> What we put in place is a plan for *future* versions of STUN to
> get
> >     >>>> biddown protection for the MAC.  That's it, no more, no less.
> >     >>>>
> >     >>>
> >     >>> Yes, but I don't believe that this will provide bid-down
> >     protection for
> >     >> the
> >     >>> MAC in the future for the reasons I indicate above.
> >     >>>
> >     >>> If you think this does something useful, please show me an
> >     example attack
> >     >>> and how this fixes it. Note that it's not generally useful to
> >     just bid
> >     >> down
> >     >>> the MAC itself unless the MAC you bid down to is weak enough to
> >     exploit
> >     >> in
> >     >>> some other way.
> >     >>
> >     >> I do not know what weaknesses will be discovered in the future.
> >     I am also
> >     >> pretty sure that the cost of not using that mechanism is very
> >     close to 0.
> >     >> What I am sure of is that the cost of reengineering a new biddown
> >     >> protection mechanism if we ever need it will be high.  We already
> >     went
> >     >> through the pains of designing one for the password algorithm, so
> >     why not
> >     >> extend it so it can be used in the aftermath of the next Snowden
> >     facepalm
> >     >> moment?
> >     >>
> >     >>> Again, happy to have a call to walk though this if that
> >     >>> helps.
> >     >>>
> >     >>> -Ekr
> >     >>>
> >     >>>
> >     >>>
> >     >>>
> >     >>>>>
> >     >>>>> You might think that there was a MAC which was easier to
> >     reverse to
> >     >> find
> >     >>>>> the original
> >     >>>>> password, but the defense you have here doesn't help with that
> >     because
> >     >>>> the
> >     >>>>> on-path attacker can do a bid-down and use the client as a MAC
> >     oracle
> >     >> for
> >     >>>>> any MAC the client supports.
> >     >>>>>
> >     >>>>> So, I still don't understand what this is supposed to do.
> Happy to
> >     >> have a
> >     >>>>> call if you
> >     >>>>> think that helps
> >     >>>>>
> >     >>>>> -Ekr
> >     >>>>>
> >     >>>>>
> >     >>>>>
> >     >>>>>
> >     >>>>> On Mon, Jun 18, 2018 at 3:40 AM, Gonzalo Camarillo <
> >     >>>>> [e-mail]
> >     <mailto:[e-mail]>> wrote:
> >     >>>>>
> >     >>>>>> Marc, Eric,
> >     >>>>>>
> >     >>>>>> what is the status of this discussion?
> >     >>>>>>
> >     >>>>>> Thanks,
> >     >>>>>>
> >     >>>>>> Gonzalo
> >     >>>>>>
> >     >>>>>> On 04/05/2018 2:35 AM, Eric Rescorla wrote:
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> On Mon, Apr 23, 2018 at 11:16 AM, Marc Petit-Huguenin <
> >     >>>> [e-mail] <mailto:[e-mail]>
> >     >>>>>>> <mailto:[e-mail] <mailto:[e-mail]>>> wrote:
> >     >>>>>>>
> >     >>>>>>>     On 04/22/2018 05:22 PM, Eric Rescorla wrote:
> >     >>>>>>>     > On Sun, Apr 22, 2018 at 2:02 PM, Marc Petit-Huguenin <
> >     >>>>>> [e-mail] <mailto:[e-mail]>
> >     <mailto:[e-mail] <mailto:[e-mail]>>>
> >     >>>>>>>     > wrote:
> >     >>>>>>>     >
> >     >>>>>>>     >>
> >     >>>>>>>     >>>>      For a request or indication message, the agent
> >     MUST
> >     >>>>>> include the
> >     >>>>>>>     >>>>      USERNAME, MESSAGE-INTEGRITY-SHA256, and
> >     >> MESSAGE-INTEGRITY
> >     >>>>>>>     >> attributes
> >     >>>>>>>     >>>>      in the message unless the agent knows from an
> >     external
> >     >>>>>> indication
> >     >>>>>>>     >>>>      which message integrity algorithm is supported
> >     by both
> >     >>>>>> agents.  In
> >     >>>>>>>     >>>>      this case either MESSAGE-INTEGRITY or
> >     >>>>>> MESSAGE-INTEGRITY-SHA256 MUST
> >     >>>>>>>     >>>>      be included in addition to USERNAME.  The HMAC
> >     for the
> >     >>>>>> MESSAGE-
> >     >>>>>>>     >>>
> >     >>>>>>>     >>> This text appears to conflict with S 7.3 of
> >     5245-bis, which
> >     >>>> says:
> >     >>>>>>>     >>
> >     >>>>>>>     >> [text missing]
> >     >>>>>>>     >>
> >     >>>>>>>     >> Hmm, no, RFC245bis is still referencing RFC5389, so
> the
> >     >>>> procedure
> >     >>>>>> for
> >     >>>>>>>     >> stunbis does not apply.
