Re: [TLS] WGLC for draft-ietf-tls-ticketrequests

"Christopher Wood" <caw@heapingbits.net> Thu, 14 November 2019 16:24 UTC

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Date: Thu, 14 Nov 2019 08:24:32 -0800
From: "Christopher Wood" <caw@heapingbits.net>
To: "Daniel Migault" <daniel.migault@ericsson.com>
Cc: "TLS@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-ticketrequests
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On Thu, Nov 14, 2019, at 8:19 AM, Daniel Migault wrote:
> Hi Chris, 
> 
> Thanks for the responses, please see my comments inline. 
> 
> Yours, 
> Daniel
> 
> On Thu, Nov 14, 2019 at 11:02 AM Christopher Wood <caw@heapingbits.net>; wrote:
> > On Thu, Nov 14, 2019, at 7:50 AM, Daniel Migault wrote:
> >  > Hi,
> >  > 
> >  > The current version is clearer than the previous one. However, as I
> >  > understand the document, it still seems very asymmetric in the sense
> >  > that it does not provide the client the ability to enforce a number. I
> >  > believe more guidances/specifications are needed on how to interpret the
> >  > count value. Interpretation is usually based on implicit assumptions of
> >  > today's usages, and explicit signaling should, in my opinion, be preferred. In
> >  > other words, I believe that long term interop will benefit from these
> >  > additional specifications.
> > 
> >  I disagree with this assessment. The document is clear on this:
> > 
> >  A supporting server MAY use TicketRequestContents.count when
> >  determining how many NewSessionTicket messages to send to a
> >  requesting client, and SHOULD place a limit on the number of tickets
> >  sent. The number of NewSessionTicket messages sent SHOULD be the
> >  minimum of the server's self-imposed limit and
> >  TicketRequestContents.count.
> > 
> >  As has been stated before, the count is a *hint* to the server, nothing more. 
> > 
> That is my point, in my opinion a hint is under specified. 

I acknowledge your opinion, but as I think I've made clear, I don't agree. Sorry!

> >  > The problem stated in the introduction is that the server needs some
> >  > information from the client in order to generate the appropriated number
> >  > of tickets. In fact the client is likely to be the one that better knows
> >  > the number of tickets to be generated, but the current text does not
> >  > enable the client to enforce that number. Instead this is entirely left
> >  > to the server.
> > 
> >  As above, I think you're misunderstanding the point of this document. Ticket requests are hints to servers.
> > 
> On the contrary, I think I understood it correctly. 
> >  > Typically, if a device does not want to have more than one ticket. It
> >  > should be able to indicate one and be sure it will only receive one
> >  > ticket. The current specification does not prevent multiple tickets to
> >  > be sent by the server. 
> > 
> >  Right, nor should it. Again, that's not a goal.
> > 
> >  > The server can ignore the count value (MAY) even 
> >  > though it is known to support the extension. This means that a server
> >  > could support the extension while still having a hard coded number of
> >  > tickets. In addition, (SHOULD) let the server determining the number of
> >  > tickets. 
> >  > 
> >  > Possible ways to address my concerns could be to limit the count value
> >  > to the number of tickets generated during the KEX, and a server MUST NOT
> >  > exceed the counter value. The text would need more guidance on how the
> >  > server SHOULD behave when emitting at different time in the KEX - after
> >  > the Finished message and after the post handshake authentication.
> > 
> >  I don't think any text changes are needed to address these comments.
> > 
> >  > The security consideration should in my opinion consider the fact that a
> >  > client over UDP/DTLS may use the count value as an amplification factor 
> >  > to have the server flooding a target. The current text only seems to
> >  > consider the computation aspect, not the bandwidth. If that cannot
> >  > happen, it might be beneficial to add it. However, when a server sends
> >  > tickets right after the Finished, it seems to me that can be used as an
> >  > attack.
> > 
> >  I'm not convinced this is useful to add. The target here is the client (attacker) that requested tickets. 
> The target is the spoofed source IP address.

That would imply the attacker is able to complete a connection with this spoofed address, no?

> >  Best,
> >  Chris
> > 
> >  _______________________________________________
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