Re: [Unbearable] I-D Action: draft-ietf-tokbind-tls13-0rtt-02.txt

John Bradley <> Thu, 29 June 2017 19:02 UTC

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From: John Bradley <>
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Date: Thu, 29 Jun 2017 15:02:05 -0400
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Cc: Leif Johansson <>, Benjamin Kaduk <>, IETF Tokbind WG <>,
To: Nick Harper <>
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Subject: Re: [Unbearable] I-D Action: draft-ietf-tokbind-tls13-0rtt-02.txt
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> On Jun 29, 2017, at 2:33 PM, Nick Harper <> wrote:
> On Thu, Jun 29, 2017 at 2:05 AM, Leif Johansson < <>> wrote:
>> On 2017-06-29 04:48, Benjamin Kaduk wrote:
>>> On Wed, Jun 28, 2017 at 03:25:13PM -0700, Nick Harper wrote:
>>>> Here's a summary of the changes since the last draft:
>>>> - If TB is accepted in 0-RTT data, keep using the early exporter for
>>>> the whole connection. There was some discussion on this in Chicago,
>>>> with more on the mailing list. Chairs, can you confirm whether we
>>>> reached consensus on the mailing list or whether we should take a hum
>>>> in Prague?
>>> I am a WG chair, but not a tokbind chair, but that question does not
>>> seem to make sense.  Consensus must be reached (or confirmed) on the
>>> mailing list, so deciding there wasn't enough feedback on the list and
>>> going to an in-room hum seems backwards, procedurally.
>> Judging consensus is sometimes tricky. I think what Nick meant was that
>> we may want to do a hum in Prague /in addition to/ seeking confirmation
>> on the list.
> It is unclear to me whether consensus was reached (hence deferring to
> the chairs on that judgement). If we don't have consensus, I'm fine
> continuing the discussion on list and in Prague. Let me check that I
> understand the process properly: first, discuss (on list and possibly
> in person), then if the discussion sounds like it's reached a
> consensus, optionally hum in person, and then confirm consensus on the
> list. Does that sound about right?

>>>> - 0-RTT TB cannot be used with externally provisioned PSKs or with a
>>>> PSK-only key exchange mode
>>>> - A new TLS extension is used for negotiating and indicating use of 0-RTT TB
>>>> - The replay indication TLS extension has been removed
>>> Some discussion on the httpbis list brought up that this document should
>>> mandate that 0-RTT token binding is only used in conjunction with
>>> a TLS stack that provides strong anti-replay protections (i.e., zero
>>> additional replays possible and one retransmission via DKG attack).  In other
>>> words, the time-based scheme of (draft-02) section 6.4 should be removed,
>>> and perhaps 6.3.1 reworded somewhat.
> I read over the "New Version Notification for
> draft-thomson-http-replay-00.txt" thread on the httpbis list. My
> understanding of the issue raised on that list is simply that there
> are attacks on 0-RTT Token Binding if there isn't a strict global
> anti-replay mechanism. There are still attacks possible on 0-RTT Token
> Binding even if there is a strict global anti-replay mechanism. I need
> to consider what additional attacks are possible without such a
> mechanism, and if any of those attacks require less privileges than
> the attacks possible with the mechanism.
>>> (It also brought up multiple peoples' sentiments that 0-RTT token binding
>>> is a bad idea in general, but this may not be procedurally the right time
>>> to have that discussion.)
>>> -Ben
> I'm aware that multiple people think that 0-RTT token binding is a bad
> idea in general. So far, the impression I've gotten of this sentiment
> is "0-RTT Token Binding is not something I have any interest in
> implementing". If the sentiment is closer to "0-RTT Token Binding is
> such a horrible thing that no one should be implementing it", then
> maybe we should discuss that sooner.

A good point,  if the attacks are still possible then supporting 0-RTT is problematic.  
On the other hand 0-RTT is a big speed boost for resumed connections.  It is hard to imagine that servers are going to disable it to use token binding over the long term.

I don’t believe the WG has any real consensus on this yet.   The draft spec is helpfull for us to discuss the issues.   

I encourage more discussion on the list on the topic of the draft and if this is the correct way to deal with 0-rtt if there are counter opinions.

John B.