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     > I hear what you're saying, but publishing two
> >     documents at the
> >     >>>>>> same time
> >     >>>>>>>     > which
> >     >>>>>>>     > make contrary recommendations about the same basic
> >     protocol is
> >     >>>>>> un-good.
> >     >>>>>>>
> >     >>>>>>>     Sure, but wouldn't it be simpler to have rfc5245bis
> >     using stunbis
> >     >>>>>>>     and have them updating their text, more than adding some
> >     tortuous
> >     >>>>>>>     text in stunbis?
> >     >>>>>>>
> >     >>>>>>>     >
> >     >>>>>>>     >
> >     >>>>>>>     >>>      The STUN usage must specify which transport
> >     protocol is
> >     >>>>>> used, and
> >     >>>>>>>     >> how
> >     >>>>>>>     >>>>      the agent determines the IP address and port
> >     of the
> >     >>>>>> recipient.
> >     >>>>>>>     >>>>      Section 8 describes a DNS-based method of
> >     determining
> >     >> the
> >     >>>>>> IP
> >     >>>>>>>     >> address
> >     >>>>>>>     >>>>      and port of a server that a usage may elect to
> >     use.
> >     >> STUN
> >     >>>>>> may be
> >     >>>>>>>     >> used
> >     >>>>>>>     >>>>      with anycast addresses, but only with UDP and
> >     in usages
> >     >>>>>> where
> >     >>>>>>>     >>>>      authentication is not used.
> >     >>>>>>>     >>>
> >     >>>>>>>     >>> Why this restriction? You should be able to use
> >     anycast with
> >     >>>>>> long-term
> >     >>>>>>>     >>> AUTH for (say) TURN.
> >     >>>>>>>     >>
> >     >>>>>>>     >> https://www.ietf.org/mail-archive/web/behave/current/
> >     >>>>>> msg03582.html
> >     >>>>>>>     <https://www.ietf.org/mail-archive/web/behave/current/
> >     >>>> msg03582.html>
> >     >>>>>>>     >>
> >     >>>>>>>     >> I think that the decision of permitting Anycast
> >     should be left
> >     >>>> to
> >     >>>>>> each
> >     >>>>>>>     >> STUN Usage.  Basic STUN Usage does not use
> >     authentication and
> >     >>>> use
> >     >>>>>> only a
> >     >>>>>>>     >> one round trip for the Binding transaction, so
> >     Unicast can be
> >     >>>>>> used.
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     >> OTOH, TURN and ICE should probably say something
> >     about that,
> >     >> so
> >     >>>> I
> >     >>>>>> added a
> >     >>>>>>>     >> new bullet point in Section 13:
> >     >>>>>>>     >>
> >     >>>>>>>     >>    o  If Anycast addresses can be used for the server
> >     in case
> >     >>>> TCP
> >     >>>>>> or
> >     >>>>>>>     >>       TLS-over-TCP, or authentication are used.
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     > Are you leaving this text in? That seems very
> confusing.
> >     >>>>>>>
> >     >>>>>>>     In isolation yes, but I think it makes sense which the
> text
> >     >> before
> >     >>>>>>>     the bullet points:
> >     >>>>>>>
> >     >>>>>>>        A STUN usage defines how STUN is actually utilized --
> >     when to
> >     >>>> send
> >     >>>>>>>        requests, what to do with the responses, and which
> >     optional
> >     >>>>>>>        procedures defined here (or in an extension to STUN)
> >     are to be
> >     >>>>>> used.
> >     >>>>>>>        A usage also defines:
> >     >>>>>>>
> >     >>>>>>>     [...]
