Re: [TLS] Encryption of TLS 1.3 content type

Nikos Mavrogiannopoulos <nmav@redhat.com> Mon, 28 July 2014 15:14 UTC

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From: Nikos Mavrogiannopoulos <nmav@redhat.com>
To: Colm =?ISO-8859-1?Q?MacC=E1rthaigh?= <colm@allcosts.net>
Date: Mon, 28 Jul 2014 17:14:16 +0200
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Subject: Re: [TLS] Encryption of TLS 1.3 content type
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On Mon, 2014-07-28 at 07:54 -0700, Colm MacCárthaigh wrote:
> On Mon, Jul 28, 2014 at 1:55 AM, Nikos Mavrogiannopoulos
> <nmav@redhat.com> wrote:
> > Are there any pointers to these attacks? Will these attacks be countered
> > with such a change? I believe not as alert messages consist of only two
> > bytes and will be distinct from any other higher protocol messages
> > transferred by the TLS record protocol.
> 
> This is not the case. Firstly, even with a stream cipher it doesn't
> distinguish the message from a 2 byte application data message. But
> most ciphers are block ciphers, and so the true length is fuzzed by
> about an order of magnitude.

On the contrary, all _new_ ciphers are stream (GCM, CCM) and every
future AEAD cipher will be a stream one (due to limitations of the AEAD
construction in TLS). Furthermore, even if you assume that only AES-CBC
is going to be used, do you really consider a 16-byte alert packet
fuzzed? Do you know many application protocols that transfer such short
packets?

regards,
Nikos