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Re: [TLS] Point validation in 1.3

Eric Rescorla <ekr@rtfm.com> Tue, 15 November 2016 07:46 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 15 Nov 2016 16:46:10 +0900
Message-ID: <CABcZeBOV7yoa=hf9fEtbrYecobvKhwZ20e_=e+zyL2ZuTvDQXQ@mail.gmail.com>
To: Watson Ladd <watsonbladd@gmail.com>
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Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] Point validation in 1.3
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On Tue, Nov 15, 2016 at 4:16 PM, Watson Ladd <watsonbladd@gmail.com> wrote:

> Hello,
>
> There has been a lot of chatter on Gitub about point validation. I think
> it's important to note that in TLS 1.3 the Triple Handshake variants
> enabled by small subgroup attacks are no longer a threat: the issue is
> reuse of ephemeral Diffie-Hellman exponents, resulting in compromise of
> what is effectively a long-term key.
>
> I would want a belt and suspenders approach: no use of ephemeral
> exponents,
>

Just to clarify, you mean "no reuse", right?

-Ekr


> and validation that points are on the curve. Order validation is
> unnecessary as the cofactor is small: in cases where it is not the curve
> probably shouldn't be used without a good reason, and I can't think of any.
>
> I know one implementation does keep ephemeral exponents indefinitely. This
> implementation also validates orders, which equals the expense of not
> regenerating ephemeral exponents.
>
> Sincerely,
> Watson
>
> _______________________________________________
> TLS mailing list
> TLS@ietf.org
> https://www.ietf.org/mailman/listinfo/tls
>
>

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