> >     >>>>>>>
> >     >>>>>>>        o  If Anycast addresses can be used for the server in
> >     case TCP
> >     >>>> or
> >     >>>>>>>           TLS-over-TCP, or authentication are used.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> What is the need for the restriction at all.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>     >>>>      transaction over UDP or DTLS-over-UDP is also
> >     >> considered
> >     >>>>>> failed if
> >     >>>>>>>     >>>>      there has been a hard ICMP error [RFC1122].
> For
> >     >> example,
> >     >>>>>> assuming
> >     >>>>>>>     >> an
> >     >>>>>>>     >>>>      RTO of 500ms, requests would be sent at times
> >     0 ms, 500
> >     >>>>>> ms, 1500
> >     >>>>>>>     >> ms,
> >     >>>>>>>     >>>>      3500 ms, 7500 ms, 15500 ms, and 31500 ms.  If
> the
> >     >> client
> >     >>>>>> has not
> >     >>>>>>>     >>>>      received a response after 39500 ms, the client
> >     will
> >     >>>>>> consider the
> >     >>>>>>>     >>>>      transaction to have timed out.
> >     >>>>>>>     >>>
> >     >>>>>>>     >>> I note that these recommendations now seem crazily
> >     long. I
> >     >>>>>> assume the
> >     >>>>>>>     >>> WG had consensus on this, but I wanted to note it.
> >     >>>>>>>     >>
> >     >>>>>>>     >> Not just the WG, also the IESG that approved RFC 5389
> >     too as,
> >     >>>> but
> >     >>>>>> for the
> >     >>>>>>>     >> addition of "or DTLS-over-UDP", this is the same text
> >     than in
> >     >>>> RFC
> >     >>>>>> 5389.
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     > Yes, I know. My point is that while they might have bee
> >     >> sensible
> >     >>>>>> when
> >     >>>>>>>     > 5389 was published they *now* seem crazily long. Did
> >     the WG
> >     >>>>>> explicitly
> >     >>>>>>>     > decide not to update them?
> >     >>>>>>>
> >     >>>>>>>     No, nobody ever suggested that there was an issue there.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> OK, well, this seems like it should be considered, then,
> >     because this
> >     >>>>>>> doesn't
> >     >>>>>>> match modern practice.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>     > This would be good to explain.
> >     >>>>>>>
> >     >>>>>>>     New text:
> >     >>>>>>>
> >     >>>>>>>        [...]
> >     >>>>>>>        containing the subjectAltName of that certificate.
> >     The test
> >     >> on
> >     >>>>>> the
> >     >>>>>>>        MESSAGE-INTEGRITY-SHA256 attribute indicates that the
> >     >>>> transaction
> >     >>>>>> is
> >     >>>>>>>        authenticated and that the client implements this
> >     >> specification
> >     >>>>>> and
> >     >>>>>>>        so can process the ALTERNATE-DOMAIN attribute.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> All right.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>     >
> >     >>>>>>>     >
> >     >>>>>>>     >>>>      o  What authentication and message-integrity
> >     mechanisms
> >     >>>>>> are used.
> >     >>>>>>>     >>>>
> >     >>>>>>>     >>>>      o  The considerations around manual vs.
> >     automatic key
> >     >>>>>> derivation
> >     >>>>>>>     >> for
> >     >>>>>>>     >>>>         the integrity mechanism, as discussed in
> >     [RFC4107].
> >     >>>>>>>     >>>>
> >     >>>>>>>     >>>>      o  What mechanisms are used to distinguish STUN
> >     >> messages
> >     >>>>>> from other
> >     >>>>>>>     >>>
> >     >>>>>>>     >>> Why is this required? It seems like that's a generic
> >     STUN
> >     >>>>>> feature.
> >     >>>>>>>     >>
> >     >>>>>>>     >> That text is identical to the text in RFC 5389.  RFC
> >     5764/7983
> >     >>>> is
> >     >>>>>> one such
> >     >>>>>>>     >> mechanism, but there is nothing that prevent another
> >     protocol
> >     >>>>>> stack to use
> >     >>>>>>>     >> a different mechanism (inference, shim, etc...)
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     > But ultimately no matter what the other protocol
> >     provides for
> >     >>>>>> demux, STUN
> >     >>>>>>>     > has its
> >     >>>>>>>     > own demux.
