Re: [TLS] Sending Custom DHE Parameters in TLS 1.3

Michael D'Errico <mike-list@pobox.com> Mon, 12 October 2020 18:06 UTC

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Date: Mon, 12 Oct 2020 14:06:31 -0400
From: "Michael D'Errico" <mike-list@pobox.com>
To: "TLS List" <tls@ietf.org>
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Subject: Re: [TLS] Sending Custom DHE Parameters in TLS 1.3
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Would it be possible to define a new dual-DH exchange
where you do one with server-supplied parameters, and
a second one with client-supplied parameters, so if one
is broken or sabotaged the connection is still secure?

Mike


On Mon, Oct 12, 2020, at 13:35, Blumenthal, Uri - 0553 - MITLL wrote:
> I suggest that custom parameters should be allowed, and documented as 
> completely under user/administrator responsibility.
> 
> Ensuring that a custom modulus is not "too small" or "too large" (etc.) 
> in that case is not your problem or your business.
> 
> 
> 
> On 10/12/20, 13:32, "TLS on behalf of Ilari Liusvaara" 
> <tls-bounces@ietf.org on behalf of ilariliusvaara@welho.com> wrote:
> 
>     On Mon, Oct 12, 2020 at 12:36:06PM -0400, Michael D'Errico wrote:
>     > 
>     > It appears that there may be a need to revert to the
>     > old way of sending Diffie-Hellman parameters that
>     > the server generates.  I see that TLS 1.3 removed
>     > this capability*; is there any way to add it back?
> 
>     The Diffie-Hellman support in TLS 1.2 is severly broken. There is no
>     way to use it safely on client side. This has lead to e.g., all the web
>     browers to remove support for it.
> 
>     There is no way to ensure that the parameters sent are not totally
>     broken, e.g.:
> 
>     - Modulus too small.
>     - Modulus too large.
>     - Modulus not prime (has been used as a backdoor!).
>     - Modulus is weak (possibly backdoored).
>     - Subgroup order does not have large prime factor.
> 
>     Even checking the third would require primality test, and primality
>     tests at relevant sizes are slow. And the fourth and fifth can not be
>     checked at all in general case.
> 
> 
>     For ECDHE, TLS 1.2 allowed server to specify custom curve to do the
>     key exchange with. Rightfully pretty much nobody implemented that.
> 
> 
>     I think TLS WG should withdraw recommendation (as flawed) on all
>     TLS_DHE_* ciphersuites.
> 
> 
>     -Ilari
> 
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