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

"Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu> Mon, 12 October 2020 17:36 UTC

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From: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
To: Ilari Liusvaara <ilariliusvaara@welho.com>, Michael D'Errico <mike-list@pobox.com>
CC: TLS List <tls@ietf.org>
Thread-Topic: [TLS] Sending Custom DHE Parameters in TLS 1.3
Thread-Index: AQHWoLYGtcjhKiOao06UF4I+ILR3K6mUOOIA//++vgA=
Date: Mon, 12 Oct 2020 17:35:21 +0000
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References: <8f57527d-efba-4d03-a3e5-f0ee33463d56@www.fastmail.com> <20201012172852.GA2560734@LK-Perkele-VII>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/fkUQJuH4m6gzPvz3w_yjAQEU3qA>
Subject: Re: [TLS] Sending Custom DHE Parameters in TLS 1.3
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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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