> >     >>>>>>>
> >     >>>>>>>     In fact I think that the reason for that item was because
> >     >>>>>>>     FINGERPRINT can also be used to demux STUN traffic, but
> >     it is
> >     >>>>>>>     optional.  So an STUN Usage needs to tell if FINGERPRINT
> is
> >     >>>>>>>     mandatory (like in ICE).
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> This should be explained.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>     >>>
> >     >>>>>>>     >>>>      that is not readily subject to offline
> dictionary
> >     >>>> attacks.
> >     >>>>>>>     >>>>      Protection of the channel itself, using TLS or
> >     DTLS,
> >     >>>>>> mitigates
> >     >>>>>>>     >> these
> >     >>>>>>>     >>>>      attacks.
> >     >>>>>>>     >>>>
> >     >>>>>>>     >>>>      STUN supports both MESSAGE-INTEGRITY and
> >     >>>>>>>     MESSAGE-INTEGRITY-SHA256,
> >     >>>>>>>     >>>>      which is subject to bid down attacks by an
> on-path
> >     >>>>>> attacker.
> >     >>>>>>>     >>>
> >     >>>>>>>     >>> By an on-path attacker who can forge HMAC-SHA1 in
> >     real-time?
> >     >>>>>>>     That's a
> >     >>>>>>>     >>> pretty serious adversary, so you should clarify here
> >     >>>>>>>     >>>
> >     >>>>>>>     >>
> >     >>>>>>>     >> New text:
> >     >>>>>>>     >>
> >     >>>>>>>     >>    STUN supports both MESSAGE-INTEGRITY and
> >     >>>>>> MESSAGE-INTEGRITY-SHA256,
> >     >>>>>>>     >>    which is subject to bid down attacks by an on-path
> >     attacker
> >     >>>>>> that
> >     >>>>>>>     >>    would strip the MESSAGE-INTEGRITY-SHA256 attribute
> >     leaving
> >     >>>>>>>     only the
> >     >>>>>>>     >>    MESSAGE-INTEGRITY attribute and exploiting a
> potential
> >     >>>>>>>     vulnerability.
> >     >>>>>>>     >>    Protection of the channel itself, using TLS or
> DTLS,
> >     >>>> mitigates
> >     >>>>>>>     these
> >     >>>>>>>     >>    attacks.  Timely removal of the support of
> >     >> MESSAGE-INTEGRITY
> >     >>>>>> in a
> >     >>>>>>>     >>    future version of STUN is necessary.
> >     >>>>>>>     >>
> >     >>>>>>>     >
> >     >>>>>>>     > I still don't understand the capabilities you seem to
> >     believe
> >     >> the
> >     >>>>>>>     attacker
> >     >>>>>>>     > has.
> >     >>>>>>>     > Can you describe the exact attack.
> >     >>>>>>>     >
> >     >>>>>>>
> >     >>>>>>>     1. Vulnerability is found in HMAC-SHA1
> >     >>>>>>>     2. Client Alice still supports M-I and M-I-256, does not
> >     know
> >     >> what
> >     >>>>>>>     version of STUN server Bob supports and so send both.
> >     >>>>>>>     3. On-path attacker removes M-I-256.
> >     >>>>>>>     5. stunbis server Bob thinks that Alice is an RFC 5389
> >     client and
> >     >>>>>>>     continue with that protocol.
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>> This seems like an extremely weak attack. In general, any
> >     protocol of
> >     >>>>>>> this type is as strong
> >     >>>>>>> as the weakest integrity algorithm it supports, so it's not
> >     that the
> >     >>>>>>> protocol has a downgrade
> >     >>>>>>> attack, but rather that the minimum algorithm supported is
> >     one you
> >     >>>> don't
> >     >>>>>>> trust as much
> >     >>>>>>> as you might like.
> >     >>>>>>>
> >     >>>>>>> -Ekr
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>>     --
> >     >>
> >
> >
> >
> >     --
> >     Marc Petit-Huguenin
> >     Email: [e-mail] <mailto:[e-mail]>
> >     Blog: https://marc.petit-huguenin.org
> >     Profile: https://www.linkedin.com/in/petithug
> >
>