Re: [Cfrg] Help with the use of contexts

Yaron Sheffer <yaronf.ietf@gmail.com> Tue, 17 January 2017 21:42 UTC

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To: Adam Langley <agl@imperialviolet.org>
References: <mailman.133.1484592340.4842.cfrg@irtf.org> <235ec588-9358-eeb1-9fa2-202409854afc@gmail.com> <CAMfhd9WhaWfwjsfWmL-meA5wrTGpC6OF0twO6gBBAMhT2fZaHw@mail.gmail.com>
From: Yaron Sheffer <yaronf.ietf@gmail.com>
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Date: Tue, 17 Jan 2017 23:41:54 +0200
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Subject: Re: [Cfrg] Help with the use of contexts
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On 17/01/17 18:12, Adam Langley wrote:
> On Tue, Jan 17, 2017 at 3:31 AM, Yaron Sheffer <yaronf.ietf@gmail.com> wrote:
>> Wide industry adoption of TLS 1.2 took around 10 years. So IMO saying "this
>> is solved in TLS 1.3" is not a good enough answer, if in the meantime we
>> will continue to see cross-protocol and cross-TLS-version attacks.
>
> If there are such attacks then they'll continue to be a problem for
> TLS 1.2 because only the Ed* schemes have the possibility of a context
> string. For ECDSA and RSA, the context still needs to be included in
> the signed message somehow. So TLS 1.2 needs a larger change than
> wiring up the context inputs of the Ed* schemes if you want to protect
> it anyway.

Correct, but this would still eliminate cross-version attacks between 
TLS 1.3 and TLS <= 1.2, for Ed*. In other words, there is (incremental) 
value in adding context opportunistically.

Thanks,
	Yaron