Re: [TLS] Working Group Last Call for draft-ietf-tls-tls13-18

[Yoav Nir <[e-mail]>]

On 3 Nov 2016, at 16:31, Martin Rex <[e-mail]> wrote:

> Since then, I've seen exactly ZERO rationale why the cleartext contenttype,
> which has existed through SSLv3->TLSv1.2 would be a problem.  With the
> removal of renegotiation from TLSv1.3, it is even less of a problem to
> keep the contenttype in the clear.

Here’s some to get this to somewhat >0:

Most TLS 1.2 connections will have a few handshake records, followed by a couple of CCS records followed by a whole bunch of application records, followed possibly by a single Alert.

You only see more handshake records in two cases:
   1. The client decided to re-negotiate. That is exceedingly rare.
   2. The server decided a renegotiation is needed so it sent a HelloRequest followed by a handshake.

With visible content type, you can tell these two flows apart. What’s more, there is rarely any reason to do #2 unless the server decided that before accessing some resource, the client needs to present a certificate. You can verify that by counting the number and adding up the size of the handshake records. Having this happen usually means a privileged resource and a privileged user. That’s quite a bit of information to leak from a supposedly secure protocol. Yes, you won’t know the user identity, but you’ll know enough to associate privileged users with IP addresses, and you’ll know at what times they accessed privileged resources as opposed to something like a landing page. That’s meta-data, but it’s quite a lot of meta-data. If we can avoid leaking that, why not?

Yoav