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

"Christopher Wood" <caw@heapingbits.net> Thu, 14 November 2019 16:01 UTC

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Date: Thu, 14 Nov 2019 08:01:34 -0800
From: "Christopher Wood" <caw@heapingbits.net>
To: "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 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. 

> 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.

> 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. 

Best,
Chris