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Re: [TLS] (strict) decoding of legacy_record_version?

Brian Smith <brian@briansmith.org> Tue, 08 November 2016 03:01 UTC

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From: Brian Smith <brian@briansmith.org>
Date: Mon, 07 Nov 2016 17:01:18 -1000
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To: David Benjamin <davidben@chromium.org>
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Cc: Matt Caswell <matt@openssl.org>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] (strict) decoding of legacy_record_version?
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David Benjamin <davidben@chromium.org> wrote:

> Once you've gotten as far as to switch to TLSCiphertext, I don't see a
> reason not to enforce. Keying on versions is problematic (which is why we
> avoided a transition to enforcement), but keying on whether the record is
> encrypted seems fine. I think it just didn't occur to us to base it on
> that. :-)
>

Since this field isn't included in the additional_data of the AEAD in TLS
1.3 any more, it isn't authenticated. That means an active MitM can use
this to transport up to 2 bytes of information hop-to-hop if the receiver
doesn't check it. That seems like a good reason to check it, and also to
check TLSCiphertext.opaque_type is application_data. Assuming this is the
reason, the reasoning should be explicitly called out because it is
non-obvious.

If that isn't a reason to do the check, then I don't think there's any
reason to mandate that implementations do it.

Cheers,
Brian
-- 
https://briansmith.org